US3712412A

US3712412A - Sound suppressing system

Info

Publication number: US3712412A
Application number: US00199879A
Authority: US
Inventors: J Hassett; W Ihde
Original assignee: Environeering Inc
Current assignee: Environeering Inc; US Filter Systems Inc
Priority date: 1971-11-18
Filing date: 1971-11-18
Publication date: 1973-01-23
Anticipated expiration: 1990-01-23

Abstract

A system for suppressing sound in a high velocity fluid stream comprising conduit means extending downstream of the source of said sound, said conduit means having a cross-sectional flow area with a transverse dimension less than one half the wave length of the sound in said fluid, and a tunable, acoustic resonator having an inlet in communication with said conduit means downstream of said source and a closed, adjustable outer end, said resonator angularly intersecting the conduit means and reflecting sound waves received through the inlet back into said conduit means to attenuate succeeding sound waves therein.

Description

United States Patent 1191 Hassett et al.

[54] SOUND SUPPRESSING SYSTEM [75] Inventors: James W. Hassett, Oak Park; William M. Ihde, La Grange, both of 111. [73] Assignee: Environeering, Inc., Skokie, I11.

[22] Filed: Nov. 18, 1971 [21] Appl. No.: 199,879

[52] U.S. C1. ..l8l/33 D,181/59, 181/64 A [51] Int. Cl. ..F01n l/02, FOln 1/16 [58] Field of Search..181/33 D, 42, 50, 48, 59, 64 A,

[56] References Cited UNITED STATES PATENTS 1,760,553 6/1930 Hewitt et al. ..181/64 A 2,027,359 1/1936 Wood et al. ....18l/64 A 2,308,886 1/1943 Mason ..181/48 2,773,553 12/1956 Henrich et al.... ....l81/42 2,990,906 7/1961 Audette ....l81/50 3,113,634 12/1963 Watters ....18l/50 3,353,626 11/1967 Cremer et al ..l81/42 1 1 Jan. 23, 1973 FOREIGN PATENTS OR APPLlCATlONS Primary Examiner-Robert S. Ward, .lr. Att0rney-Richard D. Mason et al.

[57] ABSTRACT A system for suppressing sound in a high velocity fluid stream comprising conduit means extending downstream of the source of said sound, said conduit means having a cross-sectional flow area with a transverse dimension less than one half the wave length of the sound in said fluid, and a tunable, acoustic resonator having an inlet in communication with said conduit means downstream of said source and a closed, adjustable outer end, said resonator angularly intersecting the conduit means and reflecting sound waves received through the inlet back into said conduit means to attenuate succeeding sound waves therein.

11 Claims, 6 Drawing Figures PATENTED I973 3.712.412

sum 1 or 2 INVENTOREI JAMES W HASSETT WILLIAM M. IHDE ATT'YS PATENTEDmzs I975 3.712.412

SHEET 2 OF 2 EEK FIG.5

INVENTORS. JAMES W HASS ETT WILLIAM M IHDE ATT'YS SOUND SUPPRESSING SYSTEM The present invention is directed towards a new and improved system for 'suppressing sound or noise generated from sources such as high-speed fans and the like used for moving fluid at high velocity through an exhaust stack or duct. 1

In pollution control systems wherein high efiiciency, wet-type scrubbers are utilized, high speed, high pressure fans or-blowers are generally used for moving the gas through the scrubber system. These fans create or generate considerable noise or high sound levels, which noise is often objectionable to persons in the surrounding area. Because of the size and power involved this noise or sound is relatively difficult to diffuse, or attenuate by conventional methods. The objectionable noise or sound produced by high speed fans usually resembles the sound emitted from a low-pitched siren, and has a frequency or pitch which is dependent upon the tip speed or velocity of the impeller or rotor of the fan. The sound frequency is therefore readily calculated once a fan size and speed have been chosen for a particular installation. In addition, the design parameters for the output velocity of the fan, the operating pressure, and the temperature of the fluid being moved are set up and, accordingly, the density of the flowing fluid can be calculated so that the velocity of sound in the fluid can be readily determined.

