DESCRIPTION

ACOUSTIC TRANSMISSION PATH, SPEAKER SYSTEM USING THE SAME AND TUBE MODULE FOR ASSEMBLY KIT OF ACOUSTIC TRANSMISSION PATH

Technical Field

The present invention relates to an acoustic transmission path and a speaker system using the same, in particular, relates to an acoustic transmission path having a reproducing band region extended toward a lower region side, by alternately and periodically arranging mass elements and elastic elements, and a speaker system using the same.

Background Art

Heretofore, as a method for extending a reproducing band region toward a lower region side, a system using a resonance tube, a system utilizing Helmholtz resonance using a drone cone or a port tube, and a system by combination of both systems are known.

A resonance tube type speaker system is based on principle of so-called air column resonance. According to principle of air column resonance, sound pressure enhancing effect by resonance can be obtained, at resonance frequency where sound wave emitted from a driver unit to the inside of a resonance tube becomes standing wave.

For example, Japanese published unexamined application Hei 05-284586 discloses an example of obtaining enhancing effect of low pitch sound at frequency of standing wave, which generates along an extension direction, by extending a length in one direction of a cabinet to make a closed resonance tube.

However, in a resonance tube type speaker system, resonance is possible in a plurality of frequencies corresponding to higher harmonic waves of fundamental resonance frequency, which raises a problem of, for example, generation of sound pressure enhancement, which causes uncomfortable hearing feeling in middle low pitch sound to middle pitch sound region, at the same time as enhancement of low pitch sound in fundamental resonance frequency.

Furthermore, in a resonance tube type speaker system aiming at low pitch sound reproduction, total length of a resonance tube becomes long. To make resonance to desired low frequency using a resonance tube as short as possible, for example, total length of a resonance tube is set to a length corresponding to 1/4-wavelength of sound wave in resonance frequency. Total length Y of a resonance tube corresponding to 1/4-wavelength can be given by the following numerical expression (1) , where c represents sound velocity in atmosphere; and f represents resonance frequency. Expression (1)

Y=-

4f

To avoid growing in total dimension while ensuring acoustic total length of a resonance tube, US Patent No. 5,373,564 discloses an example of a speaker system having a resonance tube folded. Similar prior art is disclosed also in Japanese published unexamined application Hei 07-131879. However, in these prior arts, a cabinet structure results in a complicated one. Furthermore, acoustic total length of a resonance tube becomes indefinite caused by generation of sound pass difference between inner and outer circumferences in a bent part, which inevitably reduces resonance intensity. In particular, in the case where air column resonance

corresponding to 1/2-wavelength is utilized, total length of an air column resonance tube is required to be twice length given by' the above numerical expression (1). For example, total length of an air column resonance tube corresponding to 1/2-wavelength resonating with sound wave of a frequency of 60 Hz reaches to 2.8 in. Therefore, even in a folded type, total length of an air column resonance tube is a big obstacle in compact sizing of total cabinet dimension of a cabinet. Such an obstacle has made substantially impossible to use an air column resonance tube corresponding to 1/2-wavelength, aiming at enhancement of low pitch sound smaller than 100 Hz, in a compact type speaker system for general domestic applications .

On the other hand, as a method for utilization of Helmholtz resonance, a bass reflex system (a phase inversion system) using a port tube and a drone cone system using a vibration plate generally called a drone cone are known.

In a bass reflex type speaker system, Helmholtz resonance is present having air in a port tube as amass element, and air in a cabinet as an elastic element. In a bass reflex type speaker system, acoustic signals with low frequency can be enhanced by setting Helmholtz resonance frequency low.

A drone cone type speaker system is one where a port tube in a bass reflex system is substituted with a mass element with a vibration plate-like shape (generally called a drone cone) . Presence of resonance, having a drone cone as a mass element and air in a cabinet as an elastic element, is completely similar as in the above bass reflex type speaker system. Japanese published unexamined application Hei 07-203576 discloses a double bass reflex system where two cabinets of a bass reflex system and a drone cone system are

connected. As is described in this Patent Literature, the double bass reflex system is a system aiming at stronger enhancing effect of low pitch sound by double action of Helmholtz resonance. In addition, a method for extending reproduction band region of a speaker system as a compositional effect of three or more different resonance frequencies, by further multiplication of resonance elements in a bass reflex system or a drone cone system, is known as a band-pass type. This technique is disclosed, for example, in Japanese published unexamined application Hei 05-14988. A port tube or a drone cone is provided in each of the partitions of a multiplexed cabinet, and each of the partitions is set so as to have different resonance frequency. Similar prior art is disclosed also in PCT application WO 01/062043 and Japanese published unexamined application Hei 05-199581. In the above-described speaker system having resonance elements multiplexed, a band-pass type acoustic filter having flat frequency characteristics is configured by combination of a plurality of resonance with different frequency. Namely, by offsetting resonance peaks having different frequencies, flattening of frequency characteristics in a passing band, and extension of apparent reproduction band region can be attained as compared with a single resonance peak. In the above speaker systems utilizing Helmholtz resonance, enhancement of low pitch sound becomes possible by setting Helmholtz resonance frequency low. However, challenge in compact sizing of a cabinet increases air elasticity in a cabinet, resulting in increase in Helmholtz resonance frequency and makes enhancement of low pitch sound difficult .

To solve problems in such a speaker system utilizing

Helmholtz resonance, FIG. 15 of the above Japanese published unexamined application Hei 07-203576 discloses an example, where enhancement of low pitch sound is supplementarily carried out by the addition of a resonance tube to a speaker system utilizing Helmholtz resonance. In addition, FIG. 1 of the above Japanese published unexamined application Hei 05-284586 discloses a complex example between Helmholtz resonance and movement of a resonance tube. However, even in the cases where a resonance tube is added to a speaker system utilizing Helmholtz resonance, total length of a resonance tube increases according to the above numerical expression (1) , therefore, there was no change in the fact that obtaining sufficient enhancing effect in low pitch sound results in growing in total dimension of a cabinet. Disclosure of the Invention

The invention is proposed focusing attention on the above-described conventional problems. It is an object of the invention to provide an acoustic transmission path with shortened acoustic total length; and furthermore, it is an object of the invention to provide a speaker system, which is capable of extending reproduction band region toward low region side, while securing total dimension compact.

According to an aspect of the invention, an acoustic transmission path has a tube part having length of the axis direction longer than inner diameter, and is open at least at one end thereof; and an acoustic vibration part alternately arranged with mass elements and elastic elements in the axis direction of the ^' tube part, at the inside of the tube part, the arrangement periodicity of the mass elements and the elastic elements makes, propagation velocity of acoustic vibration propagating in the acoustic vibration part at the inside of the tube part, set to be less than 1/3 of sound

velocity in atmosphere, in a frequency range of from 20 Hz to 200 Hz.

According to another aspect of the invention, a speaker system has (1) a tube part having length of the axis direction larger than inner diameter, and is open at least at one end thereof, (2) an acoustic transmission path having an acoustic vibration part alternately arranged with mass elements and elastic elements in the axis direction of the tube part, at the inside of the tube part, wherein, the arrangement periodicity of the mass elements and the elastic elements makes, propagationvelocity of acoustic vibrationpropagating in the acoustic vibration part at the inside of the tube part, set to be less than 1/3 of sound velocity in atmosphere, in a frequency range of from 20 Hz to 200 Hz, and (3) an electro-acoustic transducer acoustically connected to the acoustic transmission path.

According to an acoustic transmission path of the invention, the arrangement periodicity of the mass elements and the elastic elements makes, propagation velocity of acoustic vibration propagating in the acoustic vibration part at the inside of the tube part, set to be less than 1/3 of sound velocity in atmosphere, in a frequency range of from 20 Hz to 200 Hz, by which acoustic total length equivalent or longer as compared with conventional one can be secured while maintaining total dimension compact, as compared with a conventional acoustic transmission path.

According to a speaker system of the invention, the arrangement periodicity of the mass elements and the elastic elements makes propagation velocity of acoustic vibration propagating in the acoustic vibration part at the inside of the tube part, set to be less than 1/3 of sound velocity in atmosphere, in a frequency range of from 20 Hz to 200 Hz,

by which extension of reproduction band region toward lower region side becomes possible while maintaining total dimension compact, as compared with a conventional speaker system.

Brief Description of the Drawings

FIG. 1 is a cross-sectional view of an acoustic transmission path of a first embodiment of the invention.

FIG. 2 is a drawing showing a mechanical model responding to the acoustic transmission path of FIG. 1, and appearance of element vibration, which propagates this mechanical model.

FIG. 3 is a cross-sectional view of an acoustic transmission path of a second embodiment of the invention. FIG. 4 is a cross-sectional view of a speaker system of a third embodiment.

FIG. 5 is a cross-sectional view showing an example of providing a sound-absorbing material at to the speaker system of FIG. 4. FIG. 6 is a drawing schematically showing pressure amplitude of standing wave inside a tube part in the speaker system of FIG. 4.

FIG. 7 is a cross-sectional view of a speaker system of a first comparative embodiment. FIG. 8 is a cross-sectional view of a speaker system of a second comparative embodiment.

FIG. 9 is a cross-sectional view of a speaker system of a fourth embodiment of the invention.

FIG. 10 is a cross-sectional view of a speaker system of a sixth embodiment of the invention.

FIG. 11 is a cross-sectional view of a speaker system of a seventh embodiment of the invention.

FIG. 12 is a cross-sectional view of a speaker system of an eighth embodiment of the invention.

FIG. 13 is a cross-sectional view of a speaker system of a ninth embodiment of the invention. FIG. 14 is a cross-sectional view of a speaker system of a tenth embodiment of the invention.

FIG. 15 is a cross-sectional view of a speaker system of an eleventh embodiment of the invention.

FIG. 16 is a cross-sectional view of a speaker system of a twelfth embodiment of the invention.

FIG. 17 is a cross-sectional view of a speaker system of a fourteenth embodiment of the invention.

FIG. 18 is a cross-sectional view of a speaker system of a fifteenth embodiment of the invention. FIG. 19 is a cross-sectional view of a speaker system of a sixteenth embodiment of the invention.

FIG. 20 is a cross-sectional view of a speaker system of a seventeenth embodiment of the invention.

FIG. 21 is a cross-sectional view of a speaker system of an eighteenth embodiment of the invention.

Detailed Description of the Embodiment

Explanation will be given below on embodiments of the invention with reference to drawings. (A first embodiment)

FIG. 1 is a cross-sectional view of an acoustic transmission path of a first embodiment of the invention.

The acoustic transmission path 10 of the embodiment has the tube part 1 and an acoustic vibration part. The acoustic vibration part is provided with the vibration plates 3a, 3b, 3c and 3d (hereafter may be referred to generally as "vibration plates 3") , which are provided

so as to be able to vibrate at the inside of the tube part 1, and the air chambers 4a, 4b and 4c (hereafter may be referred to generally as "air chambers 4") , which are mutually partitioned in the inside of the tube part 1. Here, the vibration plates 3 function as mass element, and air in the air chambers 4 function as an elastic element . These vibration plates 3 and the air chambers 4 are arranged alternately and periodically along the axis direction of the tube part 1. Therefore, the acoustic vibration parts 3 and 4 are configured by alternate and periodic arrangement of the mass elements and the elastic elements along the axis direction of the tube part 1.

This acoustic vibration part is configured so as to propagate wave motion of predetermined acoustic vibration (hereafter referred to as "element vibration") different from normal sound wave propagating in atmosphere. In addition, the acoustic vibration part is one that makes acoustic vibration to be propagated, namely propagation velocity of element vibration, less than 1/3 of sound velocity in atmosphere, in a frequency range of from 20 Hz to 200 Hz. It should be noted that detail of the acoustic vibration part will be described later.

As described above, the acoustic transmission path 10 of the embodiment has elements such as the tube part 1, the vibration plate 3, the air chambers 4 and the like, in broad classification. Explanation will be given below on each of the elements.

The tube part 1 is, for example, a cylindrical tube. The tube part 1 is configured so as to have length in the axis direction (hereafter referred to as total length) longer than inner diameter.

The tube part 1 of the embodiment is open at both ends,

in other words has the open ends 9 at both ends . Specifically, at the both ends of the tube part 1, the end part walls 5a and 5b, having an opening, are provided. In addition, at the inner surface of the tube part 1, the separation walls βa and 6b are provided. In the embodiment, total length of the tube part 1 is 3 L, and the separation walls βa and 6b are arranged, in the same interval, at positions of distance L and 2L from the end part of the tube part 1, respectively.

It should be noted that the tube part 1 may form a tubular shape by connection of discrete short tubular modules along the axis line, or by combination of discrete semi-tubular modules, in view of providing easy assembly of the acoustic transmission path 10.

Then, explanation will be given on the vibration plates 3. The vibration plates 3 are provided via the edge member 7 on each of the end part walls 5a and 5b, or each of the separation walls βa and βb of the tube part 1. Therefore, in the embodiment, because the tube part 1 has the open ends 9 at both ends, the vibration plates 3a and 3b, as mass elements, are set to be arranged at the both ends of the tube part 1. It should be noted that the edge member 7, which supports the vibration plates 3, supports so as to be able to vibrate the vibration plates 3, and configured by a flexible material such as rubber, foamed polyurethane resin or the like, so as to be easily deformable. As a result, each of the vibration plates 3 is capable of vibrating in the axis direction of the tube part 1 within a deformation range of the edge member 7. Because each of the vibration plates 3 is arranged so as to oscillate in the axis direction of the tube part, element vibration propagates as plane wave in the tube part 1. For example, in the case where at least one vibration plate is provided with inclination against the axis direction of the

tube part 1, energies are distributed to propagation mode other than simple plane wave, resulting in weakening of resonance by reciprocating wave in the total length direction of the tube part 1. As a material of the vibration plates 3, as well as a vibrationplate of a general driver unit (an electro-acoustic transducer) , a highly rigid and light weight material such as a metal thin plate, resin-fiber fabric, high density paper or the like may be used. Shape of each of the vibration plates 3 may be non-planer shape such as a circular cone, a hemisphere or the like to ensure strength, however, due to distribution of energies to propagation mode other than simple plane wave, which generates by oscillation of the vibration plate, resonance by reciprocating wave in a total length direction of the tube part 1 may result in to be weakened. It should be noted that, in the case where the shape of the vibration plates 3 has length in the front and back directions, such as a circular cone or the like, it is desirable that unnecessary motion, such as displacement or rotation of the vibration axis or the like, is suppressed, by supporting the vicinity of the tip of the vibration plates 3 with the edge member 7, and at the same time, by the addition of an elastic support unit such as a damper or the like also at the vicinity of the rear end of the vibration plates 3. In addition, because extremely small effective vibration area of each of the vibration plates 3 as compared with cross-sectional area of the inside of the tube part 1, makes wave motion, which generates by vibration of each of the vibration plates 3, close to spherical wave, effective vibration area of each of the vibration plates 3 is preferably larger than 25% of cross-sectional area of the inside of the tube part 1, further preferably larger than 50% of

cross-sectional area of the inside of the tube part 1. On the other hand, as will be described later, in view of enhancing lowering effect of propagation velocity of acoustic vibration of an acoustic vibration part as compared with sound velocity in atmosphere, it is desirable that effective vibration area of each of the vibration plates 3 is smaller than 75% of cross-sectional area of the inside of the tube part 1.

