WO1979000286A1

WO1979000286A1 - Packless silencer

Info

Publication number: WO1979000286A1
Application number: PCT/US1978/000145
Authority: WO
Inventors: J Morgan; U Ingard; M Hirschorn
Original assignee: Industrial Acoustics Co
Priority date: 1977-11-14
Filing date: 1978-11-08
Publication date: 1979-05-31
Also published as: DE2862166D1; AU524931B2; AU4148778A; JPS6046312B2; EP0006955B1; CA1137877A; JPS54500068A; ZA786180B; IT7851879A0; EP0006955A1; IT1107751B

Abstract

The acoustical gas flow silencer field, e.g. heating, ventilating and air conditioning systems, engine intakes and exhausts, process blowers and compressors, etc. In the prior art, silencers were constructed using an absorbtive material, which was both expensive and inefficient. The device described herein is a packless silencer, and accomplishes the desired end of dampening undesired noise in a manner both more economical and efficient than heretofore known. The invention described is a resistive sheet type of duct liner or duct silencer, i.e., a liner or silencer in which acoustical flow resistance is concentrated in a thin face sheet (14a) separating the flow passage and acoustical cavity (16) the invention disclosed is a means for applying inexpensive perforated facings (14) and (14a) similar to those in a conventional packed silencer, to provide resistive sheets which are effective in terms of noise dissipation and in terms of self-noise (noise generated by flow through the flow passages).

Description

PACKLESS SILENCER

BACKGROUND OF THE INVENTION

Conventional silencers of the type in which the silencer is inserted into the flow of gas to attenuate noiae traveling in thegas stream have generally relied upon viscous friction in the pores of a cavity filler material.

A conventional silencer typically includes a duct member within which is positioned one or more silencer elements consisting of a perforated facing plate behind which is positioned a filler material, βuch as foam, rockwool, fiberglass or other fibrous acoustically absorbtive bulk material. The filler may be referred to as packing.

Because these packed duct silencers rely on absorption by the packing, the perforated facing sheet ia designed to provide optimum sound access from the flow passage to the packing material. Face sheet open face area in these silencers are typically 207. and more. The use of packing to absorb acoustical noise introduces problems in many applications. The packing tends to erode under high velocity conditions; the packing may absorb toxic or flammable substances or microorganisms; the packing is subject to attack by chemicals; and in the event of fire, some otherwise desirable packings may provide fuel or produce toxic gases. It has been known for nearly thirty years that, by using face sheets with suitable acoustic flow resistance in lieu of conventional perforated face sheets, broad band acoustical absorption could be obtained without the use of packing. In order to overcome packing problems, silencers have been designed in which the required acoustic resistance was provided by thin resistive sheets rather than by packing. The resistive sheets of these constructions have been structually self-supporting sintered materials or laminates of fabrics (metals, glass or synthetic), felts (metal, synthetic or organic) or sintered materials (metal or ceramics) - typically supported on a structural perforated sheet. These silencers have found very limited use due to their high cost. SUMMARY OF THE INVENTION

The present invention overcomes the shortcomings of the prior art by making use of a commercially available perforated face sheet having an open area in the range of 2 to 10% to provide suitable acoustic flow resistance which is enhanced by the flow present in the silencer passages as a normal consequence of its use.

By proper choice of perforation geometry in a thin sheet of stainless, cold rolled, galvanized steel, aluminum or other metallic or synthetic material, broad band noise dissipation of a useful magnitude can be obtained without the use of packing and without generating unacceptable levels of self-noise.

DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view illustrating a packless acoustic silencer of the present invention;

Figure 2 is a cross-sectional view taken along line 2-2 in Figure 1;

Figure 3 is a cross-sectional view illustrating a series arrangement of silencers in accordance with the present invention;

Figure 4 is a cross-sectional view of a silencer of Figure 1 joined with a silencer of the same type, but with cavity depth chosen to enhance performance at a higher frequency;

Figure 5 is a cross-sectional view of two silen cers of Figure 4 joined by a transition member designed to reduce restriction to air flow while further supplementing high frequency performance;

Figure 6 is a cross-sectional view of two silencere of Figure 1 joined by a transition member with a splitter;

Figure 7 is a cross-sectional view of a triple tuned silencer in which each of three modules provides broad band performance but each of which is tuned for peak performance at a different frequency; and

Figures 8-11 are graphs of various silencer performance correlations as function of octave band frequency .

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will hereinafter be described in detail a preferred embodiment of the invention, and modifications thereto, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.

Figures 1 and 2 show a packless acoustic silencer 10 which includes a four sided duct member 12. Within the duct is positioned a pair of opposed facing panels 14 having a generally flattened semi-elliptical shape. The opposing flat portions 14a of each panel are perforated to provide a plurality of holes h which open to chambers (or cavities) 16 formed behind each panel and separated by partition walls 18.

