05/248676

12/04/1973

04/28/1972

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Donaldson Company, Inc. (Minneapolis, MN)

181/228

181/248

F01N1/00; F01N1/04; F16L55/02; F16L55/033; F01N1/02; F01N1/00

181/33D,44,48,56,41,42,49,50,59,47B

3289785	Silencer with outer housing contacting inner conduit to define resonance chambers	December 1966	Walker
3382948	Mufflers with side branch tuning chambers	May 1968	Walker et al.

Wilkinson, Richard B.

Gonzales, John F.

What is claimed is

1. Apparatus for separating acoustic energy in the form of waves from a moving fluid, comprising:

2. Muffler apparatus for attenuating acoustic energy in the form of sound waves having a known frequency in a system having both sound waves and a fluid traveling therethrough, comprising:

3. The apparatus of claim 2 wherein an inlet of said duct is connected to the exhaust outlet of an engine.

4. The apparatus of claim 2 wherein sound absorbing means are mounted in said chamber to absorb acoustic energy being coupled therein.

5. The apparatus of claim 2 wherein said chamber is formed by a second duct having an outlet opening for said acoustic energy being coupled therein and wherein means are connected to said outlet opening of said second duct to dispose of said acoustic energy being coupled therein.

6. The apparatus of claim 2 wherein said duct and chamber are formed by generally parallel first and second tubes having said coupling elements in adjoining walls thereof, said first tube having open ends to permit free flow of fluid therethrough, and said second tube having closed ends and containing an acoustic energy absorbing material.

7. The apparatus of claim 2 wherein said duct and chamber are formed by first and second generally parallel, tubular members each having opposite open ends to permit free fluid flow therethrough, wherein substantially all of the acoustic energy being coupled into the second tubular member from the first tubular member continues to travel in the same direction in the second tubular member but undergoes a phase shift of 90° during the coupling process, wherein said coupling elements are sized to couple approximately one-half of the acoustic energy from said first tubular member to said second tubular member, and further including means for conveying fluid from the fluid outlet end of said first tubular member to the adjoining end of the second tubular member for free passage of fluid through said first tubular member in one direction and through said second tubular member in the opposite direction, said acoustic energy being reflected back to its source as a result of a series of coupling actions and phase shifts between said tubular members in both directions.

8. The apparatus of claim 2 wherein said coupling elements are designed to couple substantially all of said acoustic energy into said chamber, said acoustic energy undergoing a phase shift of 90° during the coupling process, wherein a second muffler apparatus is connected to an outlet of said chamber to provide an additional phase shift of 90°, and wherein means are provided to combine the resultant sound wave shifted in phase by 180° with an equal amount of the original acoustic energy to provide cancellation thereof.

9. A muffler system, comprising:

10. The apparatus of claim 9 wherein said second conduit means includes a pair of acoustic directional couplers, each of said couplers for imposing a 90° phase shift on the sound waves being transmitted therethrough.

11. The apparatus of claim 10 wherein each coupler comprises a first tubular member having an inlet and a second tubular member having an outlet, said members being mounted in side-by-side relationship with a wall therebetween, and wherein a plurality of coupling elements are provided in said wall to couple substantially all of the sound energy being carried by said first tubular member into said second tubular member.

12. The apparatus of claim 11 wherein said first and second tubular members are generally parallel members, and wherein said inlet and said outlet are on opposite ends of said members.

13. The apparatus of claim 10 wherein each coupler comprises a first tubular member having an inlet and a second tubular member having an outlet, said members being mounted in a side-by-side relationship with a wall therebetween, and wherein a plurality of coupling elements are provided in said wall to couple approximately one-half of the acoustic energy being carried by said first tubular member into said second tubular member.

14. The apparatus of claim 13 wherein said first and second tubular members are generally parallel members, and wherein said inlet of one member is disposed adjacent said outlet of the other member.