It should be noted that a plurality of mass elements, namely masses of the vibration plates 3 may all be the same, however, mass ratio among the vibration plates 3, namely ratio of maximal mass to minimal mass among the vibration plates may also be varied within a range of from 1 to 3. For example, at least one mass element may be configured so as to be lighter as compared with other mass elements. For example, to compensate the effect associated with finite number of the vibration plates 3 and the air chambers 4 , the vibration plates 3a and 3d as arranged at one end or both ends of the tube part 1, may be configured so as to have a mass of 0.5 to 1 time mass of the vibration plates 3b and 3c as mass element arranged at the inner side of the tube part 1.

Next, explanation will be given on the air chambers 4. The air chambers 4 are air chambers inside the tube part 1, and, in the embodiment, the inner space of the tube part 1 is partitionedto 3 air chambers 4a, 4b and 4c by the separation walls 6a and 6b. Therefore, each of the air chambers 4a, 4b and 4c is arranged in series along the axis line of the tube part 1. At the both sides of the each of the air chambers 4, each of the vibration plates 3 is arranged, as described above. As a result, the acoustic transmission path of the embodiment, which the vibration plates 3 as mass element and the air chambers 4 containing air as elastic elements are arranged alternately and periodically, is configured.

Explanation will be given on action effect of the acoustic transmission path of the embodiment, which is configured as above.

FIG. 2 shows the simplest mechanical model corresponding to the acoustic transmission path of the embodiment, and the lower part of FIG. 2 shows appearance of element vibration, which is wave motion propagating this mechanical model. Specifically, the vibration plates 3 in the acoustic transmission path of the embodiment correspond to mass point, and air in the air chambers 4 corresponds to a spring.

The acoustic transmission path 1 of the embodiment is one, which propagates wave motion, whose propagation velocity- is reduced, by arrangement periodicity of the above mass elements and the above elastic elements, based on principle to be described below.

As shown in FIG. 2, a simplified mechanical model corresponding to the acoustic transmission path of the embodiment is a series vibration element system, where vibration elements are arranged in a constant cycle length L, in which vibration elements have a mass point with a mass of m as a mass element and a spring with a spring constant of S as an elastic element . Equation ofmotion can be expressed bythe followingnumerical expression (2) , where displacement amount of the ^λλ nth" mass point from an equilibrium position is represented by u _n . Expression (2)

The above numerical expression (2) has a traveling-wave type solution expressed by the following numerical expression

(3) , where k represents wave number; x represents position coordinates of a mass point and a spring along an arrangement direction; and ω represents angular frequency. Expression (3) u _n =Bexp[i(kx _n -ωt)]

The fact that wave motion propagating in a model of FIG. 2 represented by the numerical expression (3), namely element vibration, shows completely different behavior from that of usual sound wave propagating in atmosphere in a usual cabinet, can be understood through dispersion relation,

"numerical expression 6", which is obtained by substituting the following numerical expressions (4) and (5) and the above numerical expression (3) into the above numerical expression

(2) . Expression (4)

U _n+1 = B exp[i(kx _n + kL - cot)]

Expression (5)

Expression (β)

In the above numerical expression (6) , ω monotonously increases with wave number k, and takes the maximal value in numerical expression (7). Expression (7)

The numerical expression (7) corresponds to fundamental resonance frequency of each of the vibration elements, as a single body, and element vibration having higher frequency than this cannot propagate. This behavior corresponds to the fact that, because each of the vibration elements with a length of L (configured by a mass point and springs in a region of L/2 at the both sides thereof) is the minimal vibration unit, in an acoustic transmission path of FIG. 2, element vibration with a wavelength of 2L (a wave number of π/L) , which is motion of adjacent vibration elements in a reversed direction, is propagation mode with the shortest wavelength (namely, the largest wave number) , and element vibration with wavelength shorter than this cannot propagate . Agraph shown at the lowerpart of FIG.2 is a plot of displacement of each of the element vibrations with a wave number of π/L, π/2L, π/3L and π/4L in positional correspondence with each of mass points IA, IB, 1C, ID and IE; and also from this graph, it can be understood that element vibration cannot take a wavelength shorter than 2L. In addition, sound velocity in atmosphere is nearly constant, however, phase velocity Vp of element vibration varies, as shown by numerical expression (8), depending on values of mass m and spring constant S. Expression (8)

As a result, by suitably setting values of mass m and spring constant S, phase velocity Vp can also be set, for example, lower than sound velocity in atmosphere. Furthermore, the above numerical expression (8) shows

that phase velocity Vp varies depending on frequency, and maximal value of phase velocity Vp is numerical expression (9) • Expression (9)

According to the mechanical model corresponding to the acoustic transmission path of the embodiment as shown above, it can be said that element vibration propagates, showing completely different behavior from usual sound wave propagating in atmosphere, in view of the following facts: wavemotion having shorterwavelength than 2L (2 times distance between adjacent mass points) substantially does not propagate; phase velocity Vp varies depending on values of mass m and spring constant S; and phase velocity Vp depends on frequency.

Explanation will be given on action effect of the acoustic transmission path 10 of the embodiment, in consideration of the above mechanical model.

In the acoustic transmission path 10 shown in FIG. 1, each of the vibration plates 3 is a mass element that is capable of oscillating in the axis direction of the tube part 1, and corresponds to a mass point in FIG.2. In addition, air inside of each of the air chambers 4, which is formed in partition by each of the vibration plates 3, is an elastic element, and corresponds to a spring in FIG. 2.

For example, excitation of the vibration plate 3a or 3b at the most outside by external sound wave results in propagation of vibration of the vibration plate 3a or 3b at the inside of the tube part 1, as the above element vibration. In this case, element vibration with wavelength, having the

open ends 9 at the both sides of the tube part 1 as antinodes, becomes standing wave in the tube part 1, and resonates with external sound wave having the same frequency. Namely, the vibration plate 3a or 3b corresponds to an opening in an air column resonance tube, and the acoustic transmission path 10 of the embodiment of FIG. 1 is configured as a resonance tube with openings at both ends .

As shown in FIG. 1, the tube part 1 of the acoustic transmission path 10 of the embodiment is set so that total length of the tube part 1 is sufficiently large, as compared with inner diameter (diameter) of the tube part 1. Therefore, as for propagation mode of element vibration in the tube part 1, plane wave, which propagates in the total length direction of the tube part 1, is dominant. It is important that element vibration in the tube part 1 is plane wave-like in obtaining strong resonance without dissipation of energies to other propagation modes. For example, different from the embodiment, setting of total length of the tube part 1 smaller, as compared with inner diameter of the tube part 1, results in weakening of resonance by reciprocating wave in the total length direction of the tube part 1, because energies are distributed to cylindrical wave mode, which propagates in the diameter direction in the tube part 1.

In addition, in the embodiment, as shown in FIG. 1, each of the vibration plates 3 is perpendicular to the axis line of the tube part 1, and each of the vibration plates 3 is arranged so as to oscillate in the axis direction of the tube part 1, thereby element vibration propagates as plane wave in the tube part 1. For example, different from the embodiment, in the case where at least one vibration plate is provided in inclination against the axis line of the tube part 1, resonance by reciprocating wave in the total length

direction of the tube part 1 results in to be weakened, because energies are distributed to propagationmode other than simple plane wave.

In addition, in the case where shape of the vibration plates 3 of the embodiment is formed in flat plate-like, element vibration can propagate as plane wave in the tube part 1, without disturbing planarity of wave surface by vibration plate shape. On the other hand, when at least one of the vibration plates 3 has non-plane-like shape such as a circular cone, a hemisphere or the like, resonance by reciprocating wave in the total length direction of the tube part 1 results in to be weakened, because energies are distributed to propagationmode other than simple plane wave, which generates by oscillation of the vibration plates 3. In addition, extremely small effective vibration area of each of the vibration plates 3, as compared with cross-sectional area of the tube part, results in wave motion, which generates by oscillation of each of the vibrationplates, very close to spherical surface wave, therefore, it is desirable that effective vibration area of each of the vibration plates to be larger than 25%, more advantageously larger than 50% of cross-sectional area of the inside of the tube part 1, for element vibration in the tube part 1 to be plane wave like. The acoustic transmission path of the embodiment shown in FIG. 1 is one where mass point m in a simplified model of FIG. 2 is substituted with mass m ₀ of the vibration plates 3, and spring constant S is substituted with elasticity Si of air inside the air chambers 4 , however, when spring constant So of the edge member 7 and mass mi of air in the air chambers 4 are considered as parasitic elements, dispersion relation of the acoustic transmission path of the embodiment is

expressed by "numerical expression 10", and phase velocity Vp is expressed by ^Xλ numerical expression 11". Expression (lθ)

Expression (ll)

Here, A represents cross-sectional area of the inside of the tube part 1; K represents volume elasticity of air; and p represents density of air.

For example, in the case where the tube part 1 is a cylindrical tube with an inner diameter of 0.12 m, and having a drone cone configured by a vibration system of a driver unit having a typical nominal caliber of 0.08 m (an effective diameter of 0.064 m) arranged in an interval of 0.15 m, each of parameter values can be set as follows: A: 0.0113 m ² , L: 0.15 m, m ₀ : 2 g, S ₀ : 790 N/m, K: 141700 N/m ² , and p: 1.29 kg/m ³ .

In addition, elasticity of the air chambers 4, which acts to the vibration plates 3, decreases in proportion to (a/A) ² , when "a" represents effective vibration area of the vibration plates 3. Namely, setting of the effective vibration area "a" of the vibration plates 3 smaller than cross-sectional area "A" of the inside of the tube part 1 substantially reduces elasticity of the air chambers 4, which

acts to the vibration plates 3, and has effect of reducing propagation velocity of element vibration, which propagates in the tube part 1. Therefore, for example, by setting effective vibration area "a" of the vibration plates 3, as mass elements, to be smaller than 75% of cross-sectional area ^λλ A" of the inside of the tube part 1, effect of reducing propagation velocity of element vibration, which propagates in the tube part 1, can sufficiently be fulfilled.

Here, by adding compensation to the above numerical expression (10) and numerical expression (11), in consideration of the fact that effective vibration area of a typical driver unit having an caliber of 0.08 m is 0.032 m ² , and therefore occupies 28.4% of cross-sectional area "A" of the tube part 1, frequency of element vibration in the shortest wavelength of 2L (a wave number of π/L) , which can propagates, is calculated to be 241 Hz, and phase velocity Vp is calculated to be 72.3 m/s. Similarly, frequency of element vibration in a wavelength of 6L (a wave number of π/3L) is calculated to be 120 Hz, and phase velocity Vp is calculated to be as slow as 108.4 m/s, about 1/3 of sound velocity in atmosphere. Thus, in the acoustic transmission path of the invention, low propagation velocity lower than

1/3 of sound velocity in atmosphere can easily be obtained.

It should be noted that the above numerical expressions (10) and (11) are for carrying out approximate prediction, on the premise of a model of an infinite and simplified acoustic vibration element sequence; and in the case where an acoustic transmission path is practically configured by finite number of acoustic vibration elements, compensation should be performed, as appropriate, to the above numerical expression (10) or (11) , or calculation results based thereon.

For example, in FIG. 1, air chambers, as elastic

elements , are present at the both sides of the vibration plates 3b and 3c positioned at the middle of a transmission path so as to sandwich the vibration plates; on the other hand, one side of the air chambers is absent as for the vibration plates 3a and 3d positioned at the open end of a transmission path, namely at the end part walls 5a and 5b having an opening, and thereby spring constant results in to be reduced nearly by half. Therefore, in a vibration element positioned at an open end, frequency of element vibration reduces to about 2 ^~1/2 at the minimum, as compared with the above numerical expressions (6) and (7) . As a result, for example, element vibration having frequency of the above numerical expressions

(7), which can propagates in other vibration elements positioned at the middle of a transmission path, does not satisfy propagation condition in a vibration element positioned at the open end 9 having reduced resonance frequency, namely, the mismatch, which wavelength becomes shorter than 2L, generates.

To reduce such mismatch, reduction of spring constant may be off set by reducing mass of the vibration plates 3a and 3d positioned at the open end 9. Namely, by reducing mass of the vibration plates 3a and 3d positioned at the open end 9, so as to suitably set within a range of from 1 time to 1/2 time mass of the vibration plates 3b and 3c positioned at the middle of a transmission path, adjustment is possible so that all of the vibration elements belonging to a transmission path, including the end part, are attuned to substantially the same frequency.

As described above, according to the acoustic transmission path of the embodiment, the following action effects can be fulfilled.

An acoustic vibration part is configured by alternate

arrangement of the vibration plates 3 as mass elements and air chambers 4 as the elastic elements, and the arrangement periodicity of mass elements and elastic elements makes, propagation velocity (phase velocity Vp) of element vibration propagating in an acoustic vibration part set to be less than sound velocity in atmosphere, according to numerical expression (8) . In particular, low propagation velocity less than 1/3 of soundvelocity in atmosphere can easilybe obtained, according to numerical expression (8) . Therefore, even a tube part having short physical total length can attain enhancing effect of low pitch sound equivalent to that in an air column having long total length.

Furthermore, partition of the inner space of the tube part 1 by the vibration plates 3b and 3c, which are arranged along the axis direction of the tube part 1, results in substantially no propagation of element vibration having a wavelength of shorter than 2L, by which acoustic vibration with unnecessary middle to high frequency can be suppressed, and only acoustic vibration with low frequency can selectively be transmitted.

Because the tube part 10 has length in the axis direction longer than inner diameter, plane wave, which propagates in the axis direction of the tube part 1, can be made a predominant state over cylindrical wave. Therefore, energy dissipation to other propagation mode such as cylindrical wave or the like can be prevented.

By arrangement of each of the vibration plates 3, as a mass element, so as to vibrate in the axis direction of the tube part 1, element vibration can be propagated as plane wave in the tube part 1, and thus weakening of resonance by reciprocating wave in the axis direction of the tube part 1 can be prevented.