Silencer 10 is adapted to be placed in a duct system, e.g. heat, ventilating and air conditioning duct. The gas flow, e.g. air, is in the direction indicated by the arrow although gas flow may also be reversed. Duct member 12 may be made of galvanized sheet metal or other materials .

Facing panel 14 is made from galvanized or stainless steel or other metallic or non-metallic, structurally stable material. Advantageously, the perforations have a hole diameter as small as is economically available from a conventional perforation punching process. A diameter of 0.032 or 0.046 inch is suitable for 26 gauge material, applicable to an air conditioning silencer; and 0.125 is suitable for 11 gauge steel which might be used in a gas turbine silencer. Advsntageouslly, the spacing of the perforations h is suπh that an open area ratio of less than 20%, preferably in the range of 2 to 10%. is achieved along the face panels. The thickness of rhe face, panel may be in the range of 26 gauge to 11 gauge (0.018 to 0.12 inch).

Lighter gauges of corrosion resistant material might be used if provision is made for structural support and stiffening. Heavier gauge might be used in some special applications, but probably with a loss of sound dissipation efficiency.

The perforated panel or sheet 14 is characterized by its hole diameter d_h, hole separation S_h and sheet thickness t. The acoustical (dynamic) impedance of the sheet Z_s, consists of a resistive part R_s and a reactive (mass reactive) part X_s The acoustical impedance of the air cavity 16 behind the sheet depends upon the depth d and the spacing between partitions S . The impedance of the cavities 16 is mainly reactive, representing a stiffness at low frequencies with a corresponding reactance X_c.

The attenuation of the silencer may be expressed in terms of an imoedance Z which is the sum of the sheet impedance Z_s and the cavity reactance.

The total resistance is equal to the sheet resistance R_s and the total reactance X is the sum of the sheet and cavitv reactance, X = X_s + X_c.

Attenuation is a complex function of R_s and X. As a design suide, it has been found that optimization of the attenuation is approximately equivalent to maximization ofthe following quantity:

Thus. R_s cannot be too small or too large and (X_s + X_c) cannot be too large.

Optimization of the resistive factor for silencers suited to the applications Dreviously noted is obtained with an acoustic flow resistance. R_s in the range of 1 to 4 ϱc where ϱc is the characteristic resistance of gas. e.g. air. -- ϱ being density and c being the speed of sound for the particular application. This resistance in prior art silencers has been provided by the viscous friction in the pores of resistive sheet materials. In the present invention, however, an optimum flow resistance is produced by interaction of mean flow in the duct with the perforated facing panel. The mechanism, through which mean flow υroduces an optimum resistance, is related to an acoustically induced deflection or "switching" of some of the mean flow in and out of the perforations. This switchinε requires energy which is taken from the sound field. This effect, first observed by C.E. McAuliffe in 1950. Study of Effect of Grazing Flow on Acoustical Characteristics of an Aperture, M.S. Thesis. Department of Naval Architecture,

M.I.T.. can be expressed as an equivalent acoustic resistance of the sheet.

In addition, the total attenuation depends on the width D of the silencer flow passage and the length L.

In utilizing a perforated sheet chosen to pro vide (in conjunction with mean flow) the desired properties for dissipation of sound, a serious problem arises which, until the present invention, prevented theuse of perforated sheets to form a packless silencer. The problem, initially referred to as "whistle", has to do with the self-noise which was produced by interaction of flow with the sound and with the perforations in the sheet.

The self-noise produced by a silencer depends on the flow speed and on the geometrical parameters of the perforated sheet.

Theoretical analysis has provided some guidelines for optimization of attenuation. However, there is at present no reliable theoretical analysis from which the level of self-noise can be predicted, and applicants have had to rely on experimental studies to establish self-noise characteristics.

A combined theoretical and experimental investigation, involving tests of over a hundred configurations, has led applicants to a range of design parameters which yield the maximum possible attenuation with self-noise acceptable even in critical HVAC applications which do not complicate, or significantly increase the cost of, the perforated resistive sheet. Experimental investigation confirmed that optimum properties of sound dissipation are obtained with perforated open areas in the range of 2.5 to 10%. A correlation of self-noise level with mean flow velocity and percent open area, and a correlation of peak self-noise frequency with mean flow velocity and the perforation geometry have been found. Discovery of a correlation of self-noise level with perforation geometry permits the reduction in self-noise of as much as 30 decibels by choices of perforation geometry that still fall within the ranee of economically producible and commercially available perforated metal sheets.

The appended graphs, Figures 8-11, illustrate some of the significant correlations that applicants have obtained. Figure 8 shows self-noise for packless silencers with various face sheet perforation diameters but otherwise of identical configuration and construction and at the same mean flow velocity. The perforated face sheet in each was 26 gauge with a 2-1/2%. open area. The perforation diameters .032, .046, .062, .078, .094. .125 and .188. The air flow speed is 1500 feet per minute (FPM).

Figure 9 shows self-noise under similar conditions as described above except that the two silencers compared have perforations of the same diameter (.125 inch) but have different perforation geometries in that thickness of the perforated sheets is different (26 gauge and 11 gauge) with flow at 1000 FPM.

