WO1979000043A1

WO1979000043A1 - Pressure variation absorber

Info

Publication number: WO1979000043A1
Application number: PCT/US1978/000042
Authority: WO
Inventors: D Traver; C Anderson; A Abelhamid
Original assignee: Carrier Corp
Priority date: 1977-07-13
Filing date: 1978-07-12
Publication date: 1979-02-08
Also published as: DE2830294B2; BR7804499A; AU3785378A; FR2397546A1; DE2830294A1; AR215323A1; AU530089B2; IT7825675A0; DE2830294C3; IN149314B; CH635899A5; FR2397546B1; JPS627400B2; ZA783845B; GB2001135B; IT1097154B; NL7807463A; NL182239C; MX146552A; JPS5439202A

Abstract

Pressure variation absorbing apparatus (10) is mounted adjacent to a moving fluid stream in the diffuser (14) of a centrifugal compressor for absorbing both acoustic and aerodynamic pressure variations. The absorbing apparatus (10) when mounted as a part of the diffuser wall of a centrifugal compressor not only reduces acoustic noise but also absorbs aerodynamic pressure variations increasing the efficiency of the compressor and simultaneously reducing the rate of flow at which surge occurs thereby enlarging the operational flow range of the compressor. A method of absorbing pressure variations in a moving fluid stream is also disclosed.

Description

PRESSURE VARIATION ABSORBER

TECHNICAL. FIELD OF THE INVENTION

This invention relates to the field of absorb¬ ing acoustic, aerodynamic and the combination of acoustic and aerodynamic pressure variations of fluctuations from a fluid stream. More particularly for absorbing acoustic, aerodynamic and the combination of acoustic and aerodynamic pressure waves from the compressible fluid passing through a diffuser in a centrifugal compressor or other si&ilar machine. ^' BACKGROUND ART

Centrifugal compressors are utilized by the re¬ frigeration industry in most large installations where a single large refrigeration._ machine is used to provide cooling, heating or both. Many methods have been attempted with varying degrees of success to limit the level of loudness of the audible noise emitted by a centrifugal re¬ frigeration machine. These methods have included encasing the motor and compressor (United States Patent No. 3,635,579); providing sound absorptive material at the in- let and outlet chambers-,of the compressor (United States Patent N<3. 3,360,193); locating a baffle in the crossover pipe of a multi-stage compressor (United States Patent No. 3,676,012) and providing an annular muffler in the discharge line of the compressor. Since large refrigeration installations consume high amounts of electrical energy every effort is made to^! increase the efficiency of the refrigeration machine to decrease the operating costs of the installation. The ab¬ sorptive apparatus herein claimed is utilized to obtain an overall efficiency increase in a refrigeration system having a centrifugal compressor.

The operational flow range of a centrifugal .^ REA(7"

OMPI compressor s norma y m e y e m n ma ow vo ume which can be produced without the occurrence of surge.

It is impractical to operate in surge due to pressure pulsations, dynamic and potentially dangerous thrust load variations and increased gas temperatures. When it is desirable to operate a centrifugal compressor under partial load it is necessary to operate the machine at sufficient flow volume to exceed the flow volume at surge notwithstanding that the partial load requirements could be met with a lesser flow rate. When operating at a flow rate which is higher than necessary to meet the load requirements, operating costs increase since the efficiency of the overall system is decreased. Even as surge is approached, aerodynamic instabilities arise introducing losses and lowering efficiency, so that operating costs increase as the surge line is approached. Hence, by decreasing the flow volume at which surge occurs the compressor can operate over a broader flow volume range and operate with a higher efficiency at flow ranges below the previously established surge volume.

Prior efforts to control the volume flow rate at which surge occurs have focused on the diffuser geometry and on providing vanes within, the diffuser to control the flow path of the fluid leaving the impeller. See "Centrifugal .Compressors ... the Cause of the Curve" by

Donald C. Hallock from Air and Gas Engineering, Volume 1, Number 1, January' 1968.

THE DISCLOSURE OF THE INVENTION It is an object of the present invention to re- duce the level of noise emitted from a centrifugal corn- compressor.

It is a further object of the present invention to increase the efficiency of a centrifugal compressor and to increase the operational range of a centrifugal com- pressor by lowering the flow volume at which surge occurs.

It is a further object of the present invention to reduce the level of the noise emitted from a moving flu stream.