It should be noted that in the case where each of the vibration plates 3, as a mass element, is formed in flat plate-like, element vibration propagates as plane wave in the tube part 1, without disturbing planarity of wave surface by vibration plate shape, by which energy distribution to propagationmode other than simple plane wave, which generates by oscillation of the vibration plates 3, can be prevented, and thus weakening of resonance by reciprocating wave in the total length direction of the tube part 1 can be prevented. By setting effective vibration area of each of the vibration plates 3, as a mass element, larger than 25%, preferably larger than 50% of cross-sectional area of the inside of the tube part 1, wave motion, which generates by vibration of each of the vibration plates 3, can be prevented from becoming similar to spherical surface wave.

By setting effective vibration area of each of the vibration plates 3, as a mass element, smaller than 75% of cross-sectional area of the inside of the tube part 1 , reduction effect of propagation velocity of element vibration, which propagates inside the tube part 1, is enhanced.

In the case where the vibration plates 3a and 3d, as mass elements, arranged at the open end of the tube part 1, are configured so as to have mass of 0.5 to 1 time the vibration plates 3b and 3c, as mass elements, arranged at the inside of the tube part 1, adjustment is possible that the vibration plates 3, as all of the vibration elements belonging to the transmission path, including the end part, and air in the air chambers 4 , are attuned to substantially the same frequency, by compensation of effect associated with finite number of the vibration plates 3 and the air chambers 4. (A second embodiment)

FIG. 3 is a cross-sectional view of an acoustic

transmission path of a second embodiment of the invention. It should be noted that in the acoustic transmission path of the embodiment, configuration is similar to that of the acoustic transmission path of the first embodiment shown in FIG. 1, except the following points: one end of the tube part is closed by an end wall; length (total length) in the axis direction of the tube part is 2L; number of the vibration plates is 2; and number of the air chambers is 2. Therefore, as for the similar configuration, the same reference number is used, and repeating explanation is omitted here.

The tube part 1 of the embodiment is closed at one end, in other words, one end is the closed end 8. Specifically, at one end of the tube part 1, a closed end wall is provided. On the other hand, the other end of the tube part 1 is open, in other words, an open end.

The vibration plate 3a is provided at other end of the tube part 1, namely at the open end of the tube part 1. In the embodiment, total length of the tube part 1 is 2L, and the vibration plate 3b is provided at position with a distance of L from the open end of the tube part 1. It should be noted that the vibration plate 3a and the vibration plate 3b may be provided via the edge member 7 at the end part wall 5 and a separation wall or the like, similarly as in the first embodiment. As a result, the vibration plate 3a and the vibration plate 3b are capable of vibrating each in the axis direction of the tube part 1.

It should be noted that it is desirable that the inside surface of the closed end wall is a flat surface, and shape of each of the vibration plates 3a and 3b is flat plate-like, for acoustic vibration, which propagates an acoustic vibration part, to propagate as plane wave at the inside of the tube part 1. In addition, from the similar viewpoint,

it is desirable that the inside surface of the closed end wall and each of the vibration plates 3a and 3b are configured so as to be orthogonal against the axis direction of the tube part 1. This is similar as in the case of the first embodiment . Next, explanation will be given on the air chambers 4a and 4b. In the embodiment, the inner space of the tube part 1 is partitioned by the vibration plate 3b into two air chambers 4a and 4b. Therefore, each of the air chambers 4a and 4b is arranged in series along the axis line of the tube part 1. In the air chamber 4a, each of the vibration plates 3a and 3b is arranged at both sides. On the other hand, in the air chamber 4b, the vibration plate 3b is arranged at one side at the air chamber 4a side, and the closed end wall is arranged at the other side. Explanation will be given on action effect of the acoustic transmission path of the embodiment configured as above .

The acoustic transmission path of the embodiment corresponds to the case where each of the vibration plates 3a and 3b is arranged at each position of adjacent mass points IA and IB, and the closed end wall is arranged at the position of 1C, in the above mechanical model shown in a graph of FIG. 2. According to FIG. 2, distance from IA to 1C corresponds to 1/2-wavelength of element vibration with a wave number of π/2L. And, by setting this element vibration, with a wave number of π/2L, to be a node at the position of IA of the open end, and to be a node also at the position of 1C of the closed end wall, the acoustic transmission path of the embodiment can resonate with element vibration having a wave number of π/2L.

On the other hand, as shown in a graph of FIG.2, element vibration with a wave number of π/L can become a node only

at a mid point of arrangement interval L (namely a position of L/2) of the vibration plates 3a and 3b. Therefore, in the acoustic transmission path of the embodiment, where the closed end 8 is arranged at a position 1C coincident with arrangement cycle of the vibration plates, element vibration with a wave number of π/L cannot become standing wave.

In general, in the case where one end of an acoustic transmission path is an open end, and the other end is a closed end, setting of total length of an acoustic transmission path in integral multiple of L does not allow element vibration with a wave number of π/L (a wavelength of 2L) to become standing wave. This point is different from the case of the first embodiment where both ends of an acoustic transmission path are open ends. Even in the embodiment, similar effect as in the case of the first embodiment is obtained.

As described above, one important characteristic of the acoustic transmission paths of the first and the second embodiments is that selective response is possible to element vibration with a specific wavelength, by configuring an acoustic transmission path into a formof an acoustic resonance tube.

Here, the acoustic transmission path of the invention, which is characterized in that element vibration with higher angular frequency than that given by the above numerical expression (7) cannot propagate, and element vibration with higher frequency than predetermined frequency is principally not present, is suitable, for example, in applications such as a resonance tube for enhancing low pitch sound of a speaker system, where response, in a resonance way, is desirable only to sound wave with low frequency, but not to sound wave with high frequency. Furthermore, use of the acoustic

transmission path of the invention is capable of configuring a compact resonance tube type speaker system having total length shortened based on propagation velocity lower than soundvelocity in atmosphere . Explanation will be given below on speaker systems as embodiments of the invention. (A third embodiment)

FIG. 4 is a cross-sectional view of a speaker system of a third embodiment.

The speaker system of the embodiment has configuration where a driver unit is acoustically connected to a similar acoustic transmission path as in the first embodiment shown in FIG. 1. Here, fundamental configuration of the acoustic transmission path itself is similar to the acoustic transmission path of FIG. 1, except that a through hole to acoustically connect the driver unit is provided at the sidewall of a tube part. Therefore, as for the similar configuration, the same reference number is used, and repeating explanation is omitted here.

As shown in FIG. 4, the speaker system 30 of the embodiment has the driver unit 20 having acoustically connected to the acoustic transmission path 10 so as not to partition the inside of the tube part 1 in the axis direction of the tube part. In particular, the driver unit 20 is acoustically connected to the acoustic transmission path 10 in the middle of arrangement of mass elements and elastic elements so as not to partition arrangement of the vibration plates 3 as mass element and air chambers 4 as elastic elements .

At the sidewall of the tube part 1 of the acoustic transmission path 10, the through hole 11 to acoustically connect the driver unit 20 is provided. In the embodiment, odd number of air chambers, specifically the air chambers Aa, 4b and 4c are arranged in series, and the through hole

11 is arranged so as to communicate the air chamber 4b arranged at the center part, among odd number of the air chambers 4a, 4b and 4c, and outside world. In particular, the through hole 11 is desirably arranged at the center part along the axis direction of the tube part 1. It should be noted that, as will be described later, a center part along the axis direction of the tube part 1 corresponds to a node position of velocity amplitude of standing wave present in the tube part 1, and the end part of the tube part 1 corresponds to an antinode position of velocity amplitude of the standing wave.

In addition, in the acoustic transmission path 10, movable range of each of the vibration plates 3 is desirably larger than ± 1.0 mm to allow sufficient oscillation as an antinode of velocity amplitude of the standing wave. However, to the vibration plates 3b and 3d at the inside which is not antinode positions of velocity amplitude of the standing wave, a smaller movable range may be set as compared with the vibration plates 3a and 3d. In addition, it is desirable that spring constants of the edge member 7 or other elastic support units (not shown) to support each of the vibration plates 3, and the vibration plates 3 themselves, when converted to Vas (equivalent volume) is larger than volume of one of the adjacent air chambers 4. Namely, it is because of making relatively high elasticity predominant of compact sized air chambers 4, as elastic elements, and thus suppressing, to small level, the increasing effect of resonance frequency by an air sealing unit such as the edge member 7 or the like. In addition, it is desirable that Qms (mechanical Q value) of a vibration system, configured by the vibration plates 3 and the air sealing unit, is larger than 3.0, in view of suitable operation as the resonance tube type speaker system 30.

The driver unit 20, which is acoustically connected to the acoustic transmission path 10 like this, has vibration surface, and is an electro-acoustic transducer that converts input electric signals to corresponding acoustic output signals. Namely, the driver unit 20 functions as a driver of a speaker. To the driver unit 20, output impedance of a driving amplifier (not shown) to drive the driver unit 20 is electrically connected. Specifically, an output impedance of the driving amplifier is connected to the both ends of a driver coil. It should be noted that Qts (total

Q value) of the driver unit 20 is desirably smaller than 0.5.

In the embodiment, the driver unit 20 is arranged at the sidewall of the tube part 1, so that the back surface thereof faces to the through hole 11, as shown in FIG. 4. The through hole 11, as described above, is arranged so as to communicate the air chamber 4b arranged at the center, with outside world, therefore, the driver unit 20 opposing to the through hole 11 is designed to be connected to the air chamber 4b arranged at the center part, among the odd number of air chambers 4a, 4b and 4c. In addition, by arrangement of the through hole 11 at the center part along the axis direction of the tube part 1, the driver unit 20 can be connected at a node position of standing wave present in the tube part 1. It should be noted that, in the embodiment, to arrange the driver unit 20 at the outside of the sidewall of the tube part 1, for example, a support member is provided at the tube part 1. The support member fixes the driver unit 20 at the outside of the sidewall of the tube part 1, and at the same time forms the driver housing 12, by partitioning the space from outside world, so as to cover the back surface side of the driver unit 20.

In addition, the speaker system of the embodiment, as shown in FIG .5 , may configure so as to have the sound-absorbing material 14 at the inside of the tube part 1. The sound-absorbing material 14 is a fiber aggregate such as natural fiber, synthetic fiber, glass fiber or the like, and is retained inside a net member formed cylinder-likely, and is arranged at the vicinity of the center axis of the tube part (hereafter referred to as "tube axis") by a support unit not shown. Explanation will be given on action effect of the speaker system 30 of the embodiment configured as above.

First, input electric signals are input from a driving amplifier, which then drives the driver unit 20 to emit acoustic output signals, namely sound wave. Sound wave emitted from the back surface of the driver unit 20 is introduced to the acoustic transmission path 20 via the through hole 11, and enhanced by resonance as the above-described element vibration.

FIG. 6 is a drawing schematically showing pressure amplitude of standing wave inside a tube part in the speaker system of FIG. 4. Pressure amplitude, as shown by a solid line in FIG. 6, forms standing wave by making an antinode at the vicinity of the center of the central air chamber 4b, and a node at the both ends of the tube part 1. Here, velocity amplitude of the standing wave, as shown by a dotted line in FIG. 6, provides a node at the vicinity of the center of the central air chamber 4b, and an antinode at the both ends of the tube part 1. Therefore, element vibration in the embodiment corresponds to element vibration with k=π/3L, shown in FIG. 2, and is also equivalent to a resonance state of an air column with openings at both ends to sound wave with total length as 1/2-wavelength. It shouldbe noted that,

in ^' an embodiment of FIG. 4, standing wave corresponding to element vibration with k=π/L, shown in FIG.2, is also allowed, and this is equivalent to a resonance state of an air column with openings at both ends, to sound wave with total length as 3/2-wavelength.

In a general air column resonance tube, however, further sets of resonance at higher harmonic waves such as 5/2-wavelength, 7/2-wavelength and the like are present, which cause acoustically uncomfortable feeling . On the other hand, in an acoustic transmission path of the invention, standing wave at frequency higher than element vibration with k=π/L, corresponding to the above numerical expression (7) , is not allowed. Therefore, the speaker system 30 of the embodiment, shown in FIG. 4, has advantage that offensive resonance of higher order than resonance equivalent to 3/2-wavelength in an air column resonance tube does not generate .

It should be noted that, in the speaker system 30 of the embodiment, sound wave with frequency higher than element vibration of k=π/L becomes sound wave having normal velocity permeating, while attenuating, through each of the air chambers 4 and each of the vibration plates 3, because there is no presence of propagation mode by element vibration. Therefore, acoustic total length of a tube for middle to high pitch sound emitted from the back surface of the driver unit 20 coincides with mechanical total length dimension of the tube. Therefore, in the resonance tube type speaker system 30 according to the embodiment, middle to high pitch sound emitted from the back surface of the driver unit 20 is discharged outside via extremely short acoustic path, by which generation of so-called "muffled sound" caused by repeated reflections at the wall surface in a long tube, like in a

conventional resonance tube type speaker system, is prevented, and thus comfortable indirect sound is recognized.

In addition, in the speaker system 30 of the embodiment, as shown in FIG. 5, emission of middle to high pitch sound can be reduced also by arrangement of the sound-absorbing material 14 at the vicinity of the tube axis. Middle to high pitch sound emitted from the back surface of the driver unit 20 is useful to be recognized as comfortable indirect sound, when emitted from the end part of the tube part 1, however, it is harmful to be recognized as offensive sound, when it forms standing wave along the diameter direction of the tube part 1 by reflection at the inner surface of the sidewall of the tube part 1. In this point, in the speaker system 30 of the embodiment, by intensive arrangement of the sound-absorbing material 14 at the axis position, where an antinode of standing wave is present along the diameter direction of the tube part 1, harmful middle to high pitch sound, forming standing wave along the diameter direction, is efficiently absorbed, and at the same time, an opening with large cross-section is ensured at the vicinity of the sidewall of the tube part 1, by which permeation can be secured for useful middle to high pitch sound to transmit along the axis direction.

By the way, in the speaker system of the embodiment, shown in FIG. 4, the driver unit 20 is acoustically connected to the acoustic transmission path 10, so as not to partition inner space of the tube part 1 in the axis direction of the tube part 1. Explanation will be given on this point by comparing the speaker system of the embodiment with a speaker system of a first comparative embodiment, shown in Fig. 7.

Also in the speaker system of the first comparative embodiment, shown in Fig.7, the driver unit 20 is acoustically

connected to the acoustic transmission path 10, however, different from the speaker system of the embodiment, inside of the acoustic transmission path 10 is partitioned in the axis direction of the tube part 1 by the driver unit 20. Specifically, in the speaker system of the first comparative embodiment, shown in Fig. 7, the vibration plate 3b in an acoustic transmission path of FIG. 1 is substituted with the driver unit 20. A problem of this speaker system of the first comparative embodiment is that acoustic vibration having discontinuous amplitude or phase is emitted before or after a vibration plate of the driver unit 20 (hereafter referred to as "driver unit vibration plate") .

