BACKGROUND AND PRIOR ART In the 1920s, discoveries were made which allowed sound waves to be recorded and reproduced with the use of electrical signals. Before that time, the record player worked on purely mechanical principles. As the pick-up needle traced the groove in the record, the mechanical vibrations of the needle were coupled to a flared horn mounted on top of the device. The vibrations were amplified by the horn, and sounds were thus produced directly, with the volume of sound reproduction being a function of the size of the acoustic horn. As a consequence, the volume and quality of sound was limited by the use of the acoustic horn and its practical size limitations.

Later, pick-ups were made which converted mechanical vibrations into electrical signals, and thus became known as transducers. Such an electrical signal could be amplified by the newly developed vacuum tube amplifier, but it could not be applied to the acoustic horn. Hence, a transducing device was needed which could convert electrical signals back into sound waves.

This is the function of the conventional loudspeaker.

The basic requirements of a dynamic loudspeaker are a circular or elliptical diaphragm or cone (also sometimes known as a dome), which is freely suspended

from a metal frame both around its edge and near its center. Attached to the center of the cone is a wound coil of wire known as a voice-coil, which is positioned between the poles of a magnet. When an electrical signal is applied to the voice-coil, a magnetic force is generated which is opposed by the permanent force of the magnet. Because the voice-coil is rigidly attached to the diaphragm, this force causes the diaphragm to move. The movements of the diaphragm closely follow the variations in the electrical signal and cause sound waves to be produced in the air. The more closely the movements of the diaphragm are able to follow the variations in the electrical signal, the more accurate will be the reproduced sound.

Certain problems arose with the radiation of sound waves of frequencies extending throughout the audible range, which is about 30 to 16,000 Hz. First, the sound from the rear surface of the diaphragm had to be isolated from the sound coming from the front surface, otherwise sounds leaving both surfaces would cancel each other out. Secondly, at high frequencies, sounds are not equally radiated in all directions but become focused in a narrow beam. In order to improve reproduction quality and efficiency at low or bass frequencies, a loudspeaker is mounted in an enclosure or cabinet.

Various types of loudspeaker enclosures are known.

A popular example is the acoustic suspension type of enclosure. An acoustic suspension enclosure is a sealed cabinet in which a loudspeaker is mounted, and which is filled with sound absorbing material to isolate sound from the rear surface of the diaphragm.

Acoustic suspension type enclosures exhibit good bass response but are inefficient in that they require more power from the amplifier to reproduce a given sound level.

Other types of known enclosures are designed to make use of the rear surface sound radiation such that it acts to enhance the sound reproduction from the loudspeaker. One example of this type of enclosure is a tuned or reflex enclosure, in which a vent or port at the front magnifies sound pressure at a specifically tuned frequency range. Another type of known enclosure is a transmission line enclosure, which has been used to guide rear surface sound radiation to the outside of the speaker cabinet in such manner that it augments the front surface radiation.

Loudspeaker arrangements for increasing efficiency and isolating rear surface radiation are also known.

One example is a so-called push-pull arrangement, as described in U. S. Patent No. 4,016,953 to Butler, incorporated by reference herein in its entirety. The push-pull arrangement of the'953 patent consists of a pair of loudspeakers mounted face to face with an airtight connection therebetween. The voice-coils of the two loudspeakers are connected in an out of phase relationship, that is, the positive terminal of the first speaker is connected to the negative terminal of the second speaker and the negative terminal of the first speaker is connected to the positive terminal of the second speaker. An audio source is then connected to the terminals of either speaker. In this way, the two loudspeakers act together as a unit. That is, when the front surface of one speaker diaphragm is deflected in one direction, the rear surface of the other diaphragm is deflected in the same direction, and vice versa. As such, the sound wave radiation from the two loudspeakers will be in the same direction.

While the known push-pull arrangement exhibits increased performance and/or power handling capacity, there remains a need in the art for achieving increased frequency response, higher efficiency, reduction of harmonic distortion, and increased presence over the

existing loudspeaker technology. ~ SUMMARY OF THE INVENTION The present invention provides a loudspeaker system which achieves a substantial improvement over the prior art with respect to each of the above- mentioned parameters, and which exhibits superior performance characteristics, enhanced spatial effects, and improved mechanical attributes over known loudspeaker designs.