It is another object of the present invention to reduce the noise level, increase the overall efficiency, and to increase the operational range and pressure rise of a centrifugal compressor without unduly impeding fluid flow or creating severe boundary layer distortions within the fluid flow path. It is yet another object of the present invention to provide absorbing apparatus which is adaptable to existing centrifugal compressors with a minimum of struc- ^• tural alterations.

It is still a further object of the present in- vention to absorb both acoustic and aerodynamic pressure variations within the fluid in communication with the ab- , sorbing apparatus.

It is also an object of the invention to prevent or delay back pressure and reverse fluid flow by means of acoustical and pressure absorbing material located in the diffusor o "a!'centrifugal compressor.

Other objects will be apparent from the descrip¬ tion to follow and the appended claims.

The above objects are achieved according to a preferred embodiment of the invention by the provision of absorbing apparatus in communication with the fluid being compressed in the diffuser section of a compressor. A porous absorbing material is mounted to form a portion of the wall surface of the diffuser. A resonant cavity is located on the opposite side of the absorbing material from the fluid in such a manner that the fluid may flow through the absorbing material into the cavity. The absorbing apparatus is annular in shape and the cavity is divided by concentric rings into a plurality of smaller cavities of by a single helical divider with periodic dams into a narrow elongated cavity or by a honeycomb or similar divider into a multiplicity of cellular type cavities, Damping material such as fiberglass is inserted into the cavity to further aid in absorbing and damping pressure variations. The absorbing material is selected to have " a flow resistance approximating the density of the fluid times the speed of sound in the fluid through the diffuser.

Fig fugal compres

therein.

Figure 2 is an enlarged partial sectional view of the invention mounted to a portion of the diffuser wall of a centrifugal compressor. Figure 3 is a partial elevational end view taken along line 3-3 of Figure 1 of the invention in a centri¬ fugal compressor showing the cavity divided into a narrow elongated cavity by a single helical divider and showing the location of the absorbing material. Figure 4 is a graph of exit pressure from a centrifugal compressor versus exit volume from a centri¬ fugal compressor shown with and without the claimed pressure variation absorber herein-and with the inlet vane angle of the compressor control vanes set at both 35 degrees and at 90 degrees.

Figure 5 is a graph of flow resistance versus absorption coefficient for air, R-ll, R-12 and R-22.

DESCRIPTION OF THE PREFERRED EMBODIMENT INCLUDING THE BEST MODE OF CARRYING OUT THE INVENTION

The following is a description of absorbing apparatus mounted in communication with the fluid in a compressor to form portion of the diffuser wall of the diffuser within a centrifugal compressor and of a method of absorbing pressure waves within the fluid. I^'t is to be understood that the invention has like applicability to any moving fluid stream whether it be^" in a centrifugal compressor, gas turbine or other dynamic head device, which converts increased dynamic head pressure created by moving blades or the like into increased static pressure. Further¬ more it would be but a design expedient dependent on the design and operating characteristics of the compressor to select which wall of the diffuser or more than one wall of the diffuser upon which to mount this apparatus. If more than one wall is selected then the diffuser on each could be arranged to absorb different frequency pressure waves.

_A multistage compressor could likewise utilize the present invention in one or more of the various compression stages.

It is to be further understood that the des- cription herein will refer to a refrigerant as the fluid _^-»

being compressed in a centrifugal compressor which is part, of an overall refrigeration machine. However, it is to be understood that the present invention will have like applicability to any compressible fluid be it a refrigerant, a gas or any other fluid. Since optimum porosity of the absorbing material is a function of the gas or fluid properties, different gases or fluids will require absorbing material of varying porosity to achieve optimum results. Referring now to the drawings, it can be seen in

Figure 1 that in a typical centrifugal cαrpr-essor refrigerant enters the oorrpressor throug refrigerant inlet 42 then travels' along a refrigerant flow path through the control vanes 44 and into impeller chamber 46. Impeller 16 mounted on shaft 48 and driven by motor 15 then accelerates the refri¬ gerant and discharges the refrigerant into diffuser 14. At the point of discharge, 15, from impeller blades 17, the refrigerant is traveling at a relatively high velocity and is at a relatively low static pressure. The refriger- ant then travels through diffuser 14 to collector 12 from which it is discharged into the remainder of the refriger¬ ation rpachi.ne. The refrigerant leaving the diffusor and entering the collector is traveling at a relatively slow velocity as compared to when it entered the diffuser and is at a relatively high static pressure as compared to the pressure when it entered the diffuser. A pressure varia¬ tion absorber 10, comprising absorbing material 20 and resonant cavity 13, is in communication with the refrig¬ erant passing through diffuser 14. Volute casting 19 is shown in Figure 1 as structurally connecting the collector, the diffuser and the impeller chamber.