In general, pressure of acoustic vibration emitted from the driver unit 20 takes a reversed phase between the front surface and the back surface of the driver unit vibration plate. Namely, when one takes plus pressure, the other takes minus pressure. Therefore, pressure distribution in the tube part 1 of acoustic vibration emitted from the driver unit 20 becomes asymmetrical distribution where polarity is reversed between the front and the rear of the driver unit vibrationplate, as shownby a solidline in FIG.7. Inaddition, velocity distribution thereof, as shown by dotted line in FIG.7, becomes symmetric distribution not smoothly connected between the front and the rear of the driver unit vibration plate. On the other hand, reflected wave present in the tube part 1, by reflection at the both ends of the tube part 1, becomes continuous and smooth distribution over the total length of the tube part 1, similarly as in the case shown in FIG. 6. Therefore, in the speaker system of the first comparative embodiment, emission wave from the driver unit 20 and reflected wave present in the tube part 1 cannot take phase relation so as to mutually enhance. Namely in the

speaker system of the first comparative embodiment, where the driver unit 20 is arranged in the middle of a transmission path, as shown in FIG. 7, presence of standing wave, having total length of a tube as a resonance target, as shown in FIG. 6, is not allowed, different from the speaker system of the embodiment.

It should be noted that arrangement of driver unit, similar to that in the first comparative embodiment, shown in FIG. 7, is common arrangement in band-pass type speaker systems disclosed in the above-described Japanese published unexamined application Hei 05-14988, PCT application WO 01/062043, and Japanese published unexamined application Hei 05-199581. Therefore, band-pass type speaker systems disclosed in these Patent Literatures are also different technique from the speaker system of the embodiment, shown in FIG. 4, from the view points that each of the acoustic masses before and after the driver unit 20 only independently forms Helmholtz resonance, and presence of standing wave, having total length of a tube as a resonance target, as shown in FIG. 6, is not allowed.

In addition, in the speaker system of the embodiment, as shown in FIG. 4, a driver unit is arranged at the outside of the sidewall of the tube part 1. Explanation will be given on this point by comparing the speaker system 30 of the embodiment with a speaker system of a second comparative embodiment, shown in Fig. 8.

Also in the speaker system of the second comparative embodiment, shown in Fig.8, the driver unit 20 is acoustically connected to the acoustic transmission path 10, shown in FIG. 1 , however, different fromthe speaker system of the embodiment, the driver unit is not arranged at the outside of the sidewall of the tube part 1.

Specifically, in the speaker system of the second comparative embodiment, both ends of the tube part 1 are open, and the driver unit 20 is provided at one end of the tube part 1, and at the other end of the tube part 1, the vibration plate 3d is provided as a mass element.

In other words, in the speaker system of the second comparative embodiment, shown in FIG. 8, the vibration plate 3a in the acoustic transmission path of FIG. 1 is substituted with the driver unit 20. In this speaker system of the second comparative embodiment, the driver unit 20 is arranged at the end of the acoustic transmission path, and the inside of the tube part 1 is not partitioned by the driver unit 20, therefore, a problem of the first comparative embodiment that presence of standing wave, having total length of a tube as a resonance target, is not allowed can be avoided. The embodiment of FIG. 8, however, has another problem, which termination condition of the tube part 1 results in being altered depending on characteristics of a driver unit, as compared with the present embodiment. For example, FIG. 8 is a cross-sectional view of a speaker system of a second comparative embodiment, similarly as in the embodiment shown in FIG. 6. In this case, velocity amplitude of standing wave, similarly as in the case of the embodiment, takes a node at the central air chamber 4b, and an antinode at the both ends of the tube part 1; therefore the vibration plate of the driver unit 20 arranged at the end of the tube part 1 is required to largely oscillate as an antinode of velocity amplitude.

However, because output impedance of a driving amplifier for the driver unit 20 is electrically connected to the driver unit 20, in the case where output impedance of the driving amplifier is low, the both ends of a voice

coil of the driver unit 20 are equivalently shorted, and strong electromagnetic brake works, which inhibits free vibration of the vibration plate of the driver unit 20. Namely, in the case where output impedance of the driving amplifier is low, the driver unit 20 arranged at the end part of the tube part 1 cannot become an antinode of velocity amplitude, therefore, presence of standing wave having total length of the tube part 1 as half-wavelength, as in the case of the embodiment shown in FIG. 6, is not allowed. Here, because it is possible that the driver unit 20 arranged at the end of the tube part 1 becomes a node of velocity amplitude, presence of standing wave, having total length of a tube as 1/4-wavelength, similarly as an air column resonance tube closed at one end, is allowed. On the contrary, in the case where output impedance of the driving amplifier is high, the both ends of a voice coil of the driver unit 20 are equivalently open, and strong electromagnetic brake does not work, which allows free vibration of the vibration plate of the driver unit 20. Namely, in the case where output impedance of the driving amplifier is high, the driver unit 20 arranged at the end of the tube part 1 canbecome an antinode of velocityamplitude. Therefore, presence of standing wave having total length of the tube part 1 as half-wavelength, as in the case of the embodiment shown in FIG. 6, is allowed. In addition, in the case where output impedance of the driving amplifier is an intermediate state, the driver unit 20 can be neither an antinode nor a node of velocity amplitude; therefore, clear resonance point is absent. Thus, the speaker system of the embodiment of FIG. 8 largely alters action and sound quality depending on kind of the driving amplifier to be combined.

Therefore, in the case of the speaker system of the

second embodiment shown in FIG. 8, although a problem, which standing wave with total length of a tube as a resonance target is not allowed, can be solved, kind of the driving amplifier to be combined is limited. Therefore, the speaker system of the embodiment shown in FIG. 4 is more preferable from the view point of having action effect that characteristics of the driver unit 20 does not depend on termination condition of a tube, and is capable of maintaining and assuring quality as an acoustic product without depending on kind of the driving amplifier to be combined.

In addition, in the speaker system 30 of the embodiment shown in FIG. 4, the driver unit 20 does not also have a role of one of the vibration plates in the acoustic transmission path 10. Explanation will be given on this point by comparing the speaker system 30 of the embodiment with the speaker system of the first comparative embodiment shown in Fig. 7, and the speaker system of the second comparative embodiment shown in Fig. 8.

In the case where the driver unit 20 also has a role of one of the vibration plates in the acoustic transmission path 10, as in the first comparative embodiment and the second comparative embodiment , resonance of an acoustic transmission path to low pitch sound requires mass of a vibration plate of the driver unit 20 to be set large, therefore, response characteristics of high pitch sound side of the driver unit is likely to deteriorate . On the other hand, in the embodiment, because the driver unit 20 does not have a role of one of the vibration plates in the acoustic transmission path 10, response characteristics of highpitch sound side of the driver unit 20 does not deteriorate.

For example, in the acoustic transmission path 10 shown in FIG. 1, the tube part 1 is a cylindrical tube with an inner

diameter of 0.10 m, and arranged therein with a vibration plate having an effective diameter of 0.064 m in an interval of 0.10 m, and each of parameter values is set as follows; A: 0.00785 m ² , L: 0.10 m, S ₀ : 790 N/m, K: 141700 N/m ² , and p: 1.29 kg/m ³ . It should be noted that compensation calculation values different from the above numerical expressions (10) and (11) are used here.

In these settings, when resonance frequency of element vibration, with a wavelength of 6L (a wave number of π/3L) with a total tube length of 0.30 m as substantially 1/2-wavelength, is desired to be, for example, 80 Hz, then mass m ₀ of a vibration plate is required to be 5 g, and when it is desired to be 60 Hz, then mass mo of a vibration plate is required tobe 1Og. These values are extremely largemasses as compared with the fact that mass mo of a driver unit having a typical nominal caliber of 0.08 m (an effective diameter of 0.064 m) , which can respond to frequency larger than 10 kHz, is about 2 g; it is extremely difficult for a vibration plate having such large mass to respond well to frequency larger than 10 kHz.

Therefore, in the speaker system of the embodiment, it is extremely advantageous that the driver unit 20 has not also a role of the vibration plates 3 of the acoustic transmission path 10, and mass of the vibration system of the driver unit 20 can be set independently from design of the acoustic transmission path 10, in view of securing good response characteristics of the driver unit over wide frequency band region.

As described above, in the speaker system 30 of the embodiment, a plurality of the vibration plates 3, having relatively large mass mo, may sometimes vibrate at the same time, and counteraction thereof could be large. Vibration

of the tube part 1 itself in the direction opposite to the vibration plates 3 by counteraction of motion of the vibration plates 3 results in energy consumption in motion not contributing to acoustic emission, and thus reduces acoustic emission efficiency.

In this point, in the speaker system 30 of the embodiment, shown in FIG. 4, by connection of the driver unit 20 at the air chamber 4b arranged at the center part, among odd number of the air chambers 4a, 4b and 4c, reduction of acoustic emission efficiency is prevented.

Namely, by connection of the driver unit 20 at the air chamber 4b arranged at the center part, among odd number of the air chambers, the acoustic transmission path 10 is symmetrically configuredwith the driver unit 20 at the center, as shown in FIG. 4. Utilization of resonance of element vibration, having total length of the tube part 1 as integer times 1/2-wavelength, shown in FIG.6, is capable of preventing reduction of acoustic emission efficiency, without inducing counteraction of the tube part 1 itself, because a pair of the vibration plates 3a and 3d at the symmetrical positions always moves in mutually opposite direction.

It should be noted that, different from the embodiment, even in the case where the driver unit 20 is not connected to the air chamber 4b arranged at the center part among odd number of the air chambers, the tube part 1 can be designed not to respond to motion of the vibration plates 3, by making mass of the tube part 1 large. However, in view of configuration of a light weight speaker system, it is desirable that configuration, where the driver unit 20 is connected to the air chamber 4b arranged at the center part, is adopted, as in the speaker system 30 of the embodiment.

In addition, in the speaker system 30 of the embodiment,

shown in FIG. 4, three air chambers 4a to 4c are arranged in series. Therefore, because the air chamber 4b containing a node of velocity amplitude is partitioned from other air chambers 4a and 4c, there is no possibility that a node position crosses over a separation wall configured by separation walls

(not shown) , along with the vibration plate 3b and 3c, and becomes unclear. In addition, because wave surface of emitted wave from the back surface of the driver unit 20 is smoothed in plain-like by the vibration plate 3b and 3c, planarity of standing wave can be maintained in a good state. Therefore, the speaker system 30 of the embodiment, shown in FIG. 4, is a more advantageous embodiment, in view of low resonance reduction of a transmission path by emitted wave from the back surface of the driver unit 20. In addition, as shown in FIG.4, connection of the driver unit 20 at a node position of velocity amplitude is capable of providing resonance amplification effect to give strong acoustic emission from tube end as an antinode of velocity amplitude while using the driver unit 20 in a state of small amplitude and small strain.

It should be noted that, in the acoustic transmission path 10 used in the embodiment, as described above, low propagation velocity of lower than 1/3 of sound velocity in atmosphere can easily be obtained, and therefore, a 1/2-wavelength resonance tube having a resonance frequency of smaller than 100 Hz, and total tube length of shorter than 0.5mcan easily be configured. For example, in the embodiment, a 1/2-wavelength resonance tube having a resonance frequency of60Hzor80Hz, and total length of only 0.3 meanbe configured. Because total length of an air column resonance tube, corresponding to 1/2-wavelength that resonates with sound wave of a frequency of 60 Hz, is 2.8 m, total length of the

speaker system 30 of the embodiment is dramatically reduced as compared with a speaker system using a usual air column resonance tube.

Here, an example is shown that was performed to confirm action effect of the speaker system 30 of the embodiment. The tube part 1 is a cylindrical tube having an inner diameter of 0.09 in, and length of the air chambers 4a, 4b, and 4c was all set to be 0.11 m. Effective diameter of each of the vibration plates was set to be 0.064 m. Mass of the vibration plates 3a and 3d arranged at the end part of the tube part 1 was set to be 16 g, and mass the vibration plates 3b and 3c arranged at the inside of the tube part 1 was set to be 2Og. Spring constant of the edge member 7 was set to be S0=790 N/m commonly to all of the vibration plates 3a, 3b and 3c. This spring constant correspond to about 1.9 L when converted to Vas (equivalent volume) , and is larger as compared with volume of the adjacent air chambers 4 to be about 0.7 L. Therefore, as an elastic element, relatively high elasticity of the compact sized air chambers 4 is predominant, resulting in suppression, to small level, of increasing effect of resonance frequency by the edge member 7 and the like.

In the embodiment, movable range of all of the vibration plates 3a, 3b and 3c was set to be ± 2.5 mm, and thus secured large value as for the vibration plate having an effective diameter of 0.064 m. This setting has effect that the vibration plates 3a and 3d arranged at the end part are capable of sufficiently oscillating as an antinode of standing wave. It should be noted that the vibration plates 3b and 3c at the inside may be set to have smaller movable range as compared with the vibration plates 3a and 3d.

Qms of a vibration system configured by the vibration plates 3 and the edge member 7 was set to be 5.2. Too small

Qms increases transmission loss of element vibration in a tube, and reduces resonance nature. Qms is desirably larger than 3.0 to provide suitable operation as a resonance tube type speaker system. It should be noted that in configuration of a transmission line type speaker system, to be described later, which does not positively utilize resonance, the range is not limited thereto.

The driver unit 20 having an effective diameter of 0.064 Ki, fS=IlO Hz and Qts=0.33 was used. In the case where volume of the air chambers 4 is smaller than 1 L, as in the embodiment, selection of the driver unit 20 having small Qts is important. Use of the driver unit having a Qts of desirably smaller than 0.5 is capable of providing flat frequency response characteristics by suppressing sharpness of resonance, in frequency near resonance point formed by a vibration system of the driver unit 20 and the air chambers 4 at the back thereof . In addition, the driver unit 20 having small Qts has advantage of being capable of forming relatively strong connection with resonance of the acoustic transmission path 10, which is set to have lower frequency than the above resonance point, because response characteristics is maintained down to a lower frequency region than the resonance point.

By adding compensation to the above numerical expression (10) and numerical expression (11), which are assumed an infinite system, in consideration of the fact that effective vibration area of each of the vibration plates 3 is 51% of cross-sectional area inside a tube, frequency of element vibration in the shortest wavelength of 2L (a wave number of π/L) , which can propagate, is calculated to be 120 Hz, and phase velocity Vp is calculated to be 26 m/s, that is reduction of velocity to about 8% of sound velocity in atmosphere. Similarly, frequency of element vibration in a

wavelength of 6L (a wave number of π/3L) is calculated to be 60 Hz, and phase velocity Vp is calculated to be 40 m/s. On the other hand, in a practical system configured by finite number of mass element, a set of resonance was observed at 107 Hz and 46 Hz by impedance measurement. At the same time, anti-resonance was observed at 120 Hz.

