TECHNICAL FIELD The present invention relates to sound muffling devices, particularly those of the type used in connection with tubes or ducts which emit sounds which one wishes to silence such as, for example, the exhaust pipes of internal combustion engines.

BACKGROUND Very early in the evolution of the internal combustion engine, it was discovered that the relatively high levels of noise emitted during operation of the engine could be controlled, to a large extent, by resonant sound muffling devices. At least as early as about a century ago, it was

discovered that a major portion of the sound emitted by an internal combustion engine exited through the tail pipe, which serves the primary purpose of exhausting spent combustion gases.

The approach toward the attenuation of these undesirably high levels of noise was to pass air exiting an engine through an acoustic filter. In principle, either high pass acoustic filters or low pass acoustic filters may be employed to muffle sounds in a duct. For example, a low pass filter is useful in order to prevent the transmission of relatively high frequency sounds. On the other hand, the low frequencies of acoustic energy which are predominant in explosive discharges, such as those created by the explosion of a gun or found in an automobile exhaust system may be filtered out using a high pass filter.

Likewise, a combination of both high pass and low pass acoustic filters may be used to achieve the elimination of noise. The elimination of noise may be viewed as generally involving the cancellation of the alternating flow of gases, representing sound transmission, while not impeding the steady flow of gas out from the exhaust system which is necessary in order to discharge spent combustion products.

As a general rule, mufflers have volumes in the range of six to eight times the piston displacement of the engine and may contain baffles with or without holes. A primary aspect of their operation involves the cancellation of sound waves by interference, usually involving breaking the waves into two parts which follow different paths and meet again out of phase before leaving the muffler. Another important aspect is that exhaust back pressure must be minimized in any muffler design, insofar as an increase of

only one psi in back pressure decreases the maximum power output of an engine by about 2.5%. About 1% of this loss is due to additional work being expended by the engine to exhaust the gases. The balance of the loss is due to the effects of increased gas pressure on volumetric efficiency.

Turning to the case of ventilating ducts, a degree of noise suppression is usually obtained by lining the ducts on at least two non-opposite walls with an efficient sound- absorbing material for a distance of three to six meters from both the inlet and the outlet. Where, due to the length of available duct, this is insufficient, additional noise suppression may be provided by introducing baffles into the duct and covering the baffles with sound-absorbing materials.

In the case of duct associated noise control systems, increased speed of air flow introduces additional noise through the generation of turbulence. This must be addressed by additional baffles and/or sound absorbing materials.

Some understanding of baffle filter systems may be obtained if we consider a quarter wavelength resonant cavity. Such a cavity, known as a Helmholz cavity is a chamber closed at one end and open at the other. Because it is a quarter wavelength in length, sounds entering the open end of the chamber pass through the chamber and are reflected back to the open end of the chamber with a phase delay of one-half a wavelength. The half wavelength delay is caused because the time of transit of the acoustic disturbance through the chamber includes a forward transmission path of one-quarter wavelength and a reflected transmission back to the open end of an additional quarter wavelength.

The result is a half wavelength or 180° phase shift in the output of the cavity with respect to the sound passing over the top of the cavity. Because the signals are phase shifted with respect to each other by 180 degrees, and because, for a first approximation, we can assume that during the emission of a particular sound, the amplitude and frequency of one wavelength of the sound is substantially identical to the amplitude and frequency of the next wavelength produced by the source. Thus, a given undulation corresponding to one wavelength is exactly cancelled by the prior undulation of the sound which one wishes to cancel. Naturally, this is only true for sound having the particular frequency which results in a quarter wavelength relationship between the Helmholz cavity and the sound. However, if the frequency is not far removed from the resonant frequency of cancellation, the cancellation effect will still ^" occur to a substantial extent.

In early automobile mufflers, the approach taken was to pass the exhaust gases over a matrix of baffles which together defined a plurality of tuned cavities. This structure acted as a filter and to a limited extent cancelled a range of sound frequencies produced by the internal combustion engine, propagated through the manifold to the tailpipe, and which would otherwise exit the engine in the form of acoustic disturbances.