The present invention provides a system for suppressing or greatly diminishing objectionable noiseor sound generated fromcommon sources such as high speed fans and the like used for moving fluids at high velocity.

It is therefore a general object of the present invention to provide a new and improved system for suppressing noise. or sound in high velocity fluid streams.

Another object of the invention is to provide a new and improved system for suppressing or reducing sound levels in high velocity fluid streams, which system is tunable or adjustable to resonate through a frequency range so that variations in frequency of the sound being generated can be handled; without material alterations of the system being required.

Another object of the present invention is to provide a sound suppressing system of the character described which is relatively low in cost, relatively small in bulk volume of space occupied yet extremely efficient in the reduction of noise or sound levels in a fluid stream.

Another object of the present invention is to provide a new and improved noise or sound suppression system capable of effectively suppressing the noise level of a high volume gas flow in an exhaust stack, duct or the like.

Another object of the present invention is to provide a new and improved noise or sound suppression system of the character described employing one or more tunable acoustic resonators of relatively small size, yet capable of operating with relatively high volume flow rates in the system.

Still another objectof the present invention is to provide a new and improved noise or sound suppression system of the character described wherein a pair of tunable sound resonators are coupled to effectively suppress unwanted sound or noise levels to a lower level.

Another object of the present invention is to provide a new and improved noise or sound suppression system for use with fluid flowing at high velocity employing means dividing the fluid flow into part streams and attenuating sound in said part streams separately of one another.

The foregoing and other objects and advantages of the present invention are accomplished in a new and improved system for suppressing sound or noise in high velocity fluid streams comprising conduit means extending downstream of a sound source. Said conduit is divided longitudinally if required to provide several separate flow areas, each with a transverse dimension less than one half the wave length of the sound to be suppressed. One or more tunable acoustic resonators, each having an open inlet in communication with the conduit means downstream of the sound source are mounted on the conduit means extended transversely outward thereof. The tunable acoustic resonators have adjustably mounted, closed outer end walls for reflecting sound waves received through the inlet back into the fluid stream flowing in the conduit means for attenuating or suppressing succeeding sound waves coming from the sound source.

Fora better understanding of the present invention, reference should be had to the following detailed description taken inconjunction with the drawings, in which:

FIG. I is a side elevational view of a new and improved sound or noise suppression system'constructed in accordance with the features of the present invention;

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

FIG. 3 is an enlarged side elevational view with a portion broken away showing intemal-details of the system of FIG. 1;

FIG. 4 is an enlarged side elevational view taken substantially along line 4-4 of FIG. 3;

FIG. 5 is a side elevational view of another embodiment of a sound suppression system constructed in accordance with the features of the present invention; and

FIG. 6 is a horizontal, transverse, cross-sectional view taken substantially along line 6-6 of FIG. 5.

Referring more particularly to the drawings and the embodiment of FIGS. 1-4, therein is illustrated a new and improved system for suppressing unwanted noise or sound constructed in accordance with the features of the present invention and indicated generally by the reference numeral I0. The system is adapted to be used with noise or sound generating sources such as high speed, high pressure, centrifugal fans, for example, a blower or fan 12 of the type commonly used in connection with pollution control systems. The fan 12 is provided for moving industrial gases and the like through a wet scrubber system or other pollution control devices and is capable of handling a relatively high volume flow rate of gas at relatively high static pressures which are required for the efficient operation of a venturi-type or orifice-type wet scrubber. The diameter of the fan rotor, the spacing of the vanes on the rotor, and the speed of rotation determine the frequency of the sound that is generated by the fan. It is believed that the sound generation occurs as the tips of the rotor vanes pass a throat portion 12a of the fan casing which is defined adjacent the inner end. of the outlet boot 14 on the fan scroll. 1 i

The pressure and temperature conditions of the gases or fluidleaving the fan outlet boot 14 are readily measured or calculated in advance and from this data the density of the flowing fluid and the speed of sound through the fluid can be determined alongwith the wave length of the sound vibrations at the particular frequency generated by the fan.