In the embodiment, a total length of only 0.33 m is capable of configuring an acoustic transmission path equivalent to 1/2 wavelength in a frequency of 46 Hz . Because total length of an air column resonance tube corresponding to 1/2-wavelength resonating with sound wave of a frequency of 46 Hz is about 3.6 m, dramatic reduction becomes possible. In addition, a usual air column resonance tube shows resonance in fundamental resonance frequency, and a plurality of frequencies corresponding to higher harmonic waves thereof, however, the embodiment is characterized in presence of resonance only at a higher harmonic wave of 107 Hz. (A fourth embodiment)

FIG. 9 is a cross-sectional view of a speaker system of a fourth embodiment of the invention. The embodiment has a structure similar to that of a speaker system of the third embodiment shown in FIG. 4, except the vibration plates 3 take embodiment so as to effectively permeate middle to high pitch sound. Therefore, as for the similar configuration to that in a speaker system of the third embodiment, the same reference number is used, and repeating explanation is omitted here.

The vibration plate 3a of the embodiment has the vibration membrane 15, and the ring-like enhancing frame 16 that enhances the peripheral part of the vibration membrane 15. The vibration membrane 15 is a paper thin membrane formed thinly, and is enhanced at the peripheral part, namely the

circumference, by the ring-like enhancing frame 16. In addition, the enhancing frame 16 is elastically supported at the inner surface of the tube part 1, by the edge member 7 , which is an air sealing unit. It should be noted that the vibration membrane 15 may also be one made of cloth, resin, metal foil or a composite body thereof, which is thinly formed at least partially, instead of paper membrane. As the enhancing frame 16, light metal such as aluminum or the like, or high strength resin material such as a fiber enhanced resin or the like may be used. The enhancing frame 16 may be added with, at the inside thereof, lattice-like or radial reinforcing crosspieces (not shown) . In addition, at the rear end of the enhancing frame 16, a plate spring-like elastic support member may be added to elastically support motion in the axis direction while restricting motion in the diameter direction. It should be noted that explanation was given on the vibration plate 3a as above, however, other vibration plates 3b, 3c and 3d also have a similar configuration.

The speaker system 30 configured as above attains the following action effects in addition to action effects similar to the speaker system of the third embodiment.

Thickness of the vibration membrane 15 may be set thin while maintaining strength required for large oscillating movement, because mass necessary to movement of the acoustic transmission path 10 is focused to the enhancing frame 16, andat the same time the circumference of the vibrationmembrane 15 is enhanced by the enhancing frame 16. Therefore, middle to high pitch sound with small amplitude can efficiently be transmitted by deformational vibration (curved deformation, local vibration) of the vibration membrane 15 having small mass, while low pitch sound with large amplitude can be transmitted by reciprocating motion of the vibration plates

that are mass element whole including the enhancing frame

16.

(A fifth embodiment)

The speaker system of the embodiment corresponds to one, where parameters including interval of the vibration plates that give length of the air chambers, and mass of the vibration plates and the like are changed, in the speaker system of the third embodiment shown in FIG. 4. In the above speaker system of the third embodiment, large peak and dip may generate in frequency characteristics, by strong resonance and anti-resonance . Therefore, in the embodiment, to weaken too strong resonance and anti-resonance, the parameters are changed. It should be noted that fundamental configuration itself of an acoustic transmission path of the embodiment is similar to that of the acoustic transmission path of the third embodiment shown in FIG. 4. Therefore, as for the similar configuration as in speaker system of the third embodiment, the same reference number is used, and repeating explanation is omitted here. In the speaker system of the embodiment, length of the air chamber 4b arranged at the center part, among the air chambers 4a, 4b and 4c arranged in series, is set longer compared with length of the air chambers 4a, and 4c arranged at the end part. Here, it is desirable that length of the air chambers 4a and 4c present at the symmetrical positions based on the center, along the axis line of the tube part 1, namely, the air chamber 4b, is set equal so as to make change in air chamber length symmetric, to extend reproducing ^' band region toward low pitch region, as well as to suppress the peak and dip by weakening strong resonance and anti-resonance. In other words, volumes of the air chambers 4a and 4c present at the symmetrical positions based on air

chamber 4b arranged at the center of the tube part 1, is set equal and makes change in air chamber volume symmetric so that volume of the air chambers becomes smaller with the arrangement toward the end part sides of the tube part 1. Also in this case, however, it is desirable that length ratio in arbitrary air chambers (interval between adjacent vibration plates) is not over 1.5, in view of maintaining propagation mode of element vibration as shown by schematic diagram shown in FIG. 2. In addition, at least one mass element 3c, among the vibration plates 3, as a plurality of mass elements, is set lighter as compared with other vibration plates 3a, 3b and 3d. In addition, in the embodiment, masses of the vibration plates themselves at the symmetrical positions with respect to the driver unit 20, as a center, are set different. Specifically, masses of the vibration plates 3a and 3d at the symmetrical positions are set different, and masses of the vibration plates 3b and 3c at the symmetrical positions are set different. In particular, in the embodiment, to flatten frequency characteristics, and to maintain strength, to some extent, of standing wave having total length of the tube part 1 as a resonance target, the vibration plates 3a and 3c having smaller mass than average mass of all of the vibration plates 3a, 3b, 3c and 3d, and the vibration plates 3b and 3d having larger mass than average mass of all of the vibration plates 3a, 3b, 3c and 3d, are alternately arranged. Also in this case, however, it is desirable that mass ratio among the vibration plates 3a, 3b, 3c and 3d, as arbitrary mass element excluding the element vibration driver unit 20, is not over 3, and more advantageously not over 2, in view of maintaining propagation of element vibration, namely resonance nature, as shown by schematic diagram in FIG. 2.

It should be noted that even in such an unequal S/rα (S represents spring constant; and m represents mass) or distance L between adjacent mass points, as parameters in the acoustic transmission path 10, use of average value of each of S/m values, and average value of each of L values in the above numerical expression ( 8 ) , is capable of designing a speaker system of the invention within a practically adjustable range, although error presents.

In addition, also in using the numerical expression (11) in consideration of parasitic elements and the like, substitution of value in square root and L with each of average values thereof is capable of designing a speaker system of the invention within a practically adjustable range.

According to the speaker system of the embodiment configured as above, also the tube part 1, having physically short total length, is capable of attaining enhancing effect of low pitch sound equivalent to a tube part having long total length, as well as suppressing significant peak and dip by weakening strong resonance and anti-resonance, thus resulting in improvement of flatness of frequency characteristics.

Here, an Example will be shown, which was carried out to confirm action effect of the speaker system 30 of the embodiment. Total length of the tube part 1 was kept same as in the case of the Example in the above third embodiment, and length of the air chambers 4a and 4c were shortened to 0.1m, and length of the air chamber 4b was extended to 0.13 m. By displacement of resonance point equivalent to a wave number of π/L in each of the air chambers 4, by changing length of the air chambers 4 in this way, resonance equivalent to a wave number of π/L as the whole tube part 1 can be weakened. Standing wave equivalent to a wave number of π/3L with total length of the tube part 1 as 1/2-wavelength is not sensitive

to change in short distance scale, and further in the Example, change in air chamber length is set symmetric; therefore, effect on standing wave equivalent to a wave number of π/3L is small. In the Example, by extending length of the air chamber 4b while keeping total length of the tube part 1, resonance point formed by the driver unit and the air chamber 4b is shifted to a little low frequency side. This setting strengthens connection between tube resonance present in lower frequency region than reproduction band region of the driver unit, and the driver unit, and acts in a way of extending reproduction band region toward low frequency side.

In addition, in the Example, masses of the vibration plates 3c and 3b were changed to 10 g and 20 g, respectively while maintaining masses of the vibration plates 3a and 3b to 16 g and 20 g, respectively, similarly as in the Example in the third embodiment. As a result, masses of the vibration plates 3a, 3b, 3c and 3dare 16g, 2Og, 1Og, and20g, respectively, and alternate arrangement is attained for the vibration plates having smaller mass than an average mass of 16.5 g of all of the vibration plates, and the vibration plates having larger mass than an average mass of 16.5 g. As a result, because resonance frequency of vibration elements including the vibration plates 3a and 3c shifts toward high frequency side, anti-resonance observed in Example 1 is set off, and dip of frequency characteristics at the vicinity of the anti-resonance point can be reduced. In addition, because asymmetry of mass distribution of whole system increases, and peak of frequency characteristics by standing wave equivalent to a wave number of π/L is reduced, frequency characteristics as a whole can be flattened with the contribution of dip reducing effect. Alternate arrangement of the light weight vibration plates 3a and 3c, and the heavy

weight vibration plates 3b and 3dprevents extreme suppression of standing wave with total length of the tube part 1 as a resonance target, and thus frequency characteristics is flattened without contraction of reproducing band region at low frequency side. On the contrary, concentration of the light weight vibration plates to one side of the tube part 1 (for example, to the side of the air chamber 4a) , and concentration of the heavy weight vibration plates to other side of the tube part 1 (for example, to the side of the air chamber 4c) extremely weakens strength of standing wave with total length of the tube part 1 as resonance target, and thus contracts reproducing band region at low frequency side, caused by mass localization in a way that the tube part 1 is substantially divided in two. It should be noted that Qms of a vibration system configured by the vibration plates 3 and the edge member 7 was set to be 3.7.

By carrying out parameter change as above, in the Example, reproducing band region was extended toward low region direction, as well as peak and dip were suppressed, and thus flatness of frequency characteristics was improved.

In the fifth embodiment as above, explanation was given on resonance suppression by changing parameters, however, also by other embodiments, resonance suppression is possible aiming at sound quality adjustment. A sixth and seventh embodiments below show such other examples of resonance suppression aiming at sound quality adjustment. (A sixth embodiment)

FIG. 10 is a cross-sectional view of a speaker system of a sixth embodiment of the invention. The speaker system of the embodiment is configured by misalignment of the tube axis of the tube part 1 in the mid-course. In other words,

the tube part 1 has a plurality of parts having axis center misaligned each other. In FIG. 10, tube axes of a part of the tube part 1 configuring the air chamber 4a, and a part of the tube part 1 configuring the air chamber 4c are have misalignment. According to suchmisalignment of the tube axes, a first group of the vibration plates configured by the vibration plates 3a and 3b, and a second group of the vibration plates configured by the vibration plates 3c and 3d mutually have deflection in a diameter direction (a perpendicular direction along a vibration direction) .

Also by misalignment of the tube axis of the tube part 1, instead of the parameter change as explained in the fifth embodiment, or together with the parameter change, resonance can be suppressed aiming at sound quality adjustment. (A seventh embodiment)

FIG. 11 is a cross-sectional view of a speaker system of a seventh embodiment of the invention. The speaker system of the embodiment is configured by bending the tube axis of the tube part 1 in the mid-course. In other words, the tube part 1 is bent at least at one position. In FIG. 11, tube axes of a part of the tube part 1 configuring the air chamber 4a, and a part of the tube part 1 configuring the air chamber 4c are bent. According to such bending of the tube axes, a first group of the vibration plates configuredby the vibration plates 3a and 3b, and a second group of the vibration plates configured by the vibration plates 3c and 3d mutually have predetermined angle without becoming parallel.

Also by bending the tube axis of the tube part 1 , instead of the parameter change as explained in the fifth embodiment, or together with the parameter change, resonance can be suppressed aiming at sound quality adjustment. (An eighth embodiment)

In the above first to seventh embodiments, explanation was given on using vibration plates as mass element. It is desirable to use vibration plates as mass element in view of compact sizing of a speaker system, and prevention of reduction of emission efficiency and generation of unnecessary noise, however, a port tube communicating between air chambers maybe used as amass element, as in the embodiment.

FIG. 12 is a cross-sectional view of a speaker system of an eighth embodiment of the invention. The speaker system of the embodiment is similar as in the case of the speaker system of the third embodiment, shown in FIG. 4, except that a port tube is used instead of using vibration plates as mass element to partition adjacent air chambers. Therefore, as for the similar configuration as in the speaker system of the third embodiment, the same reference number is used, and repeating explanation is omitted here.

The speaker system 30 of the embodiment also has the acoustic transmission path 10 and the driver unit 20. As shown in FIG.12, in the acoustic transmission path 10 of the speaker system 30 of the embodiment, the port tube 13a is used instead of the vibration plate 3b as a mass element to partition between adjacent air chambers 4a and 4b, and the port tube 13b is used instead of the vibration plate 3c as a mass element to partition between adjacent air chambers 4b and 4c. On the other hand, at the both ends of the tube part 1, the vibration plates 3a and 3b are provided, similarly as in the third embodiment .

At the inner surface of the tube part 1, the separation walls βa and 6b are provided. By the separation walls 6a and 6b, inner space of the tube part 1 is partitioned to the air chambers 4a, 4b, and 4c. The separation walls 6a and 6b partition the air chambers 4a, 4b and 4c, and also support

the port tubes 13a and 13b, respectively. The port tube 13a communicates between adjacent air chambers 4a and 4b, while the port tube 13b communicates between adjacent air chambers 4b and 4c. Not only the vibration plates 3a and 3d function as mass element, but also air in the port tubes 13a and 13b function as mass element. Because the vibration plate 3a, the air chamber 4a, the pot tube 13a, the air chamber 4a, the port tube 13b, the air chamber 4c, and the vibration plate 3d, are arranged in this order, along the axis direction of the tube part at the inside of the tube part 1, also in the acoustic transmission path 10 of the embodiment, mass elements and elastic elements are alternately and periodically arranged along the axis direction of the tube part 1. The axis lines of the port tubes 13a and 13b are in parallel to the axis line of the tube part 1. In other words, air inside the port tubes 13a and 13b, as mass element, vibrates along the axis line of the tube part 1.

The driver unit 20 is acoustically connected to the acoustic transmissionpath 10 configured as above . The driver unit 20 is acoustically connected to the acoustic transmission path 10 so as not to partition at the inside of the acoustic transmission path 10 in the axis direction of the tube part 1, similarly as in the case of the third embodiment. Specifically, at the sidewall of the tube part 1, the through hole 11 is arranged so as to communicate the air chamber 4b arranged at the center part, among the odd number of air chambers 4a, 4b and 4c, and outside world; at the outside of the sidewall of the tube part 1, the driver unit 20 is arranged matching with the position of the through hole 11. It should be noted that, it is desirable that the driver unit 20 is arranged at a node position of standing wave at the

inside of the tube part 1, by arrangement of the through hole 11 at a center part along the axis direction of the tube part, similarly as in the case of the third embodiment.

The speaker system 30 configured as above has action effect nearly the same as in the speaker system of the third embodiment .