Today, the quieting of such muffler systems is on the order of eight decibels.

Notwithstanding the numerous disadvantages of this sort of noise muffling system, modern mufflers remain substantially identical in their essentials. Generally, such prior art

mufflers are constructed of sheet metal. More particularly, such mufflers comprise an outer shell or casing made of sheet metal and a sheet metal baffle structure secured within the casing. A path for the conduction of combustion gases and attendant acoustic disturbances is provided in the muffler adjacent the various noise absorbing cavities.

Because the exhaust gases are both hot and corrosive (being the product of the combustion of gasoline) , they cause relatively quick corrosion and otherwise deteriorate the sheet metal components of the muffler. The result is that the muffler must be periodically replaced.

Still another problem with conventional mufflers is the viscous resistance which they provide to spent combustion products. Nor is the viscous resistance of the muffler of no significant effect. Rather, the resistance encountered by escaping combustion products is significant enough to adversely affect fuel efficiency and the concentration of pollutants in the exhaust gases. This is caused, in part, by the failure of the engine to exhaust spent combustion products from the cylinders with the same degree of efficiency that an internal combustion engine without a muffler achieves.

While, to some extent, the problems, involved in the rapid deterioration of automobile mufflers can be addressed through the use of relatively expensive alloys, such as certain types of stainless steel, and the use of relatively thick material, the additional cost of such high quality materials renders this uneconomical. Moreover, the additional labor costs involved in manufacturing mufflers with relatively thick sheet metal components adds cost

which clearly makes such mufflers impractical.

Likewise, while it is conceivable that a muffler design including relatively wide passages for the exhaust of combustion products and numerous cavities to cancel sounds passing over them could improve the incomplete scavenging of spent gases from the cylinders, the increase in size of a device made using such an approach would make it impractical in the environment of today's automobile. Here, space is at a premium and even the present day relatively small muffler represents a significant portion of the volume of the automobile. In any event, the muffler is ^' also often the lowest point on the automobile and thus represents the limitation on clearance over the road. In connection with this, it is noted that even in the case of diesel-engine trucks, where the problem of back pressure has required the use of relatively large mufflers and the aesthetics and size of the truck have allowed the use of large mufflers, adequate muffling of combustion noise has not been satisfactorily achieved by existing muffler systems.

SUMMARY OF THE INVENTION The invention is intended to provide a remedy. It solves the problem of how to muffle noises in a duct, such as an engine exhaust or air-conditioning duct with a simple, durable and effective device. This configuration integrates a mixing diameter with an integral microphone for improved cancellation over a wider frequency range than previous attempts. At the same time, back pressure problems are minimized thus resulting in good fuel efficiency and minimal exhaust of pollutants into the air. The same is achieved through the use of a single or multiply chambered dynamic cavity driven by an electro-

SUBSTITUTESHEET

mechanical actuator which, effectively, generates an acoustic signal used to cancel noise in the duct. The inventive muffler cavity is based upon the use of a so- called ported enclosure or symmetrically loaded system. This type of enclosure is characterized by the use of a closed rear volume, together with a front volume coupled to a radiating tuned port. This novel tuned design utilizes a single circular port driven by multiple speakers which surround the exhaust pipe to provide improved cancellation. Integrated into the port design is a mixing chamber surrounded by a circular sensing microphone. With the proper components and cavity volume and port selection high efficiency cancellation can be achieved over a 50 to 300 Hz frequency range.

As compared to previous designs a single circular port is used with multiple speakers as opposed to an array of individual ports from multiple speakers arranged around the exhaust outlet. A preferred embodiment avoids locating the microphone and anti-noise port a distance away from each other for acoustic mixing in air with limited high frequency results. The inventive system brings all of the components together at the exhaust port producing a higher degree of cancellation with higher frequency response than previous designs.

