Free flow sound attenuating device and method of using

Abstract

A sound attenuating device characterized by several novel structures which include (1) a foraminous conduit surrounded by (2) a housing having in series after the inlet end (3) a low frequency attenuator and (4) a high frequency attenuator, the conduit acting in operative association with the low frequency attenuator to enhance the effectiveness of the high frequency attenuator by providing a lower resistance to the high frequency sound entering the high frequency attenuator.

Description

BACKGROUND OF THE INVENTION

The present invention is directed to a sound and noise attenuating device for use with flowing gas systems. In particular the invention is directed to a free flow sound attenuating device wherein the sound attenuated encompasses both low and high frequency sound and to a sound attenuating device for use particularly with flowing gases of single phase. By single phase it is meant flow comprising substantially 100 percent gaseous state with little or none of the flow in a liquid state.

By low frequency it is meant frequencies 500 Hz or less, middle frequency being defined as between 500 Hz and 1000 Hz, and high frequency any frequency above 1000 Hz. The device may be used for intake and exhaust gas flow systems such as mufflers and resonators for internal combustion engines. By a low frequency attenuator it is generally meant those known in the art such as a Helmholtz type resonator, a single or multi-dimensional baffle system, retroverted systems, or expansion chambers. By high frequency attenuator it is meant those known in the art as attenuating frequencies above 1000 Hz, one example being the use of quarter-wave standing wave; cavities which are closed with acoustically absorptive foraminous material. It is noted that best results are obtained of the low frequency attenuator used is of the Helmholtz type and the high frequency attenuator is of the quarter-wave standing wave type.

By foraminous conduit it is meant any conduit having porosity such that the flow resistance across the conduit wall is properly selected relative to the acoustical impedance of the gas and sound pressure level entering the conduit. The flow resistance may be uniform or non-uniform along the conduit length, a uniform distribution being preferable because of its lower manufacturing cost.

SUMMARY OF THE PRIOR ART

Free flow or straight through sound attenuating devices are known in the art, and by free flow it is meant those devices where the flowing gas passage is direct and open with minimal back pressure developed as compared to devices where the flowing gas must pass through a multi-directional baffle system, retroverted systems, or through expansion chambers. Prior art devices have used resonators of the Helmholtz type separately or in combination with baffled systems for attenuating sound in exhaust gas flows for internal combustion engines, blower duct systems, fuel burning systems, etc. The art also discloses using a single Helmholtz resonator for tuning out a specific frequency or multiple resonators for tuning out a multiple of frequencies, such that sound accompanying such gas flows have sound removed by the action of certain frequencies resonating in the Helmholtz chamber. Whether single or multiple resonators are used the sound attenuation is dependent on the frequency characteristics of the single resonator or the sum of the frequency characteristics for multiple resonators.

For large gas flows and sound pressure levels, the prior art sound attenuating devices have necessitated very large units in absolute terms, increasing the problem of the design, handling and installation of these units.

An additional problem with respect to free flow systems is that unless large in size the efficiency of the attenuator is not capable of attenuating sound to an acceptable level to the general public or for industrial applications.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a sound attenuating device that has greater sound attenuating characteristics than the sum of the sound attenuation of the individual sound absorbers if used separately. In other words, a synergistic effect is realized by the structural combination of the present invention.

Furthermore, recognizing that prior art sound attenuators could be efficient when subjected to gas flows having a high sound pressure level, they did so at the expense of being very large in size, weight and complexity. The present invention has as one object and advantage, the provision of a sound attenuation device that because of the operative association of the foraminous conduit with the low frequency attenuator, the size, weight and complexity found with prior art structures are eliminated and a very simple free flow device can be defined. The device of the present invention accomplishes this by permitting several attenuating functions to take place in close proximity resulting in a synergistic sound attenuation characteristic.

Moreover, with prior art sound attenuators utilizing baffling, retroverted flow patterns, resonators and expansion chambers, a great number of production sequences were required in their manufacture. Accordingly, another object of this invention is to provide a sound attenuator which is simple in structure, easier to manufacture and easier to maintain than devices heretofore known in the art. In addition the device of the present invention is lower in cost and more durable in operation than prior art devices having equivalent sound pressure level and attenuation characteristics.

Furthermore, although the prior art teaches free flow devices, the structure of the present invention provides an improvement over them in its utilization of a central foraminous tube which may have an internal layer of metal fibers having a special coating. The coating applied will depend on the function it is to serve one example being a material in an oxidized or unoxidized state resistant to corrosive substances that may be present in the flowing gas. The foraminous tube operating in conjunction with a low frequency resonator also permits greater decibel attenuation in the high frequency resonator, heretobefore not known in the art or expected.