15. The apparatus of claim 10 wherein each coupler comprises a first tubular member and a second tubular member mounted in a side-by-side relationship with a wall therebetween, wherein a plurality of coupling elements are provided in said wall to couple acoustic energy from said first tubular member into said second tubular member, wherein an inlet of the first tubular member of the first coupler is connected to the discharge outlet of said first conduit, an outlet of said first tubular member is connected to said third conduit means, and an outlet of said second tubular member of the first coupler is connected to said second conduit means, said coupling openings of said first coupler being sized to transfer approximately one-half of the acoustic energy in said first tubular member into said second tubular member thereof, and wherein the second coupler has an inlet of the first tubular member thereof and an outlet of the second tubular member thereof connected in said second conduit means, and couples substantially all of the acoustic energy carried by the first tubular member thereof into the second tubular member thereof.

16. An engine exhaust muffler, comprising:

17. An engine exhaust muffler, comprising:

18. A method of separating acoustic energy in the form of waves from a moving fluid, comprising the steps of:

19. A method of removing acoustic energy in the form of sound waves within a predetermined frequency range from a flow of gas, comprising the step of passing said noise and gas through at least one conduit of a directional acoustic coupler having a pair of side-by-side conduits with acoustic coupling means therebetween.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to apparatus and method for reducing noise carried by a moving fluid, such as gas, in a duct, and more particularly relates to such method and apparatus employing directional acoustic couplers.

2. Description of the Prior Art

Despite a vast amount of effort, especially in the last 40 or 50 years, industry is still faced with severe problems of muffling noise being carried by moving fluids in ducts or conduits. Mufflers have been designed that will satisfactorily attenuate exhaust noise, but at the expense of engine performance. Most mufflers employ a series of baffles and passages that act to reflect sound waves. In addition, they often employ chambers that resonate at a specific frequency to thereby set up an impedance discontinuity that acts to reflect exhaust noises at that frequency. While these various baffles, passages and chambers are efficient sound attenuators, they act to restrict the passage of exhaust gases to thereby adversely affect engine performance.

Although most work has been done with exhaust silencing, increased attention has recently been paid to the problem of engine intake silencing. The problem is especially acute with small, two-cycle engines in which air for combustion is drawn into the engine through a duct in one direction while noise from the engine escapes through the duct in the opposite direction. Once again, silencing of the noise using the methods described above will result in reduced engine performance because of excessive restriction to air flow.

Noise can also be a problem in air conditioning systems in which large quantities of treated air are carried through a building by means of a duct system. If not properly designed, the ducts can carry unwanted noise generated by the equipment for treating the air along with the air itself. In all of these systems, there is the common problem of disposing of unwanted noise without at the same time overly restricting the flow of fluid through the duct.

To my knowledge, there has not been a significant breakthrough in the art or science of noise attenuation for many years. The recent developments of which I am aware are all variations or combinations of known technology which may offer some improvements but no breakthroughs. In contrast to these other developments, I believe that the present invention provides a major breakthrough in the technology of noise attenuation. The use of directional acoustic couplers to separate or otherwise dispose of unwanted noise in moving fluid streams is a completely new concept. After I made this discovery and recognized the advantages of the invention, searches were conducted to determine whether any pertinent prior art existed. The searches failed to develop any prior art that taught or made obvious the concept of utilizing an acoustic directional coupler as an important element of a noise attenuation system. There is no mention in the prior art of using acoustic couplers to separate or otherwise dispose of unwanted acoustic energy being carried by a moving fluid stream.

We were able to find only two publications that made reference to acoustic couplers. One article, entitled "Coupling Through a Small Aperture in a Wave Guide" by J. Van Bladel, was published in Volume 47 (1970) of The Journal of the Acoustical Society of America, page 202. The article discusses in theoretical terms the operation of a two-hole directional coupler. However, no mention is made of any practical use of the coupler. The other article, entitled "The Realization of an Acoustical Directional Coupler," by P. Lagasse, was published in J. Sound Vibration, (1971) 15 (3), 367-372. In this article, the design of an acoustic directional coupler is presented. The article discloses a coupler having a thick center wall through which the coupling elements extend, as opposed to the very thin center wall of the present invention. The author describes an application of the coupler as a device for direct measurement of the acoustical reflection coefficient of materials. No mention is made of utilizing a directional acoustic coupler in any form of sound attenuation system. Further, neither of the articles makes any reference to the separation or disposal of acoustic energy being carried by a moving fluid stream as for example, the noise being carried by exhaust gases being discharged from an engine.