Referring now to Figure 2 which is a partial en¬ larged sectional view of the diffuser and the pressure variation absorber, it can be seen that absorbing material 20, a porous high flow resistance sheet of material, is mounted to form a portion of the surface of the diffuser wall. The absorbing material could likewise be mounted on the other diffuser wali_; or on both walls. The absorb¬ ing material is mounted by means of screws 32 and by an adhesive (not shown) to a portion of volute casting 19. On the opposite side of the absorbing material from the fluid is resonant cavity 18 defined by end dividers 23 and a backplate 26. As can be seen from Figure 3 pressur variation absorber 10 is annular in shape and each end divider forms a complete ring so that^'resonant cavity 18 formed by the two end dividers, backplate 26 and the absorbing material 20 is annular in configuration althoug other configurations would be equally acceptable. It can be further seen in Figure 2 that the annular resonant cavity is divided into a series of smaller cavities by dividers 22. Dividers 22 may be a single helix with periodic solid flow barriers 33 as shown in- Figure 3 or ' comprise a series of concentric rings. A honeycomb or cellular type divider would also be satisfactory. No matter what the divider configuration a narrow cavity or series of cavities is provided. The pressure variation absorber is shown mounted within volute cavity 21 formed by various portions of volute 19. This particluar arrang merrt is structural and has no effect on the claimed inven If a narrow plurality of cavities were not pro¬ vided the refrigerant flowing through the diffuser havin a relatively low static pressure at the end of the absorb closest to the impeller and a relatively high static pressure at the end of the absorber closest to the collec would enter the pressure variation absorber closest to the co tor and flewbackwards towards the end of the pressure varia¬ tion absorber closest to the impeller. This backward flo of refrigerant would then detract from the overall effici of the unit. By providing a single helical resonant cavi with periodic flow barriers back flow due to the pressure gradient is .sufficiently small relative to the high flow resistance of the absorbing material that overall machine efficiency is not substantially affected. Back flow can similarily be limited by concentric dividers or a multi- plicity of cellular type cavities so that the incremental pressure drop in each cavity is minimal.

It can be further seen that end dividers 23 and dividers 22 are sealed to prevent fluid flow between the separate cavities. Dividers 22 and end dividers 23 are mounted to absorbing material 20 and to backplate means of an epoxy type resin. The resonant cavity 18 of the pressure variation absorber is further filled with a damping material such as fiberglass to increase the absorbing efficiency of the unit and to provide damping of possible resonating pressure waves within cavity 18. Figure 4 is an experimentally developed graph of head pressure versus flow volume for a centrifugal compressor equipped with and without the herein described invention. The graph shows both operation of the- com- pressor with the pressure variation absorber and without the pressure variation absorber. The dotted line, when the machine^'is operated without the pressure variation absorber, shows that surge occurs at a much higher flow volume than with the present apparatus. Furthermore, the graph shows the respective characteristics with the control vanes set at a 35 degree angle and with the control vanes set at a 90 degree angle. It can be seen from the graph that the operational range between the point when surge occurs and when head pressure is reduced below an operational value is greatly increased, espec¬ ially at the lower flow volumes. In addition, the pressure rise of the compressor is increased particularly for control vane settings below 90 degrees. The method of utilizing this apparatus includes locating the pressure variation absorber within the diffuser so that pressure variations in the fluid within the diffuser are absorbed. These variations include acoustical and aerodynamic waves generated by the impeller as the fluid is accelerated and those waves occurring as a result of surge and other aero- dynamic instabilities such as rotational stall as the fluid is pressurized and decelerated in the diffuser.

The precise mechanism which operates to improve the efficiency of the machine and to reduce the flow volume at which surge occurs is not fully known. It has been discovered that an absorber designed to absorb acoustic waves (which are pressure variations) and there¬ by reduce noise emitted by the machine also acts to ab¬ sorb aerodynamic pressure variations which result from surge and other aerodynamic instabilities affecting the overall efficiency of the machine. It is theorized tha^^TjRE^ r OMPI an absorber acts to restrict pressure variations resultin from either acoustic waves or aerodynamic instabilities. The efficiency improvement results from the elimination or reduction in severity of the aerodynamic instabilities A smooth flow without pressure variations not only result in a machine having a lower flow rate at which surge may occur and thereby having a greater operational range but also adds to the overall fficiency of the unit since the impeller is not forced to overcome these aerodynamic pressure fluctuations that the absorber is removing them from the system.