In particular, the present invention provides a loudspeaker system, comprising an enclosure, a transducer mounted in the enclosure and comprising at least two drivers arranged in a push-pull configuration and mounted to have an airtight coupling therebetween, and an acoustic transmission line provided within the enclosure, the transmission line being coupled to the transducer and functioning as a load on the transducer.

In an alternate embodiment, the present invention provides a transducer having at least two drivers arranged in a compound configuration and mounted to have an airtight coupling therebetween, and an acoustic transmission line coupled to the transducer and functioning as a load on the transducer.

Other features of the improved loudspeaker system of the present invention will become apparent from the following detailed description of preferred embodiments taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view of a loudspeaker system according to one preferred embodiment of the present invention; Fig. 1A is a perspective cut-away view of a loudspeaker cabinet according to one embodiment of the present invention; Figs. 2A and 2B are side views of alternate two

driver configurations according to the present invention; Fig. 3 is a side view of one alternate embodiment of the present invention comprising a four driver array; Fig. 4 is a side view of a second alternate embodiment of the present invention comprising a four driver array; Fig. 5A is a side view of an additional embodiment of the invention comprising a pair of drivers in a compound configuration; Fig. 5B is a side view of a further additional embodiment of the present invention comprising a four driver array arranged in a compound configuration; and Fig. 5C is a side view of a still further additional embodiment of the invention comprising a plurality of drivers arranged in a series/parallel compound configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures, the present invention will be described in detail. As shown in Figs. 1 and lA, a multiple driver array includes a pair of loudspeaker drivers 21 and 22 coupled in a push-pull configuration in an airtight, sealed chamber or space 23 within loudspeaker cabinet or enclosure 10. The volume of the sealed space 23 is not critical, but it must be essentially air-tight. The push-pull array is loaded into an acoustic transmission line 28, which terminates at a port 30 in the front of the enclosure 10. An opening 26 is provided in the enclosure 10 for the output of the driver array. As shown, the drivers operate together in series in a push-pull configuration, with the positive terminal of one driver connected to the negative terminal of the other driver and vice versa. An audio signal is connected to the

terminals of the driver array from a conventional crossover circuit 24.

The operation of a crossover circuit is well known in the art and thus will not be explained in detail herein. In brief summary, the crossover circuit is used to separate the audio source signal into different frequency bands or ranges and to direct the separate sub-signals to different drivers (such as woofers, midrange drivers and tweeters, for example). Fig. 1 illustrates a woofer driver array only.

The acoustic transmission line 28 as shown is a folded, tapered line, but may be straight (as shown in Figs. 2A and 2B as transmission line 28a), and can have parallel sides as well as tapered sides. However, a tapered transmission line is preferred because it provides a higher quality sound than a parallel sided line by more evenly distributing the resonant modes within the waveguide line. The taper should be decreasing from the driver end to the port end. The taper ratio (TR) is defined as the cross sectional area of the transmission line at the driver end divided by the cross sectional area of the line at the port end.

A TR of at least 1.00 functions efficiently. The best results have been obtained with a TR of n (3. 1416). In fact, it has been discovered that a TR of n improves performance even in a conventional transmission line enclosure. The reason for this is believed to be because n is a non-repeating transcendental number, it therefore enables the transmission line to break down resonant frequencies out to an infinite number of harmonics. The transmission line can be of almost any cross sectional shape, but functions most effectively where there are no right angles or parallel sides.

The acoustic transmission line 28 is preferably filled with an acoustic damping material 29 to a predetermined density. Material 29 can be a fibrous material and/or open-celled acoustic foam. Long hair

lambswool is preferred for most applications because of its high coefficient of absorption. The specific fill density is determined by the length of the transmission line, the average cross sectional area of the line, the Fs of the driver array, the Sd of the driver array, and the Qts of the driver array. The length of the transmission line is determined by the Fs of the driver array and the relative speed of the sound waves through the damping material, and can vary according to application. Typically, the length of the transmission line is set at either one quarter (1/), one half (), three quarters (3/4), or one full wavelength at the Fs of the driver array.

The damping material 29 serves three main purposes: (1) it acts as an acoustic low pass filter for the back wave of the driver array; (2) it damps resonant frequencies in the transmission line; and (3) it acts to delay the propagation of the sound waves traveling through the transmission line so that they emerge from the port 30 in proper phase relationship with the front wave of the driver array.