Aside from these two articles, it should be recognized that a vast amount of effort has been expanded in developing directional couplers for microwave circuits since the beginning of World War II. Directional couplers have long been used in waveguide systems to couple electromagnetic energy from one waveguide to another. Because of the importance of radar systems and microwave communication systems employing waveguide transmission elements, directional couplers for use in such systems have been highly developed. Two U.S. Pats. that disclose typical directional couplers for use with waveguides are the Sferrazza U.S. Pat. No. 2,820,203, issued Jan. 14, 1958 and the Hewlett U.S. Pat. No. 2,871,452 that issued Jan. 27, 1959. These patents are merely cited as being typical because many other patents and publications can be found relating to directional couplers for microwave systems.

The work I have done indicates that the basic theory of acoustic couplers and electromagnetic couplers is the same in an analytic sense although the coupling mechanisms differ in view of the differences in propagation of electromagnetic waves and sound waves. From a design standpoint, however, it appears that the theory and techniques developed in the electromagnetic coupler art can be utilized in designing directional acoustic couplers. Because the design and operation of directional acoustic couplers appear to conform to the same restrictions and mathematics as electromagnetic couplers, the designer of directional acoustic couplers can look to the electromagnetic coupler art for design assistance. The teachings of the electromagnetic coupler art will be especially helpful in designing acoustic couplers having high directivity over a relatively broad frequency range. As an example, the previously identified Sferrazza patent discloses a particular coupling element array that is said to improve the directivity of an electromagnetic coupler. The same general theory can be used for designing coupling element arrays for acoustic directional couplers.

Other publications containing useful theory on the design of directional couplers are as follows: "A Mathematical Theory of Directional Couplers," Henry J. Riblet, Proceedings of IRE, November 1947, page 1307-1313; "Directional Coupler Design Nomograms," Tore N. Anderson, Microwave Journal, May 1959, page 34-38; and "The Design of Multihole Coupling Arrays," Edward S. Hensberger, Microwave Journal, August 1959, page 38-42.

SUMMARY OF THE INVENTION

From a conceptual standpoint, the present invention is the realization that acoustic directional couplers can be utilized to control noise in the form of sound waves being transmitted through a duct or conduit also carrying a flow of fluid. By means of the present invention, it is possible to attenuate noise without substantially restricting the flow of fluid through the duct or conduit. This provides an enormous advantage over prior art systems that require the fluid flow to be restricted in order to accomplish noise attenuation. In one form of the present invention, an acoustic coupler is used to transfer or couple acoustic energy from a conduit, through coupling elements into a separate chamber provided with sound absorbing material. The acoustic energy is coupled into the chamber while minimally affecting the free flow of gases through the conduit. In another form of the invention, use is made of the capability of directional acoustic couplers to act as phase shifters, in a passive cancellation system. Passive cancellation operates by dividing the incident acoustic energy in a duct into two equal amplitude component waves. Acoustic directional couplers are then used to impose a 180° relative phase shift on one component. The two components are then recombined so that the out of phase signals canceland reflect. If a passive cancellation system is designed for use as an engine exhaust silencer, for example, the cancellation of noise can be achieved without causing any restriction to the free flow of exhaust gases through the system. In other forms of the present invention, the capabilities of directional acoustic couplers are used to reflect sound waves back toward the source without restricting flow of gases through the coupler. By means of the present invention, those concerned with designing apparatus for attenuating noise being carried by a duct or conduit are provided with a completely new technique for accomplishing their mission. As will be further described herein, the acoustic directional couplers can be used either alone or in combination with other mufflers or silencers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view in section of a symmetrical, two-element directional acoustic coupler;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 shows a system for attenuating engine exhaust noises, employing a 0db coupler;