Figure 10 shows calculated packless silencer attentuation for an effective face sheet flow resistance of 2 ϱc versus actual performance of a silencer constructed according to this invention.

Figure 11 shows attenuation of three silencers constructed according to this invention with 1. 2.5 and 7.27. perforated face open areas. This graph illustrates loss of performance with open area less than 2%.

The silencer 10 as previously discussed replaces a length of duct work in a gas passage. Although the face panels 14 are illustrated as being on opposite sides of the flow chamber, the entire flow passage may be faced with perforated face panels of the type described, e.g. rectangular or cylindrical duct with a packless duct liner.

In some applications, it may be desirable, depending upon allowable flow restriction and acoustical requirements, to arrange several silencers in series. Some of these arrangements are Illustrated in Figures 3-7, wherein corresponding numerical designations indicate corresponding elements.

Figure 3 illustrates a tandem arrangement of three silencers 10 which provide a convenient means of extending the effective leneth of the silencer through the use of standard silencer modules.

Figure 4 illustrates a combined silencer which includes a first silencer 10 and a second silencer 20 in tandem. Silencer 20 is similar in structure to silencer 10 except that its flow passage includes a splitter element 25. Splitter 25 is generally of a flattened elliptical shape and provides perforated facing panels 25a adjacent the gas flow passages. The center of splitter 25 includes cavity partitions 25b. The procedure for selecting the hole size and open area of the face sheets is as previously described. Cavity depth and flow passage width are chosen to optimize attenuation at a higher frequency for silencer 20 than for silencer 10. This combination provides better dynamic insertion loss (DIL) in some octave bands than does a combination of two silencers of configuration 10 so that design flexibility may be increased if acoustic noise in these octave bands is critical in the application.

Figure 5 illustrates a silencer combination of silencers 10 and 20 joined by a transition member 30. Member 30 provides a tapered transition from silencer 10 to silencer 20 and includes perforated face panels 34 and a centrally disposed generally triangular shaped splitter 35 having perforated facing panels 35a adjacent the flow paths and a central longitudinal partition wall 35b. The transition member 30 is useful in improving DIL in the higher frequencies and in reducing flow restriction. Figure 6 illustrates a pair of silencers 10 joined by a high frequency transition member 40. This arrangement, similar to that shown in Figurt 5, supplements DIL of similar silencers in tandem, transition member 40 includes lateral perforated facing panels 44 which define a cavity 46 with longitudinally disposed partitions 48. A central splitter 45 of flattened elliptical βhape includes perforated facing panels 45A and a longitudinal partition wall 45b. Figure 7 illustrates A triple tuned silencer arrangement wherein the flow passage width is progressively reduced by a factor of 1/2 through three silencers, as indicated in the figure. This arrangement has application in situations where even broader range DIL is desired. The arrangement includes a silencer

10' having a flow passage width of d joined to a single splitter silencer 20' having two flow passages each d/2 in width.

Finally, silencer 50 includes three splitters 55 which further divide the flow oassages to a width of d/4. Each splitter 50 includes a pair of Derforated facing panels 55a and central longitudinally disposed partition wall 55b. The duct wall also Includes perforated facing panels 54. From the above description, it will be readily apparent to those skilled in the art that other modifications may be made to the present invention without departing from the scope and spirit thereof as pointed out in the appended claims.

Claims

WE CLAIM:

1. A packless acoustic silencer comprising facing panel means separating fluid flow patha from adjacent acoustical cavities; βflid facing panels being perforated sheets having an open area in the range of 2-10 percent, said open area causing a switching action of the mean fluid flow in and out of the perforations, whereby the acoustic energy of the air flow is reduced.

2. The silencer of Claim 1, wherein said perforationβ have an effective diameter in the range of from about .032 inch to 0.125 inch respectively for sheet thicknesses from 26 gauge to 11 gauge,

3. The silencer of Claim 2, wherein said perforations are circular in shape.

4. The silencer of Claim 2, wherein said facing panel is between 1/64 to about 1/8 inch in thickness.

5. The silencer of Claim 1, further including at least one splitter element having perforated facing panels with a percent open area in the range of 2-10 per cent.

6. The silencer of Claim 1, further including a second silencer joined in tandem therewith.

7. The silencer of Claim 6, further comprising a transition silencer located between said silencer and said second silencer.

8. The silencer of Claim 7, wherein said transition silencer is tuned for optimum dynamic insertion loss spectra appropriate to the intended application.

9. A packless acoustic silencer comprising facing panel means separating fluid flow paths from adjacent acoustical cavities, said facing panels being perforated sheets having an open area in the range of 20 per cent, said open area causing a switching action of the mean fluid flow in andout of the perforations, whereby the acoustic energy of the air flow is reduced.

10. The silencer of Claim 9, wherein said per cent open area is in the range of 2 to 10 per cent.