It should be noted that the speaker system 30 of the embodiment uses the port tubes 13a and 13b as mass element, however, the port tubes 13a and 13b occupy larger volume as compared with the vibration plates, therefore, the speaker system of the third embodiment is advantageous in view of compact sizing of the speaker system 30. In addition, setting of diameter of the port tubes 13a and 13b small, to make volume of port tubes 13a and 13b small while maintaining resonance freguency of mass element, reduces acoustic emission efficiency, and also may sometimes extremely generate wind noise caused by air flow passing through the port tubes 13a and 13b. In addition, the port tubes 13a and 13b resonate as an independent air column resonance tube at acoustically unpleasant middle to high pitch sound region in particular; and thus disadvantageous as comparedwith the vibration plates . Reduction of emission efficiency and generation of unnecessary noise raise an acoustical big problem, in particular, in the case where the port tubes are arranged at the endpart of the tubepart 1 as an exposed state . Therefore, even in using the port tubes 13a and 13b, it is desirable that mass elements arranged at the end part of the tube part

1 are the vibration plates 3a and 3d, as in the embodiment.

As described above, use of the vibration plates as mass element is desirable in view of compact sizing of a speaker system, and in view of prevention of reduction of emission efficiency and generation of unnecessary noise. However,

depending on use condition, adoption of the port tubes 13a and 13b, having simple structure, as mass element, as in the embodiment, is also capable of obtaining a low propagation velocity of less than 1/3 of sound velocity in atmosphere; and a 1/2-wavelength resonance tube having a resonance frequency of smaller than 100 Hz, and total tube length of shorter than 0.5m can easily be configured, and thus total length of the speaker system of the embodiment is dramatically reduced as compared with a speaker system using a usual air column resonance tube. (A ninth embodiment)

In the above first to eighth embodiments, explanation was given on the case where a plurality of air chambers are arranged in series. It is surely desirable that a plurality of air chambers are arranged in series, in view of prevention of mass concentration per vibration plate functioning as a mass element or the like, however, even in the presence of one air chamber, as in the embodiment, a speaker system utilizing resonance of element vibration with total length of the tube part as 1/2- wavelength can be configured.

FIG. 13 is a cross-sectional view of a speaker system of a ninth embodiment of the invention. The speaker system of the embodiment corresponds to one having number of the vibration plates reduced to two, by removing two vibration plates positioned at the center part, among four vibration plates in the speaker system of the third embodiment. Therefore, whole of the inside of the tube part 1 is one air chamber. Excluding this point, the speaker system of the embodiment is similar to the speaker system of the third embodiment, shown in FIG. 4. Therefore, as for the similar configuration as in the case of the acoustic transmission path of the third embodiment, the same reference number is

used, and repeating explanation is omitted here.

The speaker system 30 of the embodiment contains the acoustic transmission path 10 and the driver unit 20. As shown in FIG.13, the acoustic transmission path 10 of the embodiment also has the tube part 1 with both ends open as open ends, similarly as in the case of the third embodiment.

At both ends of the tube part 1, the vibration plates 3a and 3b are provided as mass element, respectively. In addition, the inner space of the tube part 1 is partitioned from the outside world by these vibration plates 3a and 3b to form one air chamber 4. Because the vibration plates 3a and 3b function as mass element, and the air chamber 4 functions as an elastic element, also in the acoustic transmission path 10 of the embodiment, mass element and elastic element are alternately and periodically arranged along the axis direction of the tube part 1 at the inside of the tube part 1.

At the center part of the sidewall of the tube part, the through hole 11 is provided to acoustically connect the driver unit 20 to the acoustic transmission path 10. The driver unit 20 is arranged at the outside of the sidewall of the tube part 1 so that the back surface thereof is opposing to the through hole 11, and is acoustically connected to the acoustic transmission path 10 at the center part of the air chamber 4 via the through hole 11.

Here, diameter of the through hole 11 is desirably smaller than 1/3 of total length of the tube part 1. In particular, as in the embodiment, in the case where the center part of the tube part 1, as a node of velocity amplitude of standing wave, and the end part of the tube part 1, as an antinode of velocity amplitude of standing wave, have a structure not to be partitioned by a vibration plate or the

like, superimpose of emission at the back surface of the driver unit 20, and standing wave makes a node position of standing wave unclear, and at the same time reduces planarityof standing wave. Therefore, it is desirable that the node position of standing wave is made clear, and planarity of standing wave is improved by setting diameter of the through hole 11 to be smaller than 1/3 of total length of the tube part 1. It should be noted that, similarly as in the third embodiment, the driver unit 20 is desirably connected at the node position of standing wave in the tube part 1, similarly as in the case of the third embodiment, by arrangement of the through hole 11 at the center part along the axis direction of the tube part 1.

In addition, as shown in FIG. 13, at the vicinity of the center part along the axis direction of the tube part 1, where the driver unit 20 is provided, and also the node of velocity amplitude is provided, an sound-absorbing material composed of a similar material as in the sound-absorbingmaterial 14 of the case of the third embodiment, is arranged in focused way. The sound-absorbingmaterial 14 is a fiber aggregate such as natural fiber, synthetic fiber, glass fiber or the like, and is retained inside of net formed in cylinder-like, and is arranged at the vicinity of the tube axis by a support unit not shown. The speaker system 30 configured as above has action effects, nearly the same as in the speaker system of the third embodiment .

In addition, the node position of standing wave can be made clear, and also planarity of standing wave can be improved, by setting diameter of the through hole 11 to be smaller than 1/3 of total length of the tube part 1.

Furthermore, the sound-absorbingmaterial 14 arranged

at the vicinity of the node position, in a focused way, does not substantially have sound-absorbing effect to sound wave having low pitch sound with small velocity amplitude at the vicinity of the node position, while efficiently absorbs middle to high pitch sound wave emitted from the back surface of the driver unit 20. In FIG. 13, emission of middle to high pitch sound from the back surface of the driver unit 20 can be reduced without inhibiting resonance of a transmission path at low pitch sound by this selective sound-absorbing effect.

As described above, middle to high pitch sound emitted from the back surface of the driver unit 20 is useful to be recognized as comfortable indirect sound, when emitted from the end part of the tube part 1, however, is harmful to be recognized as offensive sound, when it forms standing wave along the diameter direction of the tube part 1, by reflection at the tube wall of the tube part 1. On the other hand, also in the speaker system of the embodiment, by arrangement, in a focused way, of the sound-absorbing material 14 at the axis position, where an antinode of standing wave along the diameter direction of the tube part 1 is present, harmful middle to high pitch sound, forming standing wave along the diameter direction, is efficiently absorbed, and at the same time, by ensuring an opening with large cross-section at the vicinity of the sidewall of the tube part 1, permeation can be secured for useful middle to high pitch sound to transmit along the axis direction.

It should be noted that, in the speaker system 30 of the embodiment, an air chamber having a node of velocity amplitude, and other air chambers are not partitioned; as compared with such speaker system 30 of the embodiment, the speaker system of the third embodiment, where an air chamber

having a node of velocity amplitude, and other air chambers are partitioned, is advantageous in view of small reduction of resonance nature of a transmission path by emission wave from the back surface of the driver unit 20. Namely, in the case where an air chamber having a node of velocity amplitude, and other air chambers are partitioned, as the speaker system of the third embodiment, there is no possibility that the node position becomes unclear. In addition, planarity of standing wave is maintained in a good state, because wave surface of emission wave from the back surface of the driver unit 20 is adjusted plane-likely by a vibration plate inside the tube part 1.

Furthermore, in the embodiment configured by only one air chamber 4, because a region corresponding to an antinode and a region corresponding to a node of standing wave are coexistent in close vicinity in the same air chamber, it is difficult to maintain clear amplitude difference by separation of the region corresponding to the antinode and the region corresponding to the node; therefore, strength of resonance also becomes relatively weak. On the contrary, the case where an air chamber having a node of velocity amplitude is partitioned from other air chambers, as in the speaker system of the third embodiment, is more advantageous, in view of amplitude difference between an antinode and a node is secured, and strong resonance can be allowed.

It should be noted that resonance with total length of the tube part 1 as 1/2-wavelength can also be utilized in the speaker system 30 of the embodiment, where number of the vibration plates is reduced to two. For example, in FIG. 13, the tube part 1 is a cylindrical tube with an inner diameter of 0.10 m, and arranged therein with a vibration plate having an effective diameter of 0.064 m, in a interval of 0.10 m,

and each of parameter values is set as follows; A: 0.00785 m ² , L: 0.30 m, S ₀ : 790 N/m, K: 141700 N/m ² , and p: 1.29 kg/m ³ . It should be noted that compensation calculation values different from the numerical expressions (10) and (11) are usedhere, andL represents distance between vibrationplates, namely total length of the tube part 1.

In these settings, when resonance frequency of element vibration of a wavelength of 2L (a wave number of π/L) with a total tube length of 0.30 m as 1/2-wavelength, is desired to be, for example, 80 Hz, then mass mo of a vibration plate is required to be 10 g, and when it is desired to be 60 Hz, then mass mo of a vibration plate is required to be 18 g. On the other hand, in the case of the speaker system of the third embodiment using element vibration of a wavelength of 6L (a wave number of π/3L) arranged with four vibration plates along a total length of the tube part 1 of the same 0.30 m, as explained in the first embodiment, mass mo of a vibration plate is required to be 5 g for a resonance frequency of 80 Hz, and mass mo of a vibration plate is required to be 10 g for a resonance frequency of 60 Hz. It should be noted that L represents distance between vibration plates, namely 1/3 of total length of the tube part 1.

Comparison of the results shows that also by the speaker system of the embodiment, a 1/2 wavelength resonance tube can easily be configured, where resonance frequency is smaller than 100 Hz, and a total length of the tube part 1 is shorter than 0.5 m; therefore total length of the speaker system of the embodiment can extremely be reduced as compared with a speaker systemusing a usual air column resonance tube, however, reduction of number of the vibration plates 3 is understood to concentrate large mass onto each of the vibration plates 3. In this way, large excess mass of the vibration plates

3 reduces transitional response characteristics. In particular, the embodiment of FIG. 13, where mass is focused at the vibration plates 3a and 3b provided at the end part of the tube part 1, where acceleration becomes maximal, is disadvantageous as compared with the case of the third embodiment. In addition, large excess mass of the vibration plates 3 reduces lifetime of a vibration support system such as the edge member 7, or a damper or the like.

Therefore, it is clear that the speaker system of the third embodiment is desirable compared with the speaker system of the embodiment, in view of reduction of transitional response characteristics, or deterioration of lifetime of a vibration support system, however, depending on conditions such as durability and the like required to transitional response characteristics or products, there is a case that a structure with reducednumber ofmass element andair chambers, as the embodiment, may be adopted. (A tenth embodiment)

FIG. 14 is a cross-sectional view of a speaker system of a tenth embodiment of the invention. In the speaker systems of the above third to ninth embodiments, explanation was given on the case where the driver unit is arranged at the outside of the sidewall of the tube part, and the driver unit and the acoustic transmission path 10 are acoustically connected directly via the through hole.

On the other hand, the speaker system of the embodiment has a port tube to connect the driver unit and inside of the tube part, and is different in acoustic connection method forthe driverunit andthe acoustictransmissionpath. Except these points, the speaker system of the embodiment has configuration similar to that of the speaker system of the third embodiment shown in FIG. 4. Therefore, as for the

similar configuration, the same reference number is used, and repeating explanation is omitted here.

As shown in FIG. 14, in the speaker system 30 of the embodiment, the driver unit 20 is acoustically connected to the acoustic transmission path 10 via the port tube 18. Specifically, the port tube 18 functions as a connection tube to connect the driver unit 20 and the inside of the tube part 1. The port tube 18 may be formed using a free-bending and flexible material. In an example shown in FIG. 14, the driver housing 12 is provided so as to cover the back surface side of the driver unit 20, andthe driver housing 12 becomes a cabinet independent from the acoustic transmission path 10. One end of the port tube 18 is communicated with the inside of the driver housing 12, while the other end of the port tube 18 is communicated with the inside of the air chamber 4b positioned at the center part. As a result, also in the speaker system 30 of the embodiment, the driver unit 20 is connected to the air chamber 4b where a node of velocity amplitude of standing wave is positioned.

The speaker system 30 of the embodiment, configured as above, has not only action effect similar as in a speaker system of the third embodiment but also the following action effects. The port tube 18 shows Helmholtz resonance at predetermined frequency, and inputs acoustic signals enhanced at this frequency, to the acoustic transmission path 10. In the speaker system 30 of the embodiment, because the driver unit 20 can be arranged independently from the tube part 1, the driver unit 20 is not limited to shape and size of the tube part 1. Therefore, has advantage in that the driver housing 12 with relatively large volume may be used, and a driver unit with relatively high Qts can be selected.

Here, an embodiment will be shown, which was carried out to confirm action effect of the speaker system 30 of the embodiment.

The driver housing 12 with an inner volume of 4.0 L was connected to a transmission path with configuration similar to the above embodiment in the third embodiment, via the free-bending port tube 18 with an inner diameter of 2 cm and a total length of 70 cm. The port tube 18 shows Helmholtz resonance at 46 Hz, and inputs acoustic signals enhanced at this frequency, to the transmission path. The embodiment is capable of using the driver housing 12 with large volume without limited to shape and size of the tube part 1, therefore, a driver unit with relatively high Qts can be selected. In the embodiment, peak of frequency characteristics at the vicinity of resonance point by the driver unit 20 and the driver housing 12 is suppressed, in using a driver unit having an effective diameter of 0.064 m, fs=100 Hz and Qts=0.6. (An eleventh embodiment)

FIG. 15 is a cross-sectional view of a speaker system of an eleventh embodiment of the invention. In the speaker system of the above third embodiment, explanation was given on the case where odd numbers of air chambers are arranged in series.

On the other hand, in the speaker system of the embodiment, even number of air chambers are arranged in series, and the driver unit is acoustically connected to at least either one of the two air chambers arranged at the center part, amongevennumberofair chambers . Excludingthis point, the speaker system of the embodiment has configuration similar to that of the speaker system of the third embodiment shown in FIG. 4. Therefore, as for the similar configuration as in the speaker system of the third embodiment, the same

reference number is used, and repeating explanation is omitted here.

As shown in FIG. 15, in the speaker system 30 of the embodiment, even number of air chambers, specifically four air chambers 4a, 4b, 4c and 4d are arranged in series. The through hole 11 is provided to at least either one of the two air chambers (in FIG. 15, the air chamber 4b) arranged at the center part, among these plurality of air chambers 4a, 4b, 4c and 4d. Therefore, the driver unit 20 is acoustically connected to two air chambers 4b and 4c arranged at the center part, among these plurality of air chambers 4a, 4b, 4c and 4d. In other words, the driver unit 20 is acoustically connected to either of the air chambers 4b and 4c adjacent to the vibration plate 3c that is arranged at the center, among a plurality of vibration plates 3a, 3b, 3c, 3d and 3e, as mass element.