Random and periodic aerodyn-amic pressure vari- ^' ations of unknown origin have also been detected within a centrifugal compressor. It is experimentally determine that the disclosed absorber also attenuates these vari¬ ations further adding to the efficiency of the overall machine.

The absorbing material 20, such as "Feltmetal" or "Fibermetal" manufacturated by Brunswick Corporation of Muskegon, Michigan or "Rigimesh" manufactured by Air¬ craft Porous Media, Glen Cove, New York, is selected so that its flow resistance approximates the density of the fluid times the- speed of sound in the fluid across the absorbing material. Hence, the absorbing material is varied according to the fluid being used or more particularly according to the particular refrigerant selected for the particular appli¬ cation. The table below shows various refrigerants, the various densities of the refrigerant leaving the impeller the various velocities of the speed of sound in the refrigerant and the consequent optimum flow resistance th absorbing material should have for each application.

(A conversion factor of 0.48823 is used to convert from

English to Metric units.) D Deennssiittyy Speed of Sound Flow

Refrigerant Resistance.

Kσ/m³ M/Sec. Rayls Tegs)

Figure 5 is a graph of the maximum normal absorption coefficient versus flow resistance for Air, R-ll, R-12, and R-22 as measured in an acoustic im¬ pedance tube. This graph is a plot of values which shows that an absorption coefficient of approximately 1.0 is obtainable by selecting the proper flow resis¬ tance for the absorbing material. The graph confirms that material having the values set forth in the table is the optimum choice to absorb pressure variations for the particular refrigerant.

The resonant cavity backing the absorbing material is designed so that its depth is-one quarter the wave length of the wave length of the lowest fre¬ quency of sound that it is desired to absorb. For example, if R-ll (trichloro-fluoromethane) is the refrigerant being used in the machine and the pressure variation absorber is designed to eliminate acoustical noise at 300 Hertz and above, then, the cavity depth should be 12.7 centimeters; the velocity of the speed of --Sound of R-ll divided by four times the frequency.

Damping material is selected for the resonant cavity so that all frequencies greater than the fre¬ quency for which the cavity is designed will be absorbed or attenuated. The damping material helps to absorb the frequencies between the resonance peaks of the design frequency thereby providing an absorber which will ab¬ sorb all frequencies from the minimum frequency increasing to the highest audible frequencies and beyond.

INDUSTRIAL APPLICABILITY OF THE INVENTION It can be seen from the above described embodiment that there has been provided an acoustic and aerodynamic pressure variation absorber which has the capability of not only absorbing acoustic waves and thereby reducing the noise level emitted by a machine (e.g. a centrifugal compressor) and/or the fluid passing therethrough but also to absorb aerodynamic pressure variations so that the efficiency of the machine is increased and the overall operational range of the machine is broadened.

Claims

C L A I. M S

1. A diffuser (14) for use with compressible fluids in a centrifugal compressor which is character¬ ized by a plurality of walls having a fluid flow path therebetween, one end of said path receiving the com¬ pressible fluid at relatively high speed and low static pressure and the other end of said path discharging the fluid at relatively low speed and high static pressure; and a pressure variation absorber (10) being mounted - in direct acoustic and aerodynamic communication with the fluid in the diffuser for absorbing both acoustic and aerodynamic pressure variations.

2. The invention as set forth in claim 1, wherein the pressure variation absorber comprises a porous absor¬ bing material (20) and a resonant cavity (18) , said absorbing material being in communication with both the fluid in the diffuser and the fluid in the cavity.

3. The invention as set forth in claim 2, wherein the absorbing material is mounted to form a portion of the walls having the fluid flowing therebetween.

4. The invention as set forth in claim 3, wherein the cavity is at least partially filled with damping material.

5. The invention as set forth in claim 4, wherein the resonant cavity is divided into a plurality of separate cavities each communicating with the absorbing material.