As shown in Fig. 1A, additional openings 26a and 26b are provided in the enclosure 10 for additional driver arrays. Opening 30a is provided for the crossover terminal plate to connect the speaker to an external amplifier.

The push-pull driver array may be configured in a face-to-face configuration as shown in Fig. 2A, with a sealed chamber 23a between the drivers, or may be arranged in a back-to-back configuration as shown in Fig. 2B, wherein the drivers 21 and 22 are enclosed in sealed chamber 23. In each case the drivers are connected to an audio signal from a crossover and the terminals of the individual drivers are connected so that the drivers act in series as a single driver.

While at least two drivers are required to achieve the push-pull transducing effect, there is no

theoretical limit to the number of drivers which may be used in the driver array. Fig. 3 illustrates an alternate embodiment which uses four drivers 21a, 22a, 21b and 22b in a linear back-to-back, face-to-face, and back-to-back arrangement respectively. Drivers 21a, 22a and 21b, 22b are enclosed in sealed chambers 23, and the drivers 22a and 21b are coupled together by an airtight chamber 23a.

Fig. 4 shows a second alternate embodiment wherein the drivers 21a, 22a, 21b and 22b are arranged as two back-to-back pairs mounted adjacently to each other in an airtight sealed chamber 23b. A larger dimensioned acoustic transmission line 28b is provided in this embodiment to accommodate the dimension of the adjacent driver pair arrangement.

The present invention achieves many advantages over the prior art push-pull transducer system as well as over conventional transmission line arrangements.

For example, the loudspeaker system according to the present invention exhibits an extended low frequency response, typically an F3 2-5 Hz below the Fs of the system. Additionally, the system has a flatter impedance curve with little or no rise at the Fs of the driver array, and also has a better transient response.

The push-pull acoustic transmission line arrangement acts more as a plane source propagator which minimizes listening room interaction, and further this arrangement loads the listening room instead of the loudspeaker enclosure. This allows music to "exist"in the listening room rather than inside the enclosure and thus provides a more vivid sound stage.

The present invention further exhibits a dramatic reduction in second order harmonic distortion over conventional acoustic transmission line systems caused by asymmetric non-linearities inherent in single driver operation. As applied to a bass driver, the present invention has a measurably lower bass frequency

response (F3) and the pitch and definition of the bass tones are significantly enhanced, resulting in more weight, authority and"slam"as compared with conventional systems. When used to reproduce midrange or high frequencies, the sound is more open and spatially vivid than obtained by conventional systems.

The present invention further enables more seamless sound integration between bass, midrange and high frequency drivers.

According to alternate embodiments of the invention, as shown in Figs. 5A-5C, simple compound drivers 21,22 are loaded into an acoustic transmission line 29, instead of push-pull compound drivers. While push-pull compound drivers exhibit better performance characteristics when large drivers and drivers with lower Qts are used, it has been found that simple compound drivers may perform better when smaller drivers or drivers with higher Qts are used.

As is understood in the art, a push-pull configuration is one specific instance of an isobaric, or constant pressure driver coupling configuration. In its general sense, an isobaric system uses two identical drivers mechanically coupled by an airtight enclosure, wherein the coils of the two drivers are connected either in parallel or in series. The volume of air trapped between the two drivers effectively connects the suspension system of the two drivers, turning the pair into a single compound driver.

As shown in Fig. 5A, a single compound driver 50 is formed by individual drivers 21 and 22 facing in the same direction, and connected by an airtight enclosure 23. The compound driver 50 is coupled to an acoustic transmission line 28 as in the first embodiment. In a preferred embodiment, the acoustic transmission line contains damping material 29. In a first variation of the simple compound driver embodiment as shown in Fig.

5B, a side-by-side four driver compound configuration

is shown, wherein series-connected drivers 21a, 21b and 22a, 22b are connected in parallel with each other, and coupled together in airtight enclosure 23.

As shown in Fig. 5C, a linear multiple driver compound configuration is shown, wherein drivers 21a and 22a are connected in parallel, and driver 22b is connected in series with the parallel pair 21a, 22a.

All three drivers are coupled together by two airtight enclosures 23a and 23b.

It is to be understood that the embodiments of the invention presented herein are shown and described for the purposes of making a full disclosure of the preferred embodiments and that the embodiments presented are thus not limiting in any way as to the scope of the present invention, which is to be defined solely by the following claims.