The speaker system 30 of the embodiment configured as above has the action effects, nearly the same as in a speaker system of the third embodiment. It should be noted that, as explained in the above third embodiment, connection of the driver unit 20 at a node position of velocity amplitude is capable of providing resonance amplification effect to give strong acoustic emission from tube end as an antinode of velocity amplitude, while using the driver unit 20 in a state of small amplitude and small strain. Here, in the tube part 1 partitioned to even number of the air chambers 4a, 4b, 4c and 4d, as the embodiment, in the case where resonance of element vibration with total length of the tube part as 1/2-wavelength is utilized, a node position of velocity amplitude of standing wave corresponds to the center vibration plate 3c, and not corresponds to the inside of the air chamber 4. However, like the speaker system

30 of the embodiment, connection of the driver unit 20 to the air chamber 3b adj acent to the vibration plate 3c is capable of obtaining resonance amplification effect pursuant to the case of the speaker system of the third embodiment. (A twelfth embodiment)

FIG. 16 is a cross-sectional view of a speaker system of a twelfth embodiment of the invention. In the speaker systems of the above third to eleventh embodiments, explanation was given on the case where the both ends of the tube part are open.

On the other hand, in the speaker system of the embodiment, one end of the tube part is closed by an end wall, and the driver unit 20 is not acoustically connected to the center air chamber, but acoustically connected to the air chamber adj acent tothe closedendwall. Excludingthis point, the speaker system of the embodiment has configuration similar to that of the speaker system of the third embodiment shown in FIG. 4. Therefore, as for the similar configuration as in the acoustic transmission path of the third embodiment, the same reference number is used, and repeating explanation is omitted here.

As shown in FIG. 16, in the acoustic transmission path 30 of the embodiment, one end of the tube part 1 is closed. In other words, one end is a closed end. Therefore, at one end of the tube part 1, the closed wall end 8 is provided. On the other hand, the other end of the tube part 1 is open; in other words, it is an open end. At this other end of this tube part 1, the vibration plate 3a is arranged.

It should be noted that the inside surface of the closed end wall 8 is desirably a flat surface, for acoustic vibration, which propagates an acoustic vibration part, to propagate as plane wave in the tube part 1. In addition, from the similar

viewpoint, it is desirable that the inside surface of the closed end wall 8 is orthogonal against the axis direction of the tube part 1.

Inner space of the tube part 1 is partitioned by the vibration plates 3b and 3c into three air chambers 4a, 4b and 4c. Therefore, the three air chambers 4 are arranged in series along the axis line of the tube part 1. In the air chamber 4c arranged at the end part, among these three air chambers 4a, 4b and 4c, the vibration plate 3c is arranged at one side, and the closed end wall 8 is arranged at the other side.

At the sidewall of the tube part 1 of the acoustic transmission path 10, the through hole 11 is arranged to acoustically connect the driver unit 20 to an acoustic transmission path. The through hole 11 is arranged so as to communicate the air chamber 4c adjacent to the closed end wall 8, with outside world. Therefore, the driver unit 20 is connected to the air chamber C adjacent to the closed end wall 8. The speaker system 30 of the embodiment, configured as above, has the following action effects.

The acoustic transmission path 10 of the speaker system 30 of embodiment corresponds to the case where each of the vibration plates 3a, 3b and 3c are arranged at the position of adjacent mass points IA, IB and 1C, respectively, and the closed end wall 8 is arranged at the position of ID, in the above mechanical model shown in FIG. 2. According to a graph of FIG. 2, distance from IA to ID allows standing wave with total tube length as 3/4-wavelength, when a wave number is π/2L. In addition, standing wave with total tube length as 1/4-wavelength, when a wave number is π/6L, is allowed.

Also in the acoustic transmission path 10 of the

embodiment, low propagation velocity less than 1/3 of sound velocity in atmosphere can be obtained, therefore, a 1/4-wavelength resonance tube, with a total length of the tube part 1 is shorter than 0.25 m, can easily be configured. Here, an Embodiment will be shown, which was carried out to confirm action effect of the speaker system 30 of the embodiment. The tube part 1 is a cylindrical tube having an inner diameter of 0.09 m, and length of the air chambers 4a, 4b, and 4c was all set to be 0.11 m. Effective diameter of each of the vibration plates 3a, 3b and 3c was set to be 0.064 m. Mass of the vibration plate 3a arranged at the end part was set to be 16 g, and mass of the vibration plates 3b and 3c arranged at the inside of the tube part 1 was set to be 2Og. Spring constant of the edge member 7 was set to be S0=790 N/m to all of the vibration plates 3a, 3b and 3c. This spring constant is about 1.9 L when converted to Vas (equivalent volume) , and larger as compared with volume of the adjacent air chambers 4a, 4b and 4c to be about 0.7 L. Therefore, as an elastic element, relatively high elasticity of the compact sized air chambers 4 is predominant, resulting in relatively small increasing effect of resonance frequency by the edge member 7 added thereto.

In the embodiment, movable range of all of the vibration plates 3a, 3b and 3c was set to be + 2.5 mm, and thus securing large value as for the vibration plates 3a, 3b and 3c, having an effective diameter of 0.064 m. This setting has effect that the vibration plate 3a arranged at the end part is capable of sufficiently oscillating as an antinode of standing wave. It should be noted that, in the vibration plates 3b and 3c at the inside, smaller movable range may be set as compared with the vibration plates 3a.

Qms of a vibration system configured by the vibration

plates 3a, 3b and 3c, and the edge member 7 was set to be 5.2. The driver unit 20 having an effective diameter of 0.064 m, fS=IlO Hz and Qts=0.33 was used.

By adding compensation to the above numerical expression (10) and numerical expression (11), which assumed an infinite system, in consideration of the fact that effective vibration area of each of the vibration plates 3a, 3b and 3c is 51% of cross-sectional area inside a tube, frequency of element vibration in a wavelength of 4L (a wave number of π/2L) , is calculated to be 85 Hz, and phase velocity Vp is calculated to be 37 m/s. In addition, frequency of element vibration in a wavelength of 12L (a wave number of π/6L) is calculated to be 31 Hz, and phase velocity Vp is calculated to be 41 m/s. On the other hand, in a practical system configured by finite number of mass element, a set of resonance was observed at 65 Hz and 24 Hz by impedance measurement.

In the embodiment, a total length of only 0.33 m is capable of configuring an acoustic transmission path equivalent to 1/4-wavelength in a frequency of 24 Hz . Because total length of an air column resonance tube corresponding to 1/4-wavelength, resonating with sound wave of a frequency of 24 Hz, is about 3.5 m, dramatic reduction becomes possible . In addition, a usual air column resonance tube shows resonance in fundamental resonance frequency, and a plurality of frequencies corresponding to higher harmonic waves thereof, however, the embodiment is characterized in presence of resonance only at a higher harmonic wave of 65 Hz. It should be noted that, in 24 Hz, the driver unit 20 has substantially no acoustic output, therefore, increase in sound pressure by resonance amplification was obtained only at 65 Hz. (A thirteenth embodiment)

In the thirteenth embodiment, what is called a

transmission line type speaker system is configured by absorption and removal of emission from a back surface by positively increasing propagation loss in a tube, in configuration of the speaker system of the twelfth embodiment shown in FIG. 16. Configuration in the embodiment is similar to that of the speaker system of the twelfth embodiment shown in FIG. 16, except that Qms of a vibration system configured by the vibration plates 3a, 3b and 3c, and the edge member 7 is reduced to positively increase transmission loss. Therefore, as for the similar configuration, the same reference number is used, and repeating explanation is omitted here .

In the speaker system of the embodiment, to reduced Qms of the vibration system configured by the vibration plates 3a, 3b and 3c, and the edge member 7, a damping material (not shown) made of silicone rubber or the like is coated at the surface of the vibration system configured by the vibration plates 3a, 3b, 3c and the edge member 7.

Object of a transmission line type speaker system as the embodiment is sound quality adjustment, and mainly to suppress low pitch sound by braking, rather than to enhance low pitch sound by resonance. In a usual transmission line type speaker system, because of increasing transmission loss in a tube by filling a sound-absorbing material in the tube, while taking a fundamental configuration of a resonance tube system, a tube part with relatively long total length is required.

On the other hand, when a speaker system of a resonance tube of the embodiment is fundamentally adopted, not only total length of the tube part 1 can extremely be shortened, but also transmission loss in a tube can easily be increased. (A fourteenth embodiment)

FIG. 17 is a cross-sectional view of a speaker system of a fourteenth embodiment of the invention. The speaker system of the embodiment is an example of a closed resonance tube where one end of a transmission path is set to be the driver unit 20, and the other end is a closed end. Namely, in the speaker system of the embodiment, the driver unit 20 is arranged at one end of the tube part 1, and closed end wall is provided at the other end of the tube part 1.

The speaker system 30 of the embodiment, configured as above, has the following action effects.

The acoustic transmission path 10 in the speaker system 30 of the embodiment corresponds to the case where the driver unit 20 is arranged at a position of the adjacent mass point IA; each of the vibration plates 3b and 3c is arranged at positions of IB and 1C, respectively; and the closed end wall 8 is arranged at a position of ID, in the mechanical model shown in FIG. 2. According to a graph of FIG. 2, distance from IA to ID allows standing wave with total tube length as 3/4-wavelength, when a wave number is π/2L. In addition, although not shown in FIG. 2, standing wave with total tube length as substantially 1/4-wavelength, when a wave number is π/6L, is allowed.

Here, an embodiment will be shown, which was carried out to confirm action effect of the speaker system 30 of the embodiment. The tube part 1 is a cylindrical tube having an inner diameter of 0.09 m, and length L of the air chambers 4a, 4b, and 4c was all set to be 0.11 m. Effective diameter of each of the vibration plates was set to be 0.064 m. Mass of the vibration plate 3a of the driver unit 20 arranged at the end part of the tube part was set to be 8 g, and mass of the vibration plates 3b and 3c arranged at the inside of the tube part 1 was set to be 10 g. Spring constant of the

edge member 7 was set to be S0=790 N/m, commonly to all of the vibration plates 3a, 3b and 3c. Here in the embodiment, movable range of all of the vibration plates 3a, 3b and 3c was set to be ± 5.0 mm, and thus securing large value as for the vibration plates, having an effective diameter of 0.064 m. This setting has effect that the vibration plate 3a arranged at the endpart is capable of sufficiently oscillating as an antinode of standing wave. It should be noted that, for the vibration plates 3b and 3c at the inside, smaller movable range may be set as compared with the vibration plates 3a.

Qms of a vibration system configured by the vibration plates 3a, 3b, 3c and the edge member 7 was set to be 5.2. The driver unit 20 having fs=50 Hz and Qts=0.28 was used. By adding compensation to the above numerical expression (10) and numerical expression (11) , which assume an infinite system, in consideration of the fact that effective vibration area of each of the vibration plates 3a, 3b and 3c is 51% of cross-sectional area in a tube, frequency of element vibration in a wavelength of 4L (a wave number of π/2L) , is calculated to be 119 Hz, and phase velocity Vp is calculated to be 52 m/s. In addition, frequency of element vibration in a wavelength of 12L (a wave number of π/6L) is calculated to be 43 Hz, and phase velocity Vp is calculated to be 57 m/s. On ^' the other hand, in a practical system configured by finite number of mass element, a set of resonance was observed at 90 Hz and 33 Hz by impedance measurement . In addition, strong anti-resonance was observed at 64 Hz.

In the embodiment, a total length of only 0.33 m is capable of configuring an acoustic transmission path equivalent to 1/4-wavelength in a frequency of 33 Hz . Because total length of an air column resonance tube corresponding

to 1/4-wavelength, resonating with sound wave of a frequency of 33 Hz, is about 2.5 m, dramatic reduction becomes possible. In addition, a usual air column resonance tube shows resonance in fundamental resonance frequency, and a plurality of frequencies corresponding to higher harmonic waves thereof, however, the embodiment is characterized in presence of resonance only at a higher harmonic wave of 90 Hz.

In the above first to fourteenth embodiments, explanation was given on the case where one acoustic transmission path and one driver unit are provided, however, an acoustic transmission path and a speaker system of the invention may have a plurality of acoustic transmission paths and a plurality of driver units. Explanation will be given, from a fifteenth to a seventeenth embodiments below, on the case of having a plurality of acoustic transmission paths. (A fifteenth embodiment)

FIG. 18 is a cross-sectional view of a speaker system of a fifteenth embodiment of the invention. The embodiment corresponds to the case added with a second driver unit 20b at the front surface of a first driver unit 20 via a second driver housing 12b, in the speaker system of the third embodiment. Here, the speaker systemitself of the embodiment is similar to that of the speaker system of the third embodiment shown, except that the second driver unit 20b is added via the second driver housing 12b. Therefore, as for the similar configuration, the same reference number is used, and repeating explanation is omitted here.

The second driver housing 12b is formed at the front surface side of the first driver unit 20. At the base end side (a side near the acoustic transmission path 10) of the second driver housing 12b, the first driver unit 20 is provided, and at the tip side of the second driver housing 12b, the

second driver unit 20b is provided.

In the case shown in FIG. 18, a wall part configuring the second driver housing 12b is bended and extended from a position where the first driver unit 20 is provided, to the vicinity of the vibration plate 3a arranged at the end part of the tube part 1, namely the center part along the axis direction of the tube part 1. Therefore, in an example shown in FIG. 18, the second driver unit 20b is arranged in close vicinity to the vibration plate 3a, which is present at one endpart of the acoustic transmission path 10. It should be noted that the second driver unit 20b may be arranged in close vicinity to the vibration plate 3d, not limited to the case of being arranged in close vicinity to the vibration plate 3a, or may be arranged within a range of sufficiently small distance, as compared with wavelength of reproduced sound wave, from both of the vibration plates 3a and 3d. In addition, the second driver unit 20b arranged in this way, is driven by electrical signals having the same phase as the first driver unit 20. Explanation will be given on action effect of the speaker system 30 configured as above.

According to the speaker system 30 of the embodiment, backpressure, which generates inside the second driver housing 12b, by the second driver unit 20b, is removed by motion of the first driver unit 20 oscillating with the same phase. In this case, backpressure of the second driver unit 20b is transmitted to the first driver unit 20 via motion of air inside the second driver housing 12b, and converted to backpressure of the first driver unit 20. Backpressure of the first driver unit 20 is transmitted to the acoustic transmission path 10 via the through hole 11, and then released outside from the vibration plates 3a and 3d provided at the

end parts of the tube part 1.

