PACKLESS SILENCER

BACKGROUND OF THE INVENTION

Conventional silencers of the type in which the silencer is inserted into the flow of gas to attenuate noise traveling in the gas 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, such as foam, rockwool, fiberglass or other fibrous acoustically absorp- tive bulk material. The filler may be referred to as packing.

Because these packed duct silencers rely on absorp-- tion by the packing, the perforated facing sheet is designed to provide optimum sound access from the flow passage to the packing material. Face sheet open face area in these silencers are typically 20% and more.

The use of packing to absorb acoustical noise intro- duces problems in many applications. The packing tends to erode under high velocity conditions; the packing 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 packings may pro- vide 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 resis¬ tance 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 perfor- ated 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 whic 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, aluminu 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 silencers of Figure 4 joined by a transition member designed to re¬ duce restriction to air flow while further supplementing high frequency performance;

Figure 6 is a cross-sectional view of two silencers 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;

Figures 8-11 are graphs of various silencer perfor¬ mance correlations as function of octave band frequency;

Figures 12 and 13 are schematic cross-sectional views of splitters with varying depth partition walls;

Figure 14 is a schematic cross-sectional view of a -silencer taken generally along line 14-14 of Figure 15 with varying depth cavity;

Figure 15 is a right end view of the silencer of Figure 14;

Figure 16 is a schematic cross-sectional view of a silencer with a zig-zag varying depth cavity; and

Figure 17 is a schematic cross-sectional view of a silencer with two symmetrical diverging-converging air flow paths.

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 embodi ment of the invention, and modifications thereto, with the understanding that the present disclosure is to be con¬ sidered as an exemplification of the principles of the invention and is not intended to limit the invention to th 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 havi 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 wal

18.

Silencer 10 is adapted to be placed in a duct system, e.g., hea , 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 material

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. Advantageously, the spacing of the per¬ forations h is such 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 the face panel may 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 dissipa¬ tion efficiency.

The perforated panel or sheet 14 is characterized 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 and a reactive (mass reactive) part X . 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 stiff¬ ness at low frequencies with a corresponding reactance X .

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

The total resistance is equal to the sheet resistance Rs and the total reactance X is the sum of the sheet and cavity reactance, X = X + X .

Attenuation is a comp ^r lex function of RS and X.

As a design guide, it has been found that optimization of the attenuation is approximately equivalent to maximization of the following quantity: --

R_

R- + (X _c + X _C ⁾ Thus, Rs cannot be too small or too large and

(Xs + Xc) cannot be too large,

Optimization of the resistive factor for silencers suited to the applications previously noted is obtained with an acoustic flow resistance, R in the range of 1 to

4 JO C where JOQ. is the characteristic resistance of gas, e.g., air, -- being density and c being the speed of sound for the particular application. This resistance i 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 produces 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 switching 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 provide (in conjunction with mean flow) the desired properties for dissipation of sound, a serious problem arises which, until the present invention, prevented the use of per¬ forated 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.

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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 per¬ forated open areas in the range of 2.5 to 10%. A correla¬ tion of self-noise level with mean flow velocity and per-, cent 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 range 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 silen¬ cers with various face sheet perforation diameters but otherwise of identical configuration and construction and at the same means flow velocity. The perforated face sheet in each was 26 gauge with a 2-l/27o 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 com¬ pared have perforations of the same diameter (.125 inch) but have different perforation geometries in that thick¬ ness of the perforated sheets is different (26 gauge and gauge) with flow at 1000 FPM.

Figure 10 shows calculated packless silencer atten¬ uation for an effective face sheet flow resistance of 2 Jύc versus actual performance of a silencer constructe according to this invention.

Figure 11 shows attenuation of three silencers con¬ structed according to this invention with 1, 2.5 and 7.2 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 face with perforated face panels of the type described, e.g., rectangular or cylindrical duct with a packless duct liner.

In some applications, it may desirable, depending upon allowable flow restriction and acoustical require- ments, to arrange several silencers in series. Some of these arrangements are illustrated in Figures 3-7, where corresponding numerical designations indicate correspond¬ ing elements.

Figure 3 illustrates a tandem arrangement of three silencers 10 which provide a convenient means of extend¬ ing the effective length of the silencer through the use of standard silencer modules.

Figure 4 illustrates a combined silencer which in¬ cludes 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 m y 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 arrange- ment, similar to that shown in Figure 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 shape includes perforated facing panels 45a and a longitudinal partition wall 45b.

Figure 7 illustrates a triple tuned silencer arrange ment wherein the flow passage width is progressively re¬ duced by a factor of 1/2 through three silencers, as indicated in the figure. This arrangement has applicatio 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 passages to width of d/4. Each splitter 50 includes a pair of perforated facing panels 55a and central longitudinally disposed partition wall 55b. The duct wall also includes perforated facing panels 54.

The splitter 35 shown in Figure 5 has, of course, a varying-depth-cavity which is acoustically active. The significance of the varying cavity depth is that it broad ens the insertion loss characteristics of the silencer, i.e., decreases peak IL and increases IL at other frequen cies in a given size silencer.

Other configurations in addition to triangular shape splitter 35 are possible. Figures 12 and 13 show two alternative embodiments for splitters 65 and 75. Each splitter has perforated facing panels 65a, 75a which form a flattened elliptical shape, much the same as splitter 25. The varying-depth-cavity configuration is provided

by fabricating the partition walls 65b, 75b to produce the varying depth. In splitter 65, partition wall 65b is a zig-zag shape so that cavity depth along the silencer goes through a sequence of enlarging and con¬ tracting. The splitter 75 has a longitudinal partition 75b which varies uniformly along the length of the splitter, i.e., the cavity undergoes a single cycle change similar to splitter 35 but with parallel perforated facing panels 75a.

The varying-depth-cavity may also be embodied in the primary facing panel configuration to achieve both the broadening of the IL and to change the stream flow direc¬ tion to eliminate "line-of-sight" through the silencer.

Figures 14 and 15 show a packless silencer 80 which includes a four side duct member 82 within which is positioned a pair of opposed facing panels 84. The panels are perforated as described above and are tapered in opposed fashion so that each panel has a long end 84a and a short end 84b at opposite ends. This arrangement produces a flow channel which is angled with respect to the longitudinal axis. The perforated facing panels 84 are supported by lateral partition walls 88 to form a plurality of different depth cavities 86 along the length of the sliencer 80. The angle of the partition walls 84 is such that when viewed from the end, see Figure 15-, the line-of-sight is blocked by a perforated facing panel 84.

Still another silencer 90 is shown in Figure 16 wherein both varying-depth-cavity and elimination of line-of-sight is achieved. Silencer 90 includes a pair of zig-zag configured, perforated panels 94 which together with partition walls 98 form a plurality of varying- depth-cavities 96 along the length of the silencer.

Additionally, the zig-zag configuration blocks the line- of-sight through the silencer.

Figure 17 shows still another silencer 100 in accordance with this invention. Silencer 100 includes two symmetrical air flow paths 103 formed by juxtaposed perforated panels 104. Panels 104 and partition walls 108, together with silencer housing 102 or longitudinal partition wall 110, form a plurality of varying-depth- cavities 106. Additionally, the line-of-sight through the silencer is blocked by the divergent-convergent flow path configuration formed.by perforated panels 104. Any quarter section of this symmetrical silencer, defined by housing 102, transverse seam 112, partition wall 110 and end edge 114, may be interchanged with any other quarter section, and each quarter section can function independently.

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.

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