It is another object of the invention to provide for a more compact rugged constructed attenuator that has improved erosive, corrosive and temperature resistant characteristics.

Additional objects and advantages of the invention will become apparent to those skilled in the art from the following discussion of the several illustrative embodiments thereof, which will be described in connection with the attached drawings, in which:

FIG. 1 shows a schematic of the basic structure of the sound attenuating device in accordance with this invention;

FIG. 2 is a section of FIG. 1 taken along lines 1--1;

FIG. 3 is a graph detailing the improvement in sound attenuation by the unit shown in FIG. 1;

FIG. 4 shows a first alternate embodiment to that shown in FIG. 1;

FIG. 5 is a graph detailing the sound attenuation by the attenuator shown in FIG. 4;

FIG. 6 shows a second alternate embodiment to that in FIG. 1;

FIG. 7 shows a section of the structure in FIG. 6 taken along lines 1--1;

FIG. 8 shows a third alternate embodiment of the sound attenuation device of the present invention;

FIG. 9 shows an embodiment of the central tube used in the present invention, sectioned along its centerline;

FIG. 10 shows a second embodiment of the central tube of the present invention, sectioned along its centerline;

FIG. 11 shows a third embodiment of the central tube of the present invention, sectioned along its centerline;

FIG. 12 shows a fourth embodiment of the central tube of the present invention, sectioned along its centerline;

FIG. 13 is a cross sectional view of another embodiment of this invention;

FIG. 14 is a cross sectional view of another embodiment of this invention;

FIG. 15 is a cross sectional view of another embodiment of this invention; and

FIG. 16 is a cross sectional view of another embodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In general terms the basic features of the preferred embodiment of this invention comprise (1) a foraminous conduit surrounded by (2) a housing having in series after the inlet end (3) a low frequency attenuator operatively associated with the conduit and (4) a high frequency attenuator operatively associated with the conduit end positioned after the low frequency attenuator to enhance the effectiveness of the high frequency attenuator by providing lower resistance to the high frequency sound entering the high frequency attenuator due to the absence of low frequency sound entering the high frequency attenuator.

FIG. 1 shows a schematic of a section of a sound attenuator according to the present invention. The attenuator 8 has a central conduit 10 having an inlet end 12 and outlet end 13 which outlet and inlet end can be connected to conventional intake or exhaust pipes according to the particular application. The attenuator 8 has a high frequency attenuator 8A and a low frequency Hz attenuator portion 8B. Alternatively, the low frequency attenuator may be a baffle system 8C, a multi-dimensional baffle system 8D, a retroverted system 8E, and an expansion chamber system 8F, shown in FIGS. 13-16 respectively, and used as desired. The low frequency attenuators of FIGS. 13-16 do not prevent the high frequency portions from being flowed through. The central conduit 10 is generally described as a foraminous tube, meaning a tube having a preselected flow resistance such that the pressure drop across the wall of the conduit is selected to match the impedance of the flowing fluid. That is, it is necessary that the conduit not be totally impervious to a flow of gas. The choice of material for the conduit may be made of a number of materials some examples being, laminated screen structure, perforated tube, metal fiber web, knitted fiber or wire material, compacted fiber, foamed metals or non-metals, glass fiber, plastic fiber, porous ceramics or combinations thereof. However, it is noted that the choice of material for the conduit will be related to its particular application. As an example, for a flow of gas at a high temperature a metal fiber web will work best whereas for gases at a low temperature a plastic may be used. In addition, for gas flows containing surges of pressure level increases in flow, a material capable of withstanding a great pressure drop across the conduit wall will be necessary. Whatever the situation it is obvious that one skilled in the art can determine the material most suitable for the particular gas flow by application of known engineering principles.

A number of different conduit strucutres may be used depending on the physical characteristics of the gas flow, four structures are shown as example in FIGS. 9, 10, 11 and 12.

FIG. 9 shows a tubular section 48 made of perforate plate having a plurality of holes 49. These holes may all have s similar or different diameters.