In accordance with the invention, engine or other exhaust noise is introduced into a mixing region with an acoustic cancellation signal where they are caused to cancel each other. A ring-shaped microphone array is disposed around the noise source and the acoustic cancellation signal, which is produced by the actuator, to generate an error signal proportional to the degree to which cancellation has not occurred. This error signal is then used to control

the signal produced by the actuator. Sensing of the sound pressure within the tubular member is done with one or more microphones where the output of the multiple microphones are combined by averaging of their individual outputs. Noise due to turbulence and other essentially random factors is cancelled through the use of a plurality of sound-sensing points.

In accordance with the preferred embodiment, a plurality of such sound-sensing points is achieved through the use of a tubular member with a plurality of sound-sensing holes disposed along its length. This tubular member is disposed concentrically with and downstream from the emission point of sound exiting the mixing region and downstream of and concentric with the acoustic output of the actuator.

This is achieved through the use of a first pipe which corresponds to the duct with noise in it being contained within a second larger pipe which is provided with the acoustic energy generated by the actuator. The use of concentric sources and a plurality of sound sensing ports in the inventive configuration results in more uniform noise cancellation, minimal mixing region size and immunity to random noise.

BRIEF DESCRIPTION OF THE DRAWINGS One way of carrying out the invention is described in detail below with reference to drawings which illustrate only one specific embodiment of the invention and in which: -

Figure 1 is a top plan view in cross section of an engine muffler constructed in accordance with the present invention;

Figure 2 is a view along lines 2-2 of Figure 1; Figure 3 is a view along lines 3-3 of Figure 1 showing the construction of the muffler in cross section; Figure 4 is a transverse cross-sectional view of the muffler illustrated in Figure 1 along the lines 4-4 of Figure 1; Figure 5 is a detail along lines 5-5 of Figure 2 illustrating the construction of a microphone assembly useful in conjunction with the muffler of Figure 1; Figure 6 is a cross-sectional view along lines 6-6 of Figure 2 illustrating the placement of a microphone within the microphone assembly; Figure 7 is a block diagram of the inventive system; Figure 8 is a diagrammatic view of an aerodynamic microphone design; Figure 9 is a diagrammatic representation of an alternative embodiment of the inventive muffler system; and Figure 10 is a view along lines 10-10 of Figure 9 illustrating the outside appearance of the muffler system of Figure 9.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to Figures 1-3, the structure of the inventive muffler 10 is seen to comprise an outer casing 12. Outer casing 12 comprises a cylindrical member 14, a forward end cap 16 and a rear end cap 18. Cylindrical member 14, forward end cap 16 and rear end cap 18 are made of a relatively inexpensive material such as plastic which is selected for mechanical strain and durability under a wide range of temperatures and other environmental factors as would be experienced by a muffler positioned at the bottom

of an automobile.

The exhaust pipe 20 is mounted within casing 12, being supported in forward end cap 16 by an insulative annular member 22.

The exhaust pipe 20 is made of steel, stainless steel or any other suitable material having a thickness sufficient to result in mechanical integrity. In addition, the exhaust pipe 20 is made thick enough to withstand the expected degree of corrosion during the life of the automobile without losing the required strength.

When installed, it is contemplated that muffler 10 will be secured to the underside of the automobile and that exhaust 20 is also secured to the automobile. Insofar as it is connected to the exhaust of the engine, the end 24 of exhaust pipe 20 • is held in position by a plurality of radial support members 26, 28, 30 and 32. Radial support members 26-32 are secured between exhaust pipe 20 and mixing chamber pipe 36, by being welded or otherwise suitably attached to both of these members. In accordance with the preferred embodiment, mixing chamber pipe 34 and support members 26-32 are all made of steel, or stainless steel or other suitable materials. Likewise, exhaust pipe 20, radial support members 26-32, and mixing chamber pipe 36 may be made of stainless steel in view of the resistance of this material to long exposures of high temperatures and the various combustion products created during the operation of the internal combustion engine.

As illustrated in Figure 1, mixing chamber pipe 34 (which may also be made of plastic) is securely mounted within rear end cap 18 by being securely attached or jam-fitted in

a circular hole 36 within end cap 18. Finally, additional support is provided by a pair of transverse radial support members 38 and 40, as illustrated in Figure 4. The transverse radial support members are made of material similar to that of support members 26-32. In addition, it is noted that transverse radial support members 38 and 40 and radial support members 30 and 32 are made of triangular shaped pieces of relatively thick sheet metal in order to provide support when forces are applied to the muffler structure in the direction parallel to the axis of symmetry of exhaust pipe 20.