In accordance with the present invention, the outlet boot 14 of the fan 12 is connected to an upright stack I or exhaust duct system generally indicated as 16, and

for this purpose a feeder duct 18 and transition section I The stack system 16 includes a plurality of stack sec:

tions 28 interconnected in end-to-end relation by collars 30 and the lowest or first is provided with a suitable clean-out opening having a removable cover or door 32 for sealing the clean-out opening during normal operation.

In accordance with the present invention, adjacent the level of the work platform 26, the sound suppression system includes a sound attenuator unit generally indicated by the reference numeral 40, which unit constructed in accordance with the features of the present invention is especially adapted to suppress to an acceptable lower level the sound or noise that is generated by the fan 12. The attenuator unit includes a cylindrical body 42 having substantially the same internal diameter as the stack sections 28 and connected at its upper and lower ends to the adjacent stack sections by means of the cylindrical connecting bands 30, as shownin FIG. 3.

' In order to prevent excessive vibrations of the body 42, as the gas flows therethrough at high velocity, several annular outwardly extending stiffening rings 44 are provided around the outside surface of the body at levels adjacent the upper and lower ends and at an intermediate level. Longitudinal stiffening ribs 46 are connected between the rings 44 to provide further strength. The transverse dimension of the stack sections 28 and the body 42 of the sound attenuator 40 are designed to be substantiallysmaller than the'calculated wave length of the sound or noise introduced into the stack. The noise or sound of the fan which is to be suppressed is introduced intothe stack system by the fluid at theopening 22 therein, indicated as level "E"- in FIG. 1 and for best results the level E" is at least two stack diameters below the body 42 of the. attenuator unit 40. In many high volume installations limiting of the stack diameter'to avalue less than'one wave length would greatly increase the friction loss and the power required to move thegasesthrough the system, so in these instances and as shown in thesystem 10 of FIGS. 1 4, the diameter"D of transverse dimension of the stack system l6 and the attenuator'body 42'may closely approach or even exceed the calculated wave length (indicated as W")' of the sound generated by the fan which is to beattenuated ,or suppressed in the system 10. In this case, the cross-sectional flow area of the body 42 and the stack system is divided by one or more longitudinallyextending planar divider walls 48, which walls effectively divide, the totalcross sectional flow area of the attenuator body into two or more separate sections (A and A, in FIG. 2). Each of the sections contains a fractional part or portion of the total flow through the system and reduces the transverse dimension of the flow containing conduit section for each part flow-to less than one half the sound wave length. It should also be noted that the longitudinal divider wall 48' extends above and below the respective ends of the sound attenuator body 42 by distances greater than one stack diameter, and accordingly, the attenuator 40 thus receives two or more completely, separate, high velocity part streams for attenuation. Because the internal diameter of the attenuator body 42 is the same as the stack sections 28, the divider wall 48 fits nicely into the adjacent stack sections. Z

The sound level in each of the part streams flowing in the divided flow areas A and A of the attenuatorbody 42 is suppressed by means of one or more tunable acoustic resonators generally indicated by the reference numeral 50. Each acoustical resonator 50 comprises a hollow cylindrical body or tubular housing 52 connected at its innerend to the body 42 and extending transversely outwardly thereof at right angles to the direction of flow in the body and perpendicular to the divider wall 48. The cylindrical resonator housings 52 are dimensioned with a diameter dsuch that the cross-sectional area of each resonator housing is approximately equal to the area of the adjacent flow section A, or A in the attenuator body 42. The resonator housings 52 are open at the inlet end in direct com munication with the interior of the body 42 which is provided with longitudinally spaced, vertically aligned resonator ports 42a Pairs of vertically spaced ports are provided on opposite sides of the longitudinal divider wall 48 for each part stream area A, and A in the body. 42. As shown in FIG. 2, the longitudinal axis of each resonator housing 52 is arranged to lie in a vertical plane perpendicular to and bisecting the center of the vertical, longitudinal divider wall 48. Accordingly, the spacing or distance between the inlet end of resonator port 42a for each resonating chamber and the reflective divider wall 48 is a maximum of the stack diameter over 2 or D/2 which isapproximately the radius'of the attenuator body and less than one half a wave length W. l As illustrated in FIGS. 1 and 3, the vertically aligned and spaced apart pairs of. resonator units 50 for each part stream are spaced apart by a vertical distance approximately. equal to one wave length W" "of the sound as it travels in the, high velocity fluidflowing through the system.