Therefore, because the speaker system 30 of the embodiment is equivalent to the case where a cabinet having infinitely large volume is connected at the back of the second driver unit 20b, use of the second driver unit 20b having small caliber andmass of a vibration system also exerts effect that efficient reproduction of low pitch sound is possible. In addition, in the speaker system 30 of the embodiment, at least a part of the acoustic transmission path 10 is utilized as a phase shifter. Namely, backpressure of the first driver unit 20, which generates as a result of removing backpressure of the second driver unit 20b, provides delay in phase thereof in a propagation process in the acoustic transmission path 10 having slower propagation velocity as compared with sound velocity in atmosphere. Therefore, it is possible to set that sound wave released outside from the vibration plates 3a and 3d provided at the end parts of the tube part 1, and sound wave released from the front surface of the second driver unit 20b are mutually in enhancing phase relation. For example, in the second driver unit 20b, when sound wave emitted from the back surface, with phase difference of 180 degree relative to sound wave emitted from the front surface, generates a phase delay of 90 degree in the acoustic transmission path 10, and then released outside from the vibration plates 3a and 3d, which are provided at the end parts of the tube part 1, it results in having a phase difference of 270 degree in total from sound wave released from the front surface of the second driver unit 20b. Therefore, enhancement of sound pressure is possible as compared with the case where wave sound is emitted singly by any one of the second driver unit 20b and the vibration plate 3a (or 3d) at the end part.

For example, in the case of configuring the acoustic

transmission path 10 having total length equivalent to 1/2-wavelength in a frequency of 46 Hz, in an example shown in FIG. 18, because acoustic length of a path from the second driver unit 20b to the vibration plate 3a corresponds to 1/4-wavelength in frequency of 46 Hz, it acts as a phase shifter of 90 degree, and is capable of providing the above enhancing effect of sound pressure. In addition, the enhancing effect of sound pressure can be obtained also in each of the frequencies where amount of a phase shift is larger than 60 degree.

As above, explanation was given on the speaker system of the embodiment, however, various modifications may be possible, and various speaker systems may be configured, as long as the second driver unit 20b is added at the front surface of the first driver unit 20, via the second driver housing 12b. (A sixteenth embodiment)

FIG. 19 is a cross-sectional view of a speaker system of a sixteenth embodiment of the invention . The speaker system of the embodiment corresponds to the case where a second acoustic transmission path 10b is added in addition to a first acoustic transmission path 10, in the speaker system of the third embodiment. Specifically, at the sidewall of the tube part 1 of the first acoustic transmission path 10, and at the sidewall of the tube part Ib of the second acoustic transmission path 10b, the opening 19 and the opening 19b are provided, respectively; and at each of the openings, the vibration plate 3j and the vibration plate 3k are provided, respectively as mass element. Excluding theses points, each of the openings provided with the vibration plate 3j and the vibration plate 3k is communicated by the connection tube Ic. Each of the configurations of the speaker system 30 of

the embodiment is similar to that of the speaker system 30 of the third embodiment or the acoustic transmission path 1 of the first embodiment. Therefore, as for the similar configuration, the same reference number is used in the explanation below, and repeating explanation is omitted here . As shown in FIG. 19, the speaker system 30 of the embodiment has the first acoustic transmission path 10 and the second acoustic transmission path 10b. The first acoustic transmission path 10 has the vibration plates 3a, 3b, 3c and 3d, and the three air chambers 4a, 4b and 4c partitioned by the vibration plates 3a, 3b, 3c and 3d. At the air chamber 4b arranged at the center part, among these air chambers 4a, 4b and 4c, the through hole 11 is provided, and the driver unit 20 is arranged at the outside of the sidewall of the tube part 1 so that the back surface thereof is opposing to the through hole 11.

In addition, at the sidewall of the tube part 1, the above opening 19 is provided at a position opposing to this through hole 11, namely the driver unit 20, and at the opening 19, the vibration plate 3j is provided. Namely, at the opening 19 of air chamber 4b arranged at the center part, the vibration plate 3j is provided.

On the otherhand, the tube part Ib of the secondacoustic transmission path 10b has the vibration plates 3e, 3f, 3g and 3h, and the three air chambers 4e, 4f and 4g, which are partitioned by the vibration plates 3e, 3f, 3g and 3h. At the air chamber 4f arranged at the center part, among these air chambers 4e, 4f and 4g, the opening 19b is provided, and the vibration plate 3k is arranged at the opening 19b . Namely, the vibration plate 3k is arranged at the opening 19b of the air chamber 4f, which is arranged at the center part.

Between the tube part 1 and the tube part Ib, the

communication tube Ic is provided to communicate the opening 19 and the opening 19b. The communication tube Ic and space partitioned by the vibration plates 3j and 3k forms a new air chamber 4j, and configures a third acoustic transmission path containing the vibration plates 3j and 3k. Therefore, the first acoustic transmissionpath 10 and the second acoustic transmission path 10b are connected via the third acoustic transmission path. Here, the vibration plates 3j and 3k have area and mass equivalent to those of the vibration plates 3b and 3c in the tube part 1, and also the air chamber 4j has cross-sectional area and air chamber length equivalent to those of the air chambers 4 of the first acoustic transmission path 10. Namely, the speaker system 30 of the embodiment has the first, the second and the third acoustic transmission paths as acoustic transmission paths, and each of the tube parts 1 and Ib of the first acoustic transmission path 10 and the second acoustic transmission path 10b are connected via the tube part Ic of the third acoustic transmission path. In the embodiment, the first acoustic transmission path 10 and the second acoustic transmission path 10b are configured as a 1/2-wavelength resonance tube with the same resonance frequency, for example, of 46 Hz.

Explanation will be given on action effect of the speaker system 30 of the embodiment, configured as above.

The third acoustic transmission path, namely a path from the back surface of the driver unit 20 to the vibration plate 3k via the vibration plate 3j , acts as a phase shifter of 90 degree in a resonance frequency of, for example, 46 Hz.

In the driver unit 20, sound wave emitted from the back surface, with phase difference of 180 degree relative to sound

wave emitted from the front surface, generates a phase delay of 90 degree in the third acoustic transmission path, and further generates a phase delay of 90 degree in the second acoustic transmission path, and then released outside from the vibration plates 3e and 3h, which are provided at the end parts of the tube part Ib in the second acoustic transmission path; thereby resulting in a phase difference of 360 degree in total, namely the same phase, between sound wave released from the whole surface of the driver unit 20, and sound wave released outside from the vibration plate 3e and 3h, by which strong effect of sound pressure enhancement can be obtained.

In this way, in the speaker system of the embodiment, the first to the third acoustic transmission paths are provided, and the first to the third acoustic transmission paths are mutually connected, so that the same phase is taken by wave sound emitted from the vibration plates 3e and 3h provided at the end parts of the second acoustic transmission path, which is one acoustic transmission path among these plurality of acoustic transmission paths, and by sound wave emitted from the front surface of the driver unit 20.

It should be noted that, in the embodiment, aiming at maximizing resonance efficiency and minimizing acoustic anti-resonance, linear acoustic transmission paths 10 and 10b are adopted, however, in the case where reduction of resonance efficiency is allowed, at least one part of the acoustic transmission paths 10 and 10b may be bent aiming at more compact sizing. For example, providing area can be reduced by bending the first acoustic transmission path 10 in an amount of from 90 to 120 degree in the center air chamber 4b, and further by bending the second acoustic transmission path in an amount of from 90 to 120 degree in the center air

chamber 4f.

(A seventeenth embodiment)

FIG. 20 is a cross-sectional view of a speaker system of a seventeenth embodiment of the invention. The speaker system of the embodiment corresponds to the case where a driver unit 20b connected to a first driver unit 20 and the same air chamber 4b, and a second acoustic transmission path connected to the back surface of the second driver unit 20b are added, in the speaker system of the third embodiment. Except the above point, each of the configurations of the speaker system 30 of the seventeenth embodiment is similar to the speaker system 30 of the third embodiment, or the acoustic transmission path 10 of the first embodiment. Therefore, as for the similar configuration, the same reference number is used in explanation below, and repeating explanation is omitted here.

As shown in FIG. 20, the speaker system 30 of the embodiment has the first acoustic transmission part 10 and the second acoustic transmissionpart 10b. The first acoustic transmission path 10 has the vibration plates 3a, 3b, 3c and 3d, and the three air chambers 4a, 4b and 4c partitioned by the vibration plates 3a, 3b, 3c and 3d. At the air chamber 4b arranged at the center part, among these air chambers 4a, 4b and 4c, the through hole 11 is provided, and the first driver unit 20 is arranged at the outside of the sidewall of the tube part 1 so that the back surface thereof is opposing to the through hole 11. The second acoustic transmission path 10b also has, similarly as the first acoustic transmission path 10, the vibration plates 3e, 3f, 3g and 3h, and the three air chambers 4e, 4f and 4g partitioned by the vibration plates 3e, 3f, 3gand3h. At the air chamber 4f arranged at the center part, among these air chambers 4e, 4f and 4g, the through

19

hole lib is provided, and the second driver unit 20b is arranged at the outside of the sidewall of the tube part Ib, so that the back surface thereof is opposing to the through hole lib.

In addition, at the same air chamber 4b with the first driver unit 20, the through hole 18 for the second driver unit 20b is provided. For example, the through hole 18 for the second driver unit 20b may be provided in a position opposing to the through hole 11 for the first driver unit

20, at the sidewall of the tube part 1. By arrangement of the front surface of the second driver unit 20b so as to face to a new through hole 18 provided in the air chamber 4b, the second driver unit 20b is acoustically connected also to the first acoustic transmission path 10. Namely, the speaker system 30 of the embodiment has the first acoustic transmission path 10 and the second acoustic transmission path 10b as the above acoustic transmissionpath, and also has the first driver unit 20 and the second driver unit 20b as the above electro-acoustic transducer; and the tube part 1 of the above first acoustic transmission path 10 connected with the above first driver unit 20, and the tube part Ib of the above second acoustic transmission path 10b are communicated via the above second driver unit 20b.

It should be noted that the second driver housing 12b is formed at the front surface of the second driver unit 20b, and the wall surface configuring this second driver housing 12b communicates each of the tube parts 1 and Ib of the first acoustic transmission path 10 and second acoustic transmission path 10b, while enclosing the second driver unit 20b. Here, the first acoustic transmissionpart 10 and second acoustic transmission member 10b are set to have different resonance frequency. In addition, the first driver unit 20

and the second driver unit 20b are driven by electrical signals with reversed phase.

Explanation will be given on action effect of the speaker system 30 of the embodiment, which is configured as above.

According to the speaker system 30 of the embodiment, by having a plurality of acoustic transmission paths, the first acoustic transmission path 10 and the second acoustic transmission path 10b, which are set to have mutually different resonance frequency, effect of sound pressure enhancement by resonance in wider frequency range, as compared with a single acoustic transmission path, can be obtained. (An eighteenth embodiment)

FIG. 21 is a cross-sectional view of a speaker system of an eighteenth embodiment of the invention. The embodiment is an example of the addition of a second acoustic transmission path to the speaker system of the third embodiment. In the embodiment, the first acoustic transmission path 10 having the vibration plates 3a, 3b, 3c and 3d, and the second acoustic transmission path 10b having the vibration plates 3e, 3f, 3g and 3h, share the air chamber 4b, in an intersection way, and are driven by the common driver unit 20.

At the air chamber 4b arranged at the center part, among the three air chambers 4a, 4b and 4c of the first acoustic transmission path 10, the through hole 11 is provided, and the driver unit 20 is arranged at the outside of the sidewall of the tube part 1, so that the back surface thereof is facing to the through hole 11. In addition, in the air chamber 4b, at the sidewall of the tube part 1 thereof, a pair of openings 19 and 19b, mutually opposing, are provided; at the opening 19, the vibration plate 3f is provided, and at the opening 19b, the vibration plate 3g is provided.

A new air chamber 4e is provided adjacent to the vibration plate 3f, and the tube part Ib extends from the sidewall of the tube part of the first acoustic transmission path 10, so as to partition this air chamber 4e from outside world. At the tip side of the tube part Ib, the vibration plate 3e is provided so as to oppose to the vibration plate 3f . Similarly, the new air chamber 4g is provided adjacent to the vibration plate 3g, and the tube part Ib extends from the sidewall of the tube part of the first acoustic transmission path 10, so as to partition this air chamber 4g from outside world. At the front side of the tube part Ib, the vibration plate 3h is provided so as to oppose to the vibration plate 3g.

According to such configuration, the vibration plates 3e, 3f, 3g and 3h, as mass element, and the air chambers 4e, 4f and 4g partitioned by these vibration plates 3e, 3f, 3g and 3h configure the second acoustic transmission path 10b. This second acoustic transmission path 10b shares the air chamber 4b arranged at the center, among three air chamber 4e, 4f and 4g, with the first acoustic transmission path 10, and the driver unit 20 is connected to this air chamber 4b shared.

Namely, the speaker system of the embodiment is characterized in having the first and the second acoustic transmission paths as the above acoustic transmission path, and each of the tube parts of the first and the second acoustic transmission paths being arranged so as to mutually share one air chamber, in an intersection way; and the above electro-acoustic transducer being connected to the air chamber shared by each of the tube parts of the first and the second acoustic transmission paths.

Here, the first acoustic transmission path 10 and the

second acoustic transmission path 10b are set so as to have different resonance frequency; namely, the speaker system 30 of the embodiment is arranged with the first acoustic transmission path 10 and the second acoustic transmission path 10b, which are set so as to have mutually different resonance frequency, and mutually share one air chamber 4b, in an intersection way; and a driver unit is connected to one air chamber 4b shared by the first acoustic transmission path 10 and the second acoustic transmission path 10b. According to the speaker system 30 of the embodiment, configured as above, by resonance of the first acoustic transmission path 10 and the second acoustic transmission path 10b in different resonance frequency, effect of sound pressure enhancement by resonance in wider frequency range, as compared with a single acoustic transmission path, can be obtained. (A nineteenth embodiment)

The embodiment relates to a tube element module for an assembly kit of the acoustic transmission path 10, as shown in FIG. 1. The tube module has a tube-like member; at least one sheet of the vibration plate 3a provided so as to vibrate in the axis direction of the tube part, at the inside of the tube part; and a first and a second joint members provided at one end and the other end of the tube part. Here, the first joint is a male joint, and the second joint is a female joint making a pair with the male joint.

For example, by preparation of a plurality of tube modules each provided with at least one sheet of the vibration plates 3 , and connecting the first and the second j oint members of a plurality of tube modules so as to be detachable, the acoustic transmission path 10 alternately arranged with the vibration plates 3, and the air chambers 4a, 4b and 4c

partitionedby the vibration plates 3, is configured. In this case too, as described in the first embodiment, the arrangement periodicity of the vibration plates 3 and the air chambers 4a, 4b and 4cmakes, propagation velocity of acoustic vibration, propagating in the acoustic vibration part 10, set to be less than 1/3 of sound velocity in atmosphere, in a frequency range of from 20 Hz to 200 Hz. Users can enjoy sound quality at their choice, by changing number of the tube modules to be connected. Explanation was given on suitable embodiments of the invention as above, however, the invention is not limited thereto, and various modifications, abbreviations and the additions may be allowed.