The isolation of heat present within exhaust pipe 20 from the remainder of the system is provided by a cylindrically- shaped layer of heat insulative material 42 which is disposed around exhaust pipe 20. Typically, this is insulating fiberglass wrap, header wrap, or an isolating air cavity. An acoustical chamber 44 is defined by a pair of inner planar walls 46 and 48, actuators 50 and 52 and a forward wall 54 with a circular concentric hole 56 in its center. On the edge of the chamber opposite the forward wall 54 is a rear wall member 58. Rear wall member 58 and forward wall 54 are both made of synthetic material such as that of outer casing 12.

Referring to Figure 4, wall 54 defines a chamber 60 which is filled with sound deadening material such as fiberglass 62 in order to change the equivalent cavity volume and improve and simplify the acoustical properties of the acoustical chamber 44. Likewise, rear wall member 58 defines a pair of chambers 64 and 66 which are filled with acoustic deadening material such as fiberglass 68 and 70 which again change the equivalent volume and simplify the operation of acoustical chamber 44 by preventing random

oscillations and resonances from interfering with the operation of the muffler.

Finally, a tubular microphone assembly 72 is provided at the end of mixing chamber pipe 4. A mixing chamber 74 (Figure 1) is defined at the end of mixing chamber pipe 34. Referring to Figure 2, a plurality of holes 76 are defined by a circular tubular member 78. Generally, holes 76 are equispaced along the circumference of member 78 and one such hole 76 is illustrated in detail in Figure 5. Typically four microphones are also equispaced within the circumference of member 78. Actual detection of sound and conversion into an electrical signal is done by these microphones 80 and 82 which have their acoustical inputs positioned within the annular cavity 84 defined by tubular member 78. Microphone assembly 72 is secured to the end of mixing chamber pipe 34 using any suitable means such as rivets, adhesive, or the like. The electrical output of these multiple microphones are combined (averaged) using a mixing circuit to provide a composite residual error signal.

The placement of microphones 80 and 82 is illustrated by the enlarged detailed diagram of Figure 6. Here microphone 80 is shown embedded in the sidewall 86 of circular tubular member 78. Microphones 80 and 82 may be positioned at a variety of angular positions depending upon whether one wishes to route the microphone cable 88 on the inside or outside of the device.

Referring to Figure 7, during operation of the inventive system, the noise generated by exhaust pipe 20 and actuators 50 and 52 is detected by tubular microphone assembly 72 which generates an error signal which is sent

to a cancellation signal generator 90. The cancellation signal generator, in turn, generates a cancellation signal which is coupled to actuators 50 and 52. A cancellation signal generator such as that marketed by several companies today may be used.

In particular, in the embodiment illustrated in Figures l- 6, an acoustical chamber 44 which is substantially completely closed except for an annular output duct region 92 defined between exhaust pipe 20 and mixing chamber pipe 34, is provided. Thus, the acoustic energy generated by actuators 50 and 52 is transformed into a concentric source which is concentric with the noise output of exhaust pipe 20. These two concentric sources are mixed together in mixing region 94 where, in the ideal case, because successive undulations are substantially completely out of phase with each other and of equal magnitude, they add together and cancel each other resulting in zero noise at the output of the exhaust system adjacent microphone assembly 72. It has been found that a mixing region 94 on the order of ten centimeters in diameter and three centimeters in length is sufficient to achieve an acceptable degree of cancellation in an automobile muffler system.

In order to permit the flow of any liquid that may have accumulated in the muffler out of the muffler, a drip hole with a short, small diameter tube 96 is provided at the bottom of the muffler, as illustrated in Figure 4. Depending upon the actual configuration of the tube microphone, it may also be desirable to put an additional drip hole 98 adjacent to the tube microphone assembly 72. From a practical standpoint, such drip holes will not affect the performance of the system in any substantial

matter from an acoustic standpoint.