In order to reduce mechanical vibration in the attenuatorunit40, a plurality of stiffening gussets 54 are provided to structurally interconnect the housings 52 of the tunable acoustic resonators and the main body 42 of the ,sound attenuator. Each tunable resonator 50 includes a closed outer end wall 56 of circular shape having a pistonlike peripheral band 58 around the outer'periphery thereof slidably disposed in thecylindrical housing 52. The clearance distance between the outside diameter of the end wall bands 58 and the inside surface of the resonator housings 52 is close enough so that heavy grease may be utilized between these two surfaces to provide a substantially airtight seal yet the clearance is large enough to permit sliding translation of the outer end walls 56 toward and away from the divider wall 48 for fine tuning of the resonators.

in this manner, the sound effective length of each resonator housing 52 is tuned or adjusted to the exact frequency of the sound or noise being produced by the fan or other sound source. The sound waves entering the resonators are reflected back toward the flow in the body 42 by the closed outer end walls 56 of the resonator and the spacing is set up to provide an exact phase relationship with respect to succeeding sound waves traveling up the stack so that cancellation or attenuation of the succeeding waves occurs, thereby greatly reducing the noise or sound level of the gas leaving the upper end of the stack system.

For the purpose of tuning the resonators 50 by adjusting the position ofthe outer end walls 56 in their respective housings $2, threaded adjusting rods 60 are provided with the inner end of the rod secured to the center of the outer end wall by means of nuts 62 or other fastening means. The rods are coaxially aligned with the housings 52 and extend outwardly to pass through a center opening in a horizontal bracket 64 extended between angle clips 66 on opposite sides of each resonator housing 52, as best shown in FIGS. 2 and 3. After adjusting the position of the end walls 56 to obtain a minimum sound level, locknuts 68 on opposite sides of the support brackets 64 are tightened to hold the tuned position. After the system has been fabricated and erected on sight and the fan 12 or other sound source is in operation, the outer end wall 56 of each acoustic resonator 50 is adjusted by means of the tuning rod 60 until the sound level or noise level, is reduced to a minimum. The locknuts 68 are then secured or tightened to hold the position of the tuned outer end walls 56 in their respective resonator or branch housings 52.

The sound suppression system 10 is designed for a particular frequency and wave length range and the diameter of the stack 16 and body 42 of the sound attenuator unit 40 is selected so that the sound effective transverse dimension is less than a half wave length for the sound source which is to be attenuated or suppressed. The stackheight is selected so that the sound attenuator unit-40 is positioned at a level not less than two stack diameters above the level of sound input to the system, as indicated by the level E" in FIG. 1 at the opening 22 in the stack system 16. 1f the volume flow rate of gas to be handled is such that in order to avoid excessive friction losses the stack diameter should be relatively large and approach or exceed the calculated wave length of the sound, then the larger diameter stack is dividedby one or more longitudinal divider walls such as the wall 48, into separate flow paths as depicted by the'areas A, and A Eachof the flow paths has a sound effective transversedimension (between the resonator port 42aand divider wall 48) less than half the calculated wave length of the sound to be attenuated. Oneor more vertically spaced resonating units 50 tunable through a design frequency range are connected to the body 42 of the attenuator in the manner described to provide an effective acoustic length for reflecting sound waves back into the primary,.vertical flow in the stack in selected phase relation for suppression or attenuation of the successive sound waves, thereby reducing the sound total intensity downstream of the attenuator unit to a much lower level. The cross-sectional area of each tunable resonator is substantially equal to the flow area of the divided segment in the body 42 in communication therewith.