FIG. 4 is a schematic diagram of a passive cancellation system employing a pair of 0db couplers;

FIG. 5 is a similar passive cancellation system employing one 3db coupler and one 0db coupler;

FIG. 6 is a similar passive cancellation system employing two 3db couplers;

FIG. 7 shows a single 3db coupler designed to act as a noise reflection device; and

FIG. 8 shows the use of a 0db coupler designed to act as a noise reflection device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like numerals are used throughout the several views to identify like elements of the invention, there is disclosed in FIG. 1 a basic form of acoustic directional coupler 10 that will be used in the various systems described herein. Coupler 10 includes a first tubular member or conduit 11 having an inlet port 1 and an outlet port 3, mounted in a side-by-side, generally parallel relationship with a second tubular member or conduit 12 having opposite open end ports 2 and 4. Conduits 11 and 12 may be of arbitrary, different cross-sectional shapes. When conduits 11 and 12 are identical in area and shape, the coupler is referred to as a symmetrical coupler. Conduits 11 and 12 are joined for a selected distance by a common, relatively thin wall 13 having at least a pair of coupling elements 14 and 15 therein. As shown in FIG. 2, common wall 13 may be relatively flat to provide sufficient common wall space for the coupling elements 14 and 15. Whether or not this is necessary will depend upon the size of coupling elements required. In a typical application of the present invention, an engine exhaust muffler, conduits 11 and 12 can be thin-walled steel tubing welded together in a side-by-side relationship with wall 13 thus being a double wall. In the preferred embodiment, coupling elements 14 and 15 are openings in the wall 13. If desired, and if the application permits, the coupling elements can be membrane covered openings. The membranes will vibrate under the influence of acoustic energy to thereby couple the energy from one conduit to the other.

If port 1 of the coupler 10 is connected to the exhaust discharge port of an engine or other device having an acoustically noisy gas effluent, the exhaust gases will freely flow through conduit 11. Most of the exhaust gases will be discharged from port 3. However, if coupling elements 14 and 15 are openings, some portion of the exhaust gases will pass upwardly through openings 14 and 15 into conduit 12 if either or both of ports 2 and 4 are open to permit exhaust gas discharge therefrom. Basically, however, conduit 11 should be designed to provide a relatively unrestricted flow path for the exhaust gases.

A typical engine emits a large amount of acoustic energy along with the exhaust gases. A portion of this acoustic energy is in the form of audible sound waves which are normally considered undesirable. The exhaust noise from a typical engine will cover a wide frequency spectrum. In some cases, it may be desirable to attenuate substantially all of the audible sound energy being emitted from the engine discharge port. In other cases, it may be desirable to attenuate only those sounds that are at the most undesirable frequency ranges. A coupler of the type shown in FIG. 1 is capable of transferring acoustic energy from one conduit to the other without unduly restricting gas flow and without absorbing more than an insignificant amount of the energy being transferred. This transfer is accomplished by positioning the coupling elements 14 and 15 along the wave propagation path. The coupling elements can be designed to transfer any amount between zero and full power from one conduit to the other. In the systems to be described herein, the couplers 10 usually transfer either one-half power or full power. The coupling notations in db give the negative of the acoustic power at port 4 relative to the power incident at port 1 in decibels. The notation 0db indicates a complete transfer of power from one conduit to the other. The notation 3db indicates a transfer of one-half of the power. Therefore, if coupler 10 of FIG. 1 were designed as a 0db coupler, for a paticular frequency or frequency range, all acoustic energy within that frequency range would be transferred from conduit 11 to conduit 12, without substantially affecting the free passage of exhaust gases through conduit 11. In a perfect system, no sound waves within the design frequency range would leave port 3.