Conventional radio speakers ruggedized for use in an active muffler may be used as actuators. Ruggedization consists of using waterproof materials such as Kevlar, or impregnated materials, with a neoprene muffler surround. In particular, an acceptable degree of performance has been achieved using circular thirteen centimeter thirty watt speakers of the type manufactured by AUDAX under Catalog No. HIF13JVX. In addition, it has been found advantageous to ruggedize the speakers through the application of a protective coating of Kevlar.

It has been found that good cancellation results for typical automotive sound pressure levels may be achieved using 20 to 50 watts of electrical power into the speaker actuators 50, 52. In principle, while a single microphone speaker will also work, the provision of two or more speakers provides some redundancy and allows a smaller enclosure to be used and would appear to improve the symmetry of the system. Generally, cancellation is achieved in the range below 800 hertz. If it is desired to achieve complementary cancellation in the range above 800 hertz, a thin steel wool liner 100 (other liner materials are also acceptable) may be positioned within exhaust pipe 20 as illustrated in Figure 4. Other traditional passive muffler attenuation methods can also be integrated in the dynamic muffler for high frequency attenuation.

The tubular ring microphone system disclosed above is both durable and has excellent performance characteristics. As can be seen, with reference to the figures, the microphones 80 and 82 are protected from the environment by being positioned within circular tubular member 78. Thus the

microphone is protected from weather and heat effects. In addition, the use of numerous holes 76 in circular tubular member 78 results in numerous individual inputs to the microphones and has the result of acoustically averaging random noise, thus drastically reducing wind and exhaust turbulence effects. The tubular configuration with the multiple microphones produces a residual error signal which is the integrated-averaged error as measured along the perimeter of the ring. For a dynamic muffler measuring this error at the zone of cancellation produces a symmetrical cancellation zone that is optimum.

The tubular microphone assembly 72 is constructed from an insulating tubular material such as plastic tube. This creates a thermally insulating medium to protect the microphone. The tubular material is perforated at regular intervals, corresponding to 30-50 holes per wavelength at the highest frequency of interest (i.e., 0.44 meters separation between holes corresponding to 600 Hz) . For best operation, the hole size needs to be small, typically around 0.062 meters. The hole size and number of holes can be varied to adjust the amount of sound pickup.

The plastic tube of the microphone assembly protects the microphones by surrounding them with a captive thermally insulating air medium. The use of open holes at the exhaust outlet provides an accurate means of sound transmission without directly exposing the microphone elements to the corrosive and hot exhaust gases.

An alternative configuration is to cover the perforations in the tubular member with thin (.001") Kapton (TM) tape. This provides all the above noted advantages of the tubular microphone while providing a sealed configuration.

In connection with this, it is noted that a variety of profiles may be used in order to minimize turbulence about holes 76. For example, turbulence reducing aerodynamic surfaces 102 and 104 may be used to reduce turbulence, as illustrated in Figure 8.

In addition, blockage of one or more of the holes in the circular microphone assembly 72 will have a less serious impact on system operation.

An alternative embodiment is illustrated in Figures 9-10. Generally, similar parts or parts performing analogous or corresponding or identical functions are numbered herein with numbers which differ from those of the earlier embodiment by multiples of one hundred.

As can be seen from the alternative embodiment of Figures 9 and 10, it is not necessary that the muffler of the present invention take a conventional form. For example, it is possible that a bumper 104 may accommodate the inventive muffler 110. In particular, an exhaust pipe 120 may feed its output to a mixing chamber 174 which, in turn, receives the acoustic output of a pair of actuators 150 and 152. Additional advantage may be obtained by providing an annular membrane 106 to receive the output of actuators 150 and 152 and couple that output generally in the directions indicated by arrows 108 while isolating the actuators from the environment.

While an illustrative embodiment of the invention has been described above, it is, of course, understood that various modifications will be apparent to those of ordinary skill in the art. Such modifications are within the spirit and scope of the invention, which is limited and defined only by the appended claims.