In theory, the sound suppression system 10 comprises a distributed acoustic impedance system. When the dimensions of the stack or other elements in an acoustic suppression system are relatively large in comparison to the actual wave length of the sound to be attenuated, it is necessary from a theoretical standpoint to treat the system as having distributed acoustic impedances, rather than lumping all of the impedances together and treating them as one constant. The system is considered in theory as one in which plain sound waves are propagated in the flowing fluid through a relatively long stack, duct or pipe in a positive upward or x direction and the ratio of the acoustic pressure to particle velocity is given by the characteristic impedance p c of the fluid medium. The acoustic impedance of any cross section S in the pipe is Z p/U p/Su p o/S where p acoustic pressure; U volume velocity; S cross sectional area; u particle velocity; p, density of medium; 0 velocity of sound in the medium; and Z acoustic impedance.

In theory of operation, when acoustic waves are moving through a rigid stack or conduit of theoretically infinite length (stack system 16), the presence of a side branch resonator (resonators 50) causes the acoustic impedance at the junction of the conduit and side branch to differ from p o/S, which is the characteristic value for'plane waves in a pipe without a side branch. This impedance change causes a reflected wave to be produced. Moreover, a portion of the incident acoustic energy in the fluid is transmitted into and usually dissipated in the side branch. Both of these factors cause a reduction in the sound energy transmitted beyond the branch filler or resonator 50. If a pipe or stack system 16 of uniformcross section 8" with a diameter D is provided with a side branch resonator 50 having an input acoustic impedance Z, and plane waves of sound represented by the equation P, A, e""""" are generated or introduced downstream of the junction between the body 42 and the branch or resonator 50, a reflected wave having the equation P, B, e"'*"" is created. If the junction point is chosen as the origin of a lateral or y" coordinate, the acoustic pressures produced in the side branch, the incoming main flow and the total wave leaving the branch are set up by the equations P, A, e P, B, e and P, A, c respectively. The pressure at the entrance to the branch may be similarly represented by P, A, e

On the assumption that the cross section dimension of the stack system is small in comparison with the calculated wave length of the sound to be suppressed, the condition of continuity of pressure is applied at the junction of the stack and resonator obtaining the equation P, P, P, P,,. The associated volume velocities in this region are represented by the equation U, P,/p

dition ofcontinuity'of-volume velocity requiresthat U U,' =-U, U bDividing this equation by the'pressure at the junction, we get the equation U, U,)/(P P,)

acousticimpedanceatthe branch due to the combined.-

effect oftheincident and reflected -waves, while 2; is the acoustic impedancein theltransmitted wave. j I 1 When the above equation'is solved for the ratio Bi/A which is the ratio of the pressure amplitude of the reflected waves to that of the-incident waves, we get the followingresult:

The transmission coefficient can then be calculated The sound power transmitted past the junction of the branches or resonators 50 and up thestack system to the outlet is zero onlywhen a, O, andthis requires that both R, and'X equal zero.-- 1 l v To determine the side branch impedance of thetunable' acoustic .chambers 50, with the housing52 closed by a rigid end wall 56, the impedance Z, O,