Coupler 10 is directional in that ideally, for a wave incident on port 1, no energy could be coupled from the incident port 1 to the auxiliary port 2. All of the acoustic energy leaves ports 3 and 4 in amounts depending upon the coupling ratio. For an acoustic directional coupler, it is desirable to have a low level of energy coupled out port 2 and a low reflection coefficient looking into port 1. The ratio of the coupled level in port 4 to that in port 2 in decibels is defined to be the "directivity" of the coupler. For symmetric couplers, it can be shown that a high directivity is equivalent to a low input reflection coefficient. A high directivity within a band of frequencies is achieved by spacing the coupling openings one-quarter wave length apart at the center frequency. In designing acoustic couplers for the systems described herein, it is desirable that the coupler be capable of transferring power effectively over a wide band of frequencies in the range of audible sound. It is also desirable that the directivity be high so that most efficient use can be made of the coupled energy. Unsymmetrical directional couplers are also applicable to some of the systems discussed herein.

FIG. 3 discloses a form of muffler in which the inlet port 1 of a duct 17 is connected to an engine exhaust port 18. The exhaust gases pass freely through duct 17 for discharge from port 3. Noise within a predetermined frequency range is coupled into a chamber 19 (corresponding to conduit 12) through a plurality of coupling elements (openings 20) in the common wall between duct 17 and chamber 19. In this embodiment, the coupling openings 20 are designed to provide full power (0db) coupling over a relatively wide range of frequencies. Adjacent pairs of openings are spaced apart one-quarter wave length for the selected frequency. The openings will then couple energy with a high degree of directionality and a low input reflection coefficient over a band of frequencies centered on the selected frequency. Further, the sizes of the holes are tapered from the ends toward the middle, with the largest openings being in the middle to prevent abrupt coupling thereby minimizing impedance discontinuities and reducing reflection.

A sound absorbing material 21 such as fiberglass is placed in chamber 19 to absorb the noise being coupled therein. The ends of chamber 19 (corresponding to ports 2 and 4) are closed. In this absorption-type filter, the acoustic energy is coupled into a side conduit or chamber where it can be absorbed without influencing the flow path through duct 17 followed by the exhaust gases. If desired, several absorption-type filters like that shown in FIG. 3, each designed for a particular frequency, can be connected in series to effectively cover a wider frequency range. A number of these absorption-like filters can be connected in series, with each filter covering very effectively a rather narrow range of frequencies, to thereby cover in total a relatively wide frequency range, without causing any substantial restriction to the flow of exhaust gases through the duct 17. If desired, the absorption-type filter shown in FIG. 3 can also be connected for use in combination with standard muffler systems.

A low-reflection absorption-type filter, an example of which is shown in FIG. 3, has the advantage of isolating the absorbing action from the engine and from the exhaust gas flow path. This allows realization of the maximum reflection coefficients of reflective (reactive) devices, such as conventional mufflers, irrespective of their location in the exhaust system, so long as the absorption filter is between the reflective device and the engine.

A practical 0db coupler will remove over 90 percent of the acoustic energy (transmitted power reduced by 10db) over some band of frequencies, and over 99 percent of the energy (transmission reduced by 20db) over a narrower band. For example, a laboratory model has given 10db reduction over a 370Hz band for a design frequency of 800Hz (a 46 percent band width) and 20db reduction over a 100Hz band (12.5 percent band width). The test also showed that over the frequency range from 700Hz to 1,000Hz, the amplitude reflection coefficient looking into port 1 was less than 0.08. Thus, a practical 0db coupler can be designed as a very low reflection device.

Referring again to FIG. 1, a very useful inherent property of symmetrical acoustic couplers, as herein described, is that the wave leaving port 4 lags in phase 90° with respect to the wave leaving port 3. Thus, in a symmetrical 3db coupler, in which one-half the energy is coupled into conduit 12, the acoustic energy leaving port 4 will be 90° out of phase with respect to the remaining acoustic energy leaving port 3. A symmetrical acoustic coupler can thus be used as a phase shifter.

