Conventional silencer* 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 silen¬ cer elements consisting of a perforated facing plate be¬ hind which is positioned a filler material, βuch as foam, rockwool, fiberglass or other fibrous acoustically absorb- tive 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 pack¬ ing may absorb toxic or flammable substances or micro¬ organisms; the packing is subject to attack by chemicals; and in the event of fire, some otherwise desirable pack¬ ings may provide fuel or produce toxic gases. It has been known for nearly thirty years that, by using face sheets with suitable acoustic flow resis¬ tance in lieu of conventional perforated face sheets, broad band acoustical absorption could be obtained with¬ out the use of packing. In order to overcome packing problems, silen¬ cers have been designed in which the required acoustic

resistance was provided by thin resistive sheets rather than by packing. The resistive sheets of these con¬ structions have been structually self-supporting sintered materials or laminates of fabrics (metals, glass or syn- thetic), 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 shortcom¬ ings of the prior art by making use of a commercially available perforated face sheet having an open area in the range of 2 to 107. 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 ob¬ tained 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-

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cers of Figure 4 joined by a transition member designed to reduce restriction to air flow while further supple¬ menting high frequency performance;

Figure 6 is a cross-sectional view of two silen- cere 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 fre¬ quency.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT While this invention is susceptible of embodi¬ ment in many different forms , there is shown in the draw¬ ings and will hereinafter be described in detail a pre¬ ferred embodiment of the invention, and modifications thereto, with the understanding that the present disclo- ^' sure 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 silen¬ cer 10 which includes a four sided duct member 12. With- in 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 per¬ forated 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, struc¬ turally stable material. Advantageously, the perfora- tions have a hole diameter as small as is economically available from a conventional perforation punching pro¬ cess. 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. Advsn- tageouflly, the spacing of the perforations h is suπh that an open are* ratio of less than 207-, preferably in th ^p range of 2 to 107. 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 spe¬ cial applications, but probably with a loss of sound dissipation efficiency.

The perforated panel or sheet 14 is character¬ ized by its hole diameter d, , hole separation S, and sheet thickness t. The acoustical (dynamic) impedance of the sheet Z , consists of a resistive part R _B and a reactive (mass reactive) part Xs _* The acoustical im- pedance 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 reac¬ tive, representing a stiffness at low frequencies with a corresponding reactance X .

The attenuation of the silencer may be express¬ ed in terms of an imoedance Z which is the sum of the sheet impedance Zs„ and the cavity ^J reactance.

The total resistance is equal to the sheet re- sistance 8„ and the total reactance X is the sum of the

εheet and cavitv reactance, X ^■ Xβ„ + Xc...

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

R_

^R s" ⁺ < ^X s ^{+ X} c

Thus. Rs cannot be too small or too large and ^ ^X s ^{+ X} c^ ^cannot b ^{β t0} ° large.

Optimization of the resistive factor for silen¬ cers suited to the applications Dreviously noted is ob¬ tained with an acoustic flow resistance. Rs„ ^■ - in the range of 1 to 4 <?r. where θc is the characteristic re- siεtance of gas. e.g. air. -- t ¹ being density and c being the speed of sound for the particular application. This resistance in prior art silencers has been provid¬ ed by the viscous friction in the pores of resistive sheet materials. In the. present invention, however, an optimum flow resistance is produced bv 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 de- flection 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 Aper- ture, M.S. Thesis. Department of Naval Architecture,

M.I.T.. can be expressed as an equivalent acoustic re¬ sistance of the sheet.

In addition, the total attenuation depends on the width π of the Rilencer flow passage and the length .

In utilizing a perforated sheet chosen to pro-

vide (in conjunction with mean flow) the desired pro¬ perties for dissipation of sound, a serious problem arises which, until the present invention, prevented theuse of perforated sheets to form a packless βilen- cer. 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 per¬ forations 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 guide¬ lines 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 inves¬ tigation, involving tests of over a hundred configura- tions , has led applicants to a range of design para¬ meters which yield the maximum possible attenuation with self-noise acceptable even in critical HVAC appli¬ cations which do not complicate, or significantly in¬ crease the cost of,the perforated resistive sheet. Experimental investigation confirmed that op¬ timum properties of sound dissipation are obtained with perforated open areas in the range of 2.5 to 107.. A correlation of self-noise level with mean flow velo¬ city 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 geo¬ metry 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

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and commercially available perforated metal sheets.

The appended graphs, Figures 8-11, illustrate some of the significant correlations that appli¬ cants 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 2-1/27. 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 condi¬ tions 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 resis- tance of 2 ^c 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 27..

The silencer 10 as previously discussed re¬ places 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 des¬ cribed, e.g. rectangular or cylindrical duct with a packless duct liner.

In some applications, it may be desirable, de¬ pending 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 lenizth 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 silen¬ cer 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 proce¬ dure 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 attenua¬ tion 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 tn these octave bands is critical in the appli- cation.

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.

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Figure 6 illustrates a pair of silencers 10 joined by a hi?h frequency transition member 40. This arrangement, similar to that shown in Figurt 5, supple¬ ments 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 el¬ liptical 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 progres¬ sively reduced by a factor of 1/2 through three silen¬ cers, as indicated in the figure. This arrangement has application in situations where e^en 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 per¬ forated facing panels 54. From the above description, it will be readily apparent to those skilled in the art that other modifi¬ cations may be made to the present invention without departing from the scope and spirit thereof as pointed out in the appended claims.