.P,=,Bei-"" l"" =BlA=l H Y Q As indicated by the above equations, thevpressure amplitude-of the reflected wave, is equal tothat of the incident wave, and at the end wall the incident and reflected pressures are'always in phase. On the otherhand, the volume currents are always 180 out of phase at this. position, so thatth'e, end wall corresponds to a nodal position of volume velocity, The particle velocity is analogous to current in, the electrical sense and the pressures .are analogous, to voltage. Moving an odd number of half wave lengths away from the rigidend walls'56, the volume'velocity,is almaximum and the pressure becomeszero'. Thisis thecondition of zero im-. pedance necessary. for Z, 0. (Z, R +jX 0) in theory the foregoing is a situation wherein the diameteror'transversedimension of the stack system is less than half the wavelength of the sound. However, when the diameter of the stacltapproaches a half wave length, the transmission coefficientv a, approaches 1, and the effectiveness of the closed side branch is lost. It is therefore necessaryto alter the stackby reducing the sound'effective transverse dimensiontoless thanhalf a wavelength. 1 e 1 The'present-invention provides a system whereindepending on the volume throughout, the stack systemis divided into two or-more separatesegments or part streams'by planarlongitudinal divider. members. The,

separateflowsections thus formed may then have side branchresonatorsSO of smaller dimensions..when the divider walls are spaced less than. half a wavelength from the inletopenings of the side branchresonators,

thevcoefflcient of sound transmission downstream beyond the resonators is greatly reducedso thatthe sound level is appreciably diminished or attenuated, I

The foregoing is believed to generally explainv the theory-ofoperation of the sound suppression system It) ofthe presentinvention.

Referring .nowlQtoFlG. S, .therein. is illustrated another embodimentof av sound or noise suppression systemconstructed in accordance ,with the'featurcsof the present invention and referredto generally bythe reference number .In theory of operation the system 110 is similarto the previouslydescribed system 10 and employs an elongated stack system ll6 formed of separatestacksections 128 of adiameter D" which may be equal to or considerably,larger than the wavelength ,Wiof the sound which is introduced into the stack system at alevel several diameters below a sound attenuator unit l40from asource such as a high, speed fan (not shown). In accordance with the present invention, the attenuator, unit includes acylindrical body l42-divided longitudinally intoa plurality of separate flow segments A A A andA by longitu dinaldivider-walls 148 which are arranged to angularly intersectone another asshown in FIG. 6. The totalflow area in the stack 1 16 and attenuatorbody 142 is divided into a plurality of separateflow segments, each of.

which .is provided. with a pair of vertically aligned and spaced apart, tunable acoustic resonators 50, which are similar tothe. resonators 50 previously described. The

cross-sectional area of each resonator, housing l 52 is substantially equal to the flow area of the respective part-streams A A A or- A which the resonator r As illustrated in FIG 5, each pair of tunable acoustic resonators 15f) are spaced apart vertically by a center line distance substantially equal to one wavelength W of the sound to be attenuated andeach, of the branchresonator housings 152 is open at its inner end indirect communication with a resonator .wall 142a port provided in the wall of the attenuatorbody. 1:42. The outer end of each resonator housing isiclosed off by a rigidsound reflectiveouter end wall 156 which is adjustably positioned for tuning to a particular frequency inorder to provide for the desired sound attenuation effect as previous ly described. 1

As illustratedinFlG. 6,-it will be seen that the longitudinal divider plates 148 intersectone another at the axial center of the stack system 116 and thus each pair form adjacentradial divider wall surfaces from a V- shaped sound reflectingtrough facing directly opposite the ports 142a formed in the cylindrical attenuator housing 142..The sound effective distance between the reflecting trough surfaces andthe opposing ports 142a isless than one-half a wavelength as in the prior em} bodiment l0 which utilizes a single planar dividing wall 48. Each port 142a is incommunication withthe inlet end of one of the tunable aeousticresonator units 150 which forms a closed end branch with respect to the main flow. By moving and tunably'adj'usting the outer end wall 156 in each branch resonator lsllrthe desired phase relationship between the reflected sound waves developed in the branch resonators and the oncoming waves moving up the stack system 116 are established, and the sound or noise level is attenuated as previously explained.

By division of the relatively large diameter stack system 116 into a plurality of separate flow sections A,, A A A etc., with the longitudinal divider walls 148, the transverse sound effective dimension of each of the separate flow sections is reduced to a relatively small ratio or value in comparison to the wavelength "W" of the sound even though the stack diameter is large. Extremely effective sound attenuation or cancelling action occurs in the system 110 even though the dimensions of the system are large in comparison to the wavelength W of the sound to be attenuated.