6. . The invention as set forth in claim 1, wherein the absorbing material has a flow resistance approxima¬ ting the product of the fluid density times the speed of sound in the fluid in the diffuser. 1 , In a centrifugal compressor or other dynamic head device for supplying a compressible fluid under pressure and having a housing containing a fluid path including an inlet port (42) and an impeller chamber

(46); an impeller (26) rotatably mounted within the impeller chamber so that fluid enters into the impeller chamber and is accelerated by the impeller; a diffuser

(14) which receives the accelerated fluid from the _^-~. impeller; and a collector (12) for discharging fluid f _O

Vtyxm, under pressure received from the diffuser, the improve¬ ment characterized by a pressure variation absorber (10) being mounted in direct aerodynamic and acoustic communication with the fluid in the diffuser for absorbing both acoustic and aerodynamic pressure vari¬ ations.

8. The invention as set forth in claim 7, wherein the pressure variation absorber comprises a porous absorbing material (20) and a resonant cavity. (18) , said absorbing material being in communication with both the fluid in the diffuser and the fluid in the cavity.

9. The invention as set forth in claim 8, wherein the absorbing material is mounted to form a portion of the diffuser wall surface.

10. The invention as set forth in claim 9, wherein the resonant cavity is at least partially filled with an acoustic and aerodynamic damping material.

11. The invention set forth in claim 10, wherein the resonant cavity is divided into a plurality of separate cavities each communicating with the absorbing material.

12. The invention as set -forth in claim 7, wherein the absorbing material has a flow resistance approximating the product of -the fluid density times the speed of sound in the fluid in the diffuser-

13. The invention as set forth in claim 10, wherein the resonant cavity is divided by a helical plate into a single helical cavity.

14. Apparatus for reducing aerodynamic and acoustic pressure variations within a moving stream of compressible fluid which is characterized by walls formed to provide a flow path for a stream of fluid, at least a portion of a wall of the flow path being constructed of porous material (10) permeable to the fluid in the flow path; and a resonant cavity (18) located adjacent the porous material on the side of the porous material away from the flow path, said porous material being capable of allowing fluid flow between the flow path and the resonant cavity.

15. The apparatus as set forth in claim 14, wherein the flow resistance of the

the product of the fluid density and the velocity of sound in .the fluid.

16. The apparatus as set forth in claim 15, wherein the depth of the resonant cavity is equal to one quarter of the wavelength of the lowest design frequency of pressure variations for which a coefficient of absorption of one is desired.

17. The apparatus of claim 14, wherein the resonant cavity includes means (22) to divide the cavity into a series of smaller cavities to reduce the fluid flow within the resonant cavity resulting from pressure variations within the fluid stream moving along the flow path.

18. The apparatus of claim 17, wherein the series of smaller cavities is at least partially filled with pressure variation damping material to further feduce pressure variations within the fluid stream.

19. A method for increasing the efficiency of a centrifugal compressor while reducing the audible noise emitted from the compressor and for providing a greater operational flow range of the compressor by reducing the flow volume at which surge occurs which is character¬ ized by the steps of accelerating a compressible fluid with an impeller (16); receiving the accelerated fluid in a diffuser (14) wherein the fluid is slowed to increase its static pressure; collecting the fluid in a collector (12) at high pressure; and absorbing both aerodynamic pressure variations and acoustic pressure variations within the diffuser.

20. The invention as set forth in claim 19, wherein the step of absorbing pressure variations in¬ cludes locating a porous absorbing material (20) in communication with the fluid; and providing a resonant cavity (18) in communication with the absorbing material whereby fluid flows between the cavity and the diffuser through the absorbing material.

21. The invention as set forth in claim 20, and further including the step of damping the aerodynamic and acoustic pressure variations within the cavity.

22. The invention as set forth in claim 21, and further including the step of dividing the resonant cavity to provide a plurality of smaller cavities each in communication with the absorbing material.

23. The invention as set forth in claim 22, wherein the step of locating a porous absorbing material includes the step of selecting a porous material so that its flow resistance approximates the density of the fluid multi¬ plied by the speed of sound in the fluid.

24. A method for increasing the flow efficiency of a compressible fluid stream while reducing the audible noise emitted from the fluid and for providing a greater operational flow range by reducing acoustic aerodynamic pressure variations which is characterized by the steps of selecting an aerodynamic and acoustic pressure vari- ation absorbing material (20) having a flow resistance equal to the density of the fluid times the speed of sound of the fluid; locating the absorbing material in communi¬ cation with the fluid; and providing a resonant cavity (18) in communication with the absorbing material so that fluid may flow through the absorbing material into the cavity.

25. The invention as set forth in claim 24 and further including the step of damping the aerodynamic and acoustic pressure variations within the cavity.