The phase shift capabilities of the acoustic coupler can be used to achieve sound attenuation by cancellation. FIG. 4 discloses one configuration for achieving a 180° relative phase shift for internal cancellation. The exhaust gases and acoustic noise from the engine enter the inlet of a first conduit 25 which is provided at its outlet end with a Y-junction 26. Y-junction 26 divides at least the sound energy (but not necessarily the exhaust gases) into two generally equal parts for separate transmission through second conduit means 27 and third conduit means 28. Connected in series with second conduit means 27 are a pair of 0db acoustic couplers 10. The couplers 10 are shown schematically, but the construction of each is generally the same as shown in FIG. 1. The port numbering system is also the same. Ports 2 and 3 of each coupler 10 are blocked off and may contain absorbers so that both the gases and sound energy enter port 1 and leave port 4. Because these are symmetrical 0db couplers, all of the acoustic energy incident on port 1 of each coupler leaves the coupler through port 4 shifted in phase by minus 90° relative to port 3. The sound waves leaving port 4 of the downstream coupler are made to be 180° out of phase with respect to the sound waves in third conduit means 28 by properly adjusting the length of conduit means 28. The out of phase sound waves in conduit means 27 and 28 are recombined at a downstream Y-junction 29 which leads to a single outlet conduit 30. If the system is designed so that the sound waves in conduit means 27 and 28 are of equal amplitude, they will recombine and cancel each other at the Y-junction 29. Thus, in a perfectly designed system, for a particular frequency range, no audible sound would leave port 30. The sound waves cancel in the sense that they are reflected back toward the source from the Y-junction 29 and are eventually dissipated as heat. However, it can be seen that the exhaust gases are free to flow through the system without restriction. In a practical system, the sound waves being propagated through conduit means 27 will suffer a phase change merely from traveling a distance through the various conduits. In like manner, some phase change will occur to the sound waves passing through conduit means 28. The length of conduit means 28 should therefore be adjusted so that there is a net 180° phase difference between the waves recombining at the Y-junction 29 and so that the rates of change of phase with frequency of the two conduits is as close to being equal as possible. It should again be noted that the Y-junction 26 is designed to split the sound energy into substantially equal halves. However, the exhaust gas flow is not necessarily split into two equal parts. Depending upon their relative restrictions to gas flow, a portion of the gas will flow through conduit means 27 and the remaining portion will flow through conduit means 28 but these gas flows will be recombined at Y-junction 29 for common discharge through outlet conduit 30.

FIG. 5 discloses another form of cancellation system utilizing one symmetrical 3db coupler and one symmetrical 0db coupler. In this system, all of the exhaust gas flow and sound energy enters port 1 of the 3db coupler. Port 2 is blocked off and may contain absorbers. One-half of the sound energy leaves port 3 and the other half leaves port 4. The coupler has imposed a 90° phase lag on the energy leaving port 4 relative to that leaving port 3. The sound energy and gases leaving port 4 enter port 1 of the 0db coupler. Ports 2 and 3 of the 0db coupler are blocked. All of the energy entering port 1 of the 0db coupler leaves port 4, shifted in phase a total of minus 180°. The out of phase sound waves are again recombined for cancellation at the downstream Y-junction. Adjustments in conduit length are required for achieving the 180° relative phase and nearly equal phase slopes.

Another system for achieving internal cancellation is shown in FIG. 6. Once again the sound energy is divided into two equal components. One component is carried to a recombination point 31 without shifting its phase. The other component is transmitted thorugh a pair of 3db couplers in series to obtain a minus 180° phase shift. The component to be shifted enters port 1 of the first 3db coupler and leaves port 2 thereof. Ports 3 and 4 are blocked. With this type of connection, essentially all of the energy entering port 1 eventually leaves port 2 with a relative phase of minus 90° relative to the outgoing wave at port 1. The operation is as follows. Two symmetrical 3db directional couplers connected end-to-end result in a 0db coupler having the required 90° phase property of symmetrical couplers. A blocked pipe results in a unity reflection coefficient with a phase angle equal to zero degrees. Thus, blocking the pipes as shown in FIG. 6 makes the 3db couplers behave exactly as a 0db coupler would except that port 2 becomes the output port with a relative phase angle of 90° relative to the outgoing wave at port 1. The sound energy leaving port 2 of the first 3db coupler enters port 1 of the second coupler where the same sequence occurs. Once again, the two components are recombined at the output Y-junction for cancellation thereof. In the system of FIG. 6, the conduit means not containing the two couplers has been lengthened so as to maintain a net 180° phase difference and best equality of phase slopes at the point of recombination.