As the present invention has been described by reference to several embodiments thereof, it will be apparent that numerous other modifications and embodiments will be devised by those skilled in the art which will fall within the true spirit and scope of the present invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

l. A system for suppressing sound generated at a source in a high velocity fluid stream comprising conduit means extending downstream of said source, said conduit means having a cross-sectional flow area with a transverse dimension less than one half the wavelength of the sound in said fluid, and a tunable acoustic resonator having an inlet in communication with said conduit means downstream of said source and angularly intersecting the same for reflecting waves received through the inlet back into said conduit means to attenuate the succeeding sound waves therein.

2. The system for suppressing sound as set forth in claim 1 wherein said tunable acoustic resonator comprises a tubular sleeve with an open inlet end connected to said conduit means, and an outer end wall mounted for sliding movement in said sleeve toward and away from said open end, and means for adjustably positioning said outer end wall in said sleeve.

3. The system of claim 1 wherein said conduit means comprises a tubular stack having a maximum transverse dimension of approximately one wavelength of said sound in said fluid and longitudinal divider means in said stack extended upstream and downstream of said inlet of said resonator and providing a reflective surface opposite said opening spaced less than a half wavelength therefrom in a direction transversely of said stack.

4. The system of claim 3 including another of said tunable acoustic resonators having its inlet facing an opposite side of said divider means.

5. The system of claim 4 wherein said longitudinal divider means includes a plurality of divider walls, pairs of said divider walls forming divergent reflective surfaces, and a plurality of said resonators mounted on said stack, each resonator with its inlet end positioned to face one pair of said divergent reflective surfaces.

6. The system of claim 1 wherein said inlet of said tunable resonator is positioned downstream of said source in said conduit means by a distance at least eq l71al to double the value of said majordime nsion.

The system of claim 4 wherein said acoustic resonators are dimensioned to have a transverse crosssectional area of approximately the same value as the adjacent cross-sectional flow areas in said stack as divided by said longitudinal divider means.

8. The system of claim 1 including in combination another of said tunable acoustic resonators mounted downstream of said first mentioned resonator on said conduit means.

9. The system of claim 2 wherein said sleeve comprises a cylindrical tube open at said outer end; said outer end wall comprising a circular disk with a cylindrical rim around the periphery thereof disposed in slidingly sealed concentric relation within said tube; said adjusting means comprising a bracket diametrically extended across said open outer end of said tube and longitudinal adjustment means coaxially aligned in said tube interconnecting said bracket and said circular disk.

10. The system of claim 9 wherein said resonator sleeve extends outwardly of said conduit at right angles thereto and longitudinal divider means in said conduit having a surface facing the inlet end of said sleeve and normal to the longitudinal axis thereof, said surface spaced from said inlet end of said sleeve by a distance less than one-half said wavelength.

11. The system of claim 8 wherein said tunable resonators are spaced apart in the direction of fluid flow in said conduit means by a distance approximating 1 wavelength of said sound.

Claims

1. A system for suppressing sound generated at a source in a high velocity fluid stream comprising conduit means extending downstream of said source, said conduit means having a crosssectional flow area with a transverse dimension less than one half the wavelength of the sound in said fluid, and a tunable acoustic resonator having an inlet in communication with said conduit means downstream of said source and angularly intersecting the same for reflecting waves received through the inlet back into said conduit means to attenuate the succeeding sound waves therein.

6. The system of claim 1 wherein said inlet of said tunabLe resonator is positioned downstream of said source in said conduit means by a distance at least equal to double the value of said major dimension.

7. The system of claim 4 wherein said acoustic resonators are dimensioned to have a transverse cross-sectional area of approximately the same value as the adjacent cross-sectional flow areas in said stack as divided by said longitudinal divider means.