^• 26. A method for increasing the flow efficiency of .a compressible fluid stream by reducing acoustic and aerodynamic pressure variations within the stream which is characterized by the steps of channelling the fluid stream along a flow path; interrupting fluid flow along the flow path by mounting a resonant cavity (18) in direct communication therewith so that fluid may flow between the cavity and the flow path; and limiting the amount of fluid flow between the cavity and the flow path by locating a porous absorbing material (20) between the cavity and the flow path whereby pressure variations within the fluid stream are absorbed without significantly reducing the fluid flow along the flow path.

27. The method of claim 26, wherein the step of limiting the fluid flow includes selecting an absorbing material

product of the fluid density times the velocity of sand in the fluid.

28. The method of claim 26, and further including the step of damping pressure variations within the resonant cavity by at least partially filling said cavity with a damping material so that the pressure vari¬ ations in the fluid stream are further reduced.

I_JURE

OMPI AMENDED CLAIMS

(received by the International Bureau on 12 July 1978 (12.07.78))

1. In a centrifugal compressor or other dynamic head, device for supplying a compressible fluid under pressure and having a housing containing a fluid path including an inlet port (42) and an impeller chamber (46); an impeller (26) rotatably mounted within the impeller chamber so that fluid enters into the impeller chamber and is accelerated by ' the impeller; a diffuser (14) which receives the accelerated fluid from the impeller; and a collector (12) for discharg¬ ing fluid under pressure received from the diffuser, the im¬ provement characterized by a pressure variation absorber

(10) being mounted in direct aerodynamic and acoustic com¬ munication with the fluid in the diffuser for absorbing both acoustic and aerodynamic pressure variations.

2. The invention as set forth in claim 1, wherein the pressure variation absorber comprises a porous absorbing material (20) and a resonant cavity (18) , said absorbing material being in communication wit both the fluid in the diffuser and the fluid in the cavity.

3. The invention as set forth in claim 2, wherein the absorbing material is mounted to form a portion of the dif¬ fuser wall surface.

4. The invention as set forth in claim 3, wherein the resonant cavity is at least partially filled with an acoustic and aerodynamic damping material.

5. The invention set forth in claim 4, wherein the resonant cavity is divided into a plurality of separate cavities each communicating with the absorbing material.

6. The invention as set forth in claim 1, wherein the absorbing material has a flow resistance approximating the product of the fluid density times the speed of sound in the fluid in the diffuser.

7. The invention as set forth in claim 4, wherein the resonant cavity is divided by a helical plate into a single helical cavity. - _ _^yQ^~ 8. A method for increasing the efficiency of a centrifugal compressor while reducing the audible noise emitted from the compressor and for providing a greater operational flow range of the compressor by reducing the flow volume at which surge occurs which is characterized by the steps of accelerating a compressible fluid with an im¬ peller (16); receiving the accelerated fluid in a diffuser (14) wherein the fluid is slowed to increase its static pressure; collecting the fluid in a collector (12) at high pressure; and absorbing both aerodynamic pressure variations and acoustic pressure variations within the diffuser.

9. The invention as set forth in claim 8, wherein the step of absorbing pressure variations includes locating a porous absorbing material (20) in communication with the fluid; and providing a resonant cavity (18) in communication with the absorbing material whereby fluid flows between the cavity and the diffuser through the absorbing material.

10. The invention as set forth in claim 9, and further including the step of damping the aerodynamic and acoustic pressure variations within the cavity.

11. The invention as set forth in claim 10, and further including the step of dividing the resonant cavity to provide a plurality of smaller cavities each in communication with the absorbing material.

12. The invention as set forth in claim 11, wherein the step of locating a porous absorbing material includes the step of selecting a porous material so that its flow re¬ sistance approximates the density of the fluid multiplied by the speed of sound in the fluid.

-^RE

O P STATEMENTUNDERARTICLE19

1. Enclosed is a placement sheet for pages 10, 11, 12, and 13 of the International Application. Page 14 of the original application is' hereby cancelled. The amend¬ ment involves the^* cancellation of originally filed claims 1 to 6, 14 to 18 and 24 to 28 together with the renumbering of original claims 7 to 13 and 19 to 23 and the associated changes in the dependences- set forth in these claims. The language of the claims maintained in the application has not changed. .

2. The amendments have been made to reduce the number of independent claims in the application to those independent claims, together with their dependent claims, which most clearly expressed the inventive advance provided by the present invention.