FIG. 10 shows a tubular metal structure section 50 made of metal fiber web or mesh structure, which web and mesh structures may be produced by processes known in the art as described in U.S. Pat. No. 3,505,038; 3,127,668 and 3,469,297 each of which is incorporated by reference. The mesh and web structures can be made of metal fiber made in accordance with the descriptions given in U.S. Pat. No. 3,394,213; 3,505,038; 3,505,039; 3,698,863; 3,379,000 and 3,277,550 each of which is also herein incorporated by reference. It is also possible to provide chopped fibers useful in the tubular structure in accordance with U.S. Pat. No. 3,504,516. The above listed patents being owned by the assignee of the present invention.

In addition, it is also possible to use coated metal fibers in accordance with U.S. Pat. No. 3,698,863 and 3,505,038. Other ways of producing the fibers and fiber webs for the tube of this invention are known in the prior art and such other references are not excluded by the citing of the above references which are given as example only.

It is noted that the tubular structure made of fiber web or mesh as described above is desirable to other structures, since it has been found that this structure provides the greatest amount of frictional losses for optimum sound absorption.

FIG. 11 shows a tube made of a screen-like material 51 which may be used either separately or in combination with the tube shown in FIG. 9, the controlling factor being whether the outer shell 11 (See FIG. 1) of the structure has sufficient rigidity to provide support for the screen-like tube. An equivalent form for the screen-like material would be a highly perforated tube or mat structure tubular in shape and having an external diameter approximately equal to the internal diameter of the tube.

FIG. 12 shows a preferred embodiment of the present invention comprising a conduit 10, wherein a conduit 52 (in accordance with the conduit shown in FIG. 9) has ports 53 of similar diameter shown large for purposes of illustration. It also has metal, organic or ceramic fibers 54 provided on the internal surface thereof, the fibers having a protective coating of oxidation-catalyst such as nickel, platinum, aluminum oxide, copper oxide, etc.

Referring to FIG. 1 and FIG. 2 whatever the form of the central conduit used, it is surrounded by and secured to an inner wall 21 closing cavities 19, with an outer shell 11 enclosing this inner wall except for the inlet and outlet ends projecting a short distance for easy attachment to the device with which it will be used. It is noted that the foraminous portion of conduit 10 is foraminous only for that portion of the conduit associated with cavities 19, this being indicated between points c and b. The remaining portions of conduit 10 being non-porous to fluid flow except for ports 15. The outer shell 11 is of a conventional nature, one example being sheet metal.

The flowing gas having concomitant sound to be attenuated enters the sound attenuator 8 at the inlet end 12. The gas comes into contact with low frequency Helmholtz resonator ports 15 which use chamber 17 for resonating. Although it was defined earlier in the specification what is generally accepted as definition for low, middle, and high frequency, the cut-off range for low, medium and high frequency is relative and will depend upon the application. The gas stream then passes by annular high frequency, quarter wavelength depth tuned cavities 19 which communicate with the central tube through the tube's foraminous wall 20. The alpha character a indicates the depth of the quarter wavelength cavity.

FIG. 3 is a graph plotting attenuation of sound power in db verses frequency of the sound with the area under the curves representing the amount of sound absorbed. If for example the low frequency resonator was tuned, by appropriate adjustment of chamber volume 17 and port diameter 15 well known in the art, to 250 Hz the response curve would be that designated by lines L. A high frequency resonator line duct will have the typical curve shown as line H, usually tuned somewhere above 1000 Hz. From theory and practice in the art it would be expected and predicted that if a sound absorber containing two attenuating means, one tuned to low frequencies and one tuned to high frequencies were to be used in one structure the resulting curve would follow line L for frequencies just above 250 Hz and then follow line A, a transition phase, and continue along line H in normal fashion for the high frequencies. What was found with the device of the present invention, however, was a synergistic effect. For low frequencies the curve looks like that expected in the art, line L, but for higher frequencies the response follows curve I attenuating the sound in a gas flow approximately 10 db higher than expected. By definition in the art a decibel increase in sound power of 3 decibels is equivalent to doubling the sound power level. Therefore, a reduction of 10 decibels (db) in sound power level is equivalent to reducing the sound power level to one-tenth of the original level. This one improvement of the invention and it is significant in that none of the prior art structures have disclosed or suggested this possibility. The improvement is believed to be the result of the elimination of low frequency sound from interacting with the foraminous tube and thereby enhancing the effectiveness of the tube at high frequencies. Specifically, the wall of the foraminous conduit provides a certain resistance to the flow of high frequency sound to resonant cavities 19. This resistance is increased if low frequency sound is present along with the high frequency sound. There is a further increase in resistance at the wall of the tube if the low frequencies are at high sound pressure levels. Maximum attenuation will occur in thin-walled foraminous tubes backed by quarter wavelength cavities, when the tube wall resistance is effectively equal to the characteristic impedance of the fluid flowing through the tube. The acoustical impedance of fluid gases can range between 3 - 400 cgs, rayls, the following given as example only with impedance values for other gases available in standard reference books:

air at 6 psia and 1000.degree. F - 10 rayls.