Although the sound waves are cancelled at the output Y-junctions of FIGS. 4-6, it should be understood that the acoustic energy does not simply disappear. The energy is actually reflected back to the source where it eventually dissipates through conversion to heat.

FIGS. 4-6 are meant to be representative of the systems employing acoustic couplers as phase shifters to provide internal noise cancellation. It is not intended that these three figures represent the only ways in which internal cancellation can be achieved by using couplers as phase shifters. There are other combinations using one, two or more couplers following the same general concept that would work equally well.

Another form of muffler system utilizing an acoustic coupler is shown in FIG. 7. I refer to this type of muffler as a "bandstop filter." The acoustic coupler 10 is again shown schematically with the same port numbering system being retained. Once again, the exhaust gases and noise enter port 1. In a band-stop filter of this kind, ports 3 and 4 are connected by a curved conduit 32 permitting the free flow of gases and sound waves between ports 3 and 4. Port 2 is left open and represents the exhaust discharge opening. The exhaust gases are provided with a uniform, unrestricted gas flow from port 1 through port 3, conduit 32, port 4 and port 2. With respect to the sound waves entering port 1, however, the bandstop filter of FIG. 7 reflects all of the sound waves back to the source. The two numerals (1.0) at port 1 represent unit amplitude acoustic waves propagating into and out of the filter, thus the case of perfect reflection is shown. The numeral (0.0) at port 2 indicates that no sound energy within the design frequency band leaves port 2.

A 3db coupler is used in the bandstop filter of FIG. 7. The starting point is the realization that two 3db couplers connected end-to-end comprise a composite 0db coupler. If the conduits connecting the two 3db couplers are crossed so that port 3 of the first connects to port 2 of the second, and so on, then the composite coupler will have all its energy appearing at port 3. Then, realize that if ports 4 and 3 of the first coupler are connected togehter, the same situation results but all the acoustic energy is now propagating out of port 1 (the image of port 3 of the composite cross-connected coupler). Thus, connecting ports 3 and 4 of a symmetrical 3db coupler as shown results in a reflective, bandstop filtering device. The net result is that all of the energy is reflected back toward the source where it gradually dissipates in the form of heat. An advantage of this bandstop configuration is that the flow of exhaust gases from the engine can follow a direct path without having to pass through a coupling structure. This design minimizes engine horsepower loss caused by an unwanted pressure drop. If desired, a number of these bandstop filters, each designed for a particular frequency range, can be connected in series to cover a broad band of noise frequencies.

FIG. 8 illustrates how a 0db coupler is used as a band-stop filter. In this case, all of the ports are open, except port 4, which is blocked. Because this is a 0db coupler, a single unit of sound energy port 1 is completely coupled to port 4. The energy is reflected from port 4, which is blocked, and on the return pass is completely coupled back to port 1. Again, the sound energy is completely coupled or reflected back toward the source. This bandstop design allows a straight-through, no-bend (port 1-port 3) path for exhaust gases and has almost no back pressure, even by comparison with the previous 3db bandstop design. The tradeoff is that greater length is usually required for a 0db coupler design.

Although the above disclosure has been written in terms of achieving directionality by spacing the coupling elements one-quarter wave length apart, it should be noted that any odd quarter wave length coupler will be highly directional. The spacing of the coupling elements may be one-quarter wave length or any odd multiple thereof. A spacing of three quarter-wave lengths may be needed when attempting to directionally couple high frequency acoustic energy. At high frequencies, coupling elements spaced one-quarter wave length apart are too close for effective directional coupling. When used herein the phrase "odd multiple" includes one-quarter wave length as well as multiples of 3, 5, 7, etc.

Although many of the embodiments described above have been described in terms of their use in engine exhaust systems, it should be understood that their use is not so limited. Identical or similar devices and systems can be used, for example, as air duct noise attenuators or as engine intake silencers.