hydrogen at 14.7 psia and 0.degree. C - 11.4 rayls,

air at 14.7 psia and 0.degree. C - 42.86 rayls,

air at 55 psia and 800.degree. F - 100 rayls,

air at 115 psia and 400.degree. F - 250 rayls,

air at 100 psia and 100.degree. F - 300 rayls,

Freon -- 22 at 76 psia and 150.degree. F - 380 rayls (the condition of Freon inside of a sealed refrigeration unit)

The matching resistance of the foraminous portion of conduit 10 is also measured in units of rayl being defined as the change in sound pressure level drop (measured in dynes/cm.sup.2) across the conduit wall divided by the velocity of fluid flow (measured in cm/sec). A unit of rayl is then dynes-sec/cm.sup.3.

It has been shown that if the effective resistance of the conduit wall is too low, the quarter-wave standing wave will be established but maximum sound absorption will not occur. If the conduit wall resistance is too high the quarter-wave standing wave will not be strongly established and sound absorption will again be low. The prior art literature indicates that a high sound pressure level high frequency sound present will cause the effective acoustical impedance (resistance) of the material used to increase; "high" pressure level for high frequency sound being defined as above 130 db, with particular concern in the 140-160 db range. Any high frequency sound pressure level below 130 db being defined as low. It has been found by the inventor, that the effective resistance of the material used also increases if low frequency sound at sound pressure levels of 110-120 db are present. The result is that with a broad spectrum of sound the low frequencies present reduce the ability of the foraminous material to absorb energy at the high frequencies since the material will exhibit a higher effective resistance. This higher resistance is great enough to seriously reduce the effect of the high frequency quarter-wave standing wave cavities and correspondingly reduce absorption. What is believed to be happening in the present invention is as follows. It was found that resistance of the foraminous tube 10 changes with the frequency and sound pressure level of sound passing therethrough. If low frequencies are present the wall of the tube in effect acts as high resistance. However, if low frequencies are absent the resistance of the tube wall decreases. The sound attenuator of the present invention utilizes this effect for synergistic results. At the inlet end the low frequencies in the flowing gas communicate through ports 15 to the low frequency resonator chamber 17, low frequencies resonate in chamber 17 with the result that the low frequencies are substantially attenuated. The high frequencies in the flowing gas then encounters the high frequency resonators 19. Since the low frequencies are now absent, the high frequencies pass through a low resistance wall, establish standing waves and are attenuated. The high frequencies can then be attenuated with greater efficiency in the high frequency resonators which is the improvement shown as curve I in FIG. 3. If the low frequencies were present with the high when encountering chambers 19, the low frequencies would cause the tube wall to exhibit a high resistance. The high resistance of the wall however will prevent the high frequencies from entering chambers 19 efficiently and thereby result in lower attenuation, which is the curve H in FIG. 3. There is no suggestion or disclosure in the prior art that an interaction exists between the different frequency ranges such that the presence of low frequencies in a flowing gas will cause a high frequency resonator of the type described to be less capable and efficient in attenuating the high frequencies. This effect has not heretobefore been recognized or utilized.

Another embodiment of the attenuator shown in FIG. 1 is that shown in FIG. 4 which is identical to the attenuator in FIG. 1, but which has in addition a middle frequency Helmholtz attenuator indicated by chamber 18. Tube 10 communicates with chamber 18 by ports 16 to attenuate the middle frequency generally designated in the art as between 500 and 800 Hz. The response of this attenuator is shown in FIG. 5 where line H-1 would be expected according to the teachings of the prior art and where line I-1 is the improvement according to the structure of the present invention.

In the above description, frequencies of 0 to 500, 500-1000 Hz and 1000 Hz and above were used in defining the low, medium and high frequency ranges respectively. The particular frequencies generated by a sound source, will vary with style, size, etc. of the application. Thus, the frequencies necessary and useful in the above structures will vary. The maximum efficiency of the attenuator of the present invention will thus depend upon the exact frequency characteristics of the sound source. Utilizing this data the frequencies to be used for tuning the low, medium and high resonators can be calculated by standard formulae as described in "The Theory of Sound," by John William Strut, Baron Rayleigh, published in 1894 and republished by Dover Publications, Inc. in 1945.

It has been noted earlier that for large gas flows and sound pressure levels, the prior art attenuating devices have necessitated very large units in absolute terms. As example, a centrifugal compressor moving a fluid comprising air at a temperature of 300.degree. F, pressure of 15 psi, and flow velocity of 145 to 290 ft/sec. required a prior art muffler (attenuator) 18 inches in diameter by 86 inches long to reduce the sound level below 90 db, whereas with the present invention for air at the given temperature, pressure and flow velocity a unit only 141/2 inches in diameter by 291/2 inches long is required. A volumetric reduction of 449% over the prior art device.

As another example, for a compressor application for a given gas flow at a given temperature, pressure and flow velocity, a selected attenuation in db required a Burgess BEO-10 unit 30 inches in diameter by 136 inches long whereas a structure by the present invention achieved equivalent results with a unit 22 inches in diameter by 32 inches long. A volumetric reduction of 790% over the prior art.

The effect of the Helmholtz resonators used in this invention is maximized by knowing the frequencies of the system to be treated. Variations in gas flow volume and sound pressure level will obviously necessitate increases or decreases in the size and design of the attenuator. However, adjustments for these parameters are well known in the art.

FIGS. 6 and 7 show another alternate embodiment of the present invention wherein a shell 36 surrounds and is secured to a foraminous tube 31 having an inlet 32 and outlet 38. FIG. 7 more clearly shows the low frequency resonators chamber 41 communicating with the gas flow by ports 37. The high frequency standing wave cavities 42 communicate with the gas flow by the porous nature of the tube. In this embodiment the gases enter at inlet 32 and come in contact with a low frequency resonator chamber 41 through ports 37. The gas stream flows by annular high frequency cavities 42 through the tube's foraminous wall 43, and then continues to flow and contact simultaneously a series of both low frequency resonators and high frequency standing wave cavities. Use of this structure having a plurality of structures as shown in FIG. 1 is required where the sound pressure level of the gas is very high. In other words, every sound attenuating structure has a certain efficiency in attenuating the sound accompanying gases flowing therethrough. The magnitude of sound accompanying the gas flow is directly related to the sound pressure level. If the sound pressure level is very high, an attenuator, say for example having a given amount of efficiency, may not attenuate a sufficient amount of sound to bring it to an acceptable level. Accordingly, it will be necessary to flow the gas through a second unit to further attenuate the sound. The structure in FIG. 6 is for this purpose, indicating an attenuator having a plurality of single attenuators similar to that shown in FIG. 1 for applications where large sound pressure levels are encountered.

FIG. 8 shows a third embodiment of the present invention having a double, parallel ducted structure having a shell 23 provided with an inlet 24 and outlet 25. There are two foraminous tubes 27 communicating with a low frequency resonator 30 through ports 28 and with high frequency standing wave cavities 32 through the porosity of the tubes walls. It is noted that the foraminous portion of tubes 27 extend for only those portions of tubes 27 associated with cavities 32. The remaining portions of tubes 27 being non-pervious to fluid flow except for ports 28 and 29. In addition, and similar to FIG. 4, a middle frequency resonator 31 is provided and communicates with tube 27 through ports 29. The resonators, depending on the system on which it will be used, will be tuned in accordance with the above discussion regarding frequency design characteristics and the mesh or web structure discussed previously. This embodiment is useful for higher volumes of gas flow.

It is also noted that the depth a of the high frequency resonators (see FIG. 1) is measured from the outer wall E of the foraminous material. These resonators being preferably tuned by making a an odd multiple of one-quarter wave lengths of the high frequency sound. This will cause the sound pressure level at the tube wall to be substantially at zero pressure with the sound velocity being at its maximum.

While the foregoing description sets forth the principles of the invention in connection with specific resonators known in the art, the terms and expressions employed are terms of description in the art, and not of limitation, with no intention in using such terms to exclude any equivalent of the structures described. It is also understood that the description is made only by way of example and not as a limitation of the scope of the invention as set forth in the general aspects thereof and in the accompanying claims.

Claims

What I claim is:

1. A device for attenuating sound in a fluid flowing therefrom, the fluid having a known acoustical impedance measured in rayls, comprising a housing surrounding a conduit having an inlet and an outlet, the conduit having in series after the inlet:

first means for attenuating low frequency sound entering the device; and

second means for attenuating high frequency sound entering the device,

the second means having a foraminous duct in series with said conduit, the duct having a preselected acoustical resistance measured in rayls substantially the same value in rayls as the impedance of the flowing fluid, the first means attenuating the low frequency sound to a level sufficiently low to enable the foraminous duct to effectively function at its preselected acoustical resistance.

2. A device as recited in claim 1 wherein the device is a free flow device.

3. A device as recited in claim 2 wherein the fluid is a flowing gas.

4. The device as recited in claim 1 wherein the first means is a Helmholtz resonator.

5. The device as recited in claim 4 wherein the first means is tuned to 250 Hz.

6. The device as recited in claim 1 wherein the first means is a baffle system.

7. The device as recited in claim 6 wherein the baffle system is a multi-dimensional baffle system.

8. The device as recited in claim 1 wherein the first means is a retroverted system.

9. The device as recited in claim 1 wherein the first means is an expansion chamber.

10. The device as recited in claim 1 wherein the second means is a quarter-wave standing wave cavity.

11. The device as recited in claim 10 wherein the depth a of the standing wave cavity is an odd multiple of one-quarter wavelengths of the sound pressure level.

12. The device as recited in claim 11 wherein the second means is tuned above 1000 Hz.

13. The device as recited in claim 1 wherein the foraminous duct has an effective impedance in the range 3-30 cgs rayls.

14. The device as recited in claim 1 wherein the foraminous duct has an effective impedance in the range 31-100 cgs rayls.

15. The device as recited in claim 1 wherein the foraminous duct has an effective impedance in the range 101-400 cgs rayls.

16. The device as recited in claim 1 wherein the foraminous duct is a laminated screen structure.

17. The device as recited in claim 1 wherein the foraminous duct is a perforated tube.

18. The device of claim 17 wherein the perforated tube is made of metal with metal fiber in the openings.

19. The device as recited in claim 1 wherein the foraminous duct is a metal fiber web structure.

20. The device as recited in claim 19 wherein the metal fiber web structure has the inside surface thereof covered with a layer of fibers having a protective coating.

21. The device as recited in claim 20 wherein the fibers are made of metal.

22. The device as recited in claim 20 wherein the fibers are of organic material.

23. The device as recited in claim 20 wherein the fibers are of ceramic material..

24. The device as recited in claim 20 wherein the protective coating is an oxidation catalyst.

25. The device as recited in claim 1 further including third means, located in series after the second means, for attenuating middle frequency sound entering the device.

26. The device as recited in claim 25 wherein the third means is a Helmholtz resonator.

27. The device as recited in claim 25 wherein the third means is tuned within the range 500 Hz to 1000 Hz.

28. The device as recited in claim 1 further including in series after the second means,

a plurality of quarter-wave standing wave cavities surrounding a portion of the periphery of the foraminous duct;

at least one Helmholtz resonator surrounding the plurality of standing wave cavities and communicating with the foraminous duct by ports located in the conduit at those portions not communicating with the standing wave cavities.

29. The device as recited in claim 1 wherein the foraminous duct comprises two foraminous tubes parallel to each other.

30. The device of claim 1 wherein the second means is a laminar absorber.

31. A method for attenuating the level of sound in a fluid flowing through a muffler, the fluid having a known acoustical impedance measured in rayls, comprising the steps of:

a. providing a high frequency sound attenuator for absorbing frequencies over 1000 Hz, the attenuator having an acoustical resistance in rayls matched to the acoustical impedance of a fluid entering the muffler; and

b. providing a low frequency sound attenuator to absorb sound having a frequency of under 1000 Hz to a level sufficiently low to enable the high frequency attenuator to effectively function at its preselected acoustical resistance.

32. The method as recited in claim 31 wherein the low frequency attenuator is a Helmholz resonator.

33. The method of claim 30 wherein the high frequency attenuator is a foraminous tube.

34. The method of claim 33 wherein the foraminous tube is a mesh of metal fibers.

35. The method of claim 33 wherein the tube is a woven metal fiber tube.

36. The method of claim 35 wherein the woven metal fiber tube has the inside surface thereof covered with a layer of metal fibers coated with an oxidation catalyst.

37. The method as recited in claim 30 wherein the high frequency attenuator includes a quarter wave standing wave cavity.

38. The method of claim 37 wherein the depth a of the standing wave cavity is an odd multiple of onequarter wavelengths of the sound pressure level.