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INVENTOR WILHELI'1 S. EVERETT

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ATTORNEY, 2,990,907 United States Patent Office Patented July 4, 1961

1 2 acoustic spectrum without introducing any substantial 2,990,907 back pressure into the system. ACOUSTIC FILTER Wilhelm S. Everett, 1349, E. Main St., Santa P.aula, Calif. I have devised an acoustic mute which absorbs se­ Filed June 11, 1959, Ser. No. 819,779 lected frequencies of the acoustic speotrum, which may 6 Claims. (CI. 181-54) 5 be at the resonance frequencies of ,the acoustic system into which it is placed. Also, especially where the acous­ This invention relates to an acoustic filter to be em­ tic spectrum contains a material proportion of the acous­ ployed in connection with a stream of compressible fluid, ,tic energy in relatively low frequencies and/or relatively to limit or decrease the fraction of the energy of the high frequencies, I may cut off such high frequency or stream present in frequencies in the audible range. 10 low frequency wave or both low frequency and high fre­ This application is a continuation in part of applicant's quency waves. By so doing I may obtain a more uni­ application Serial No. 526,725, filed August 5, 1955, form distribution of acoustic energy over the whole acous­ which is now abandoned. tic spectrum. I accomplish these results by introducing As is well known, streams of compressible fluids, such into the system acoustic resonators whose frequency re- ,as gases flowing in confined ohannels, such as pipe sys­ 15 sponse is in 'the frequency region in which noise sup­ ,tems, upon intake from and discharge into the atmos­ pression is desired. phere generate a great deal of noise depending on their These ,acoustic resonators have a capacitance and in­ velocity and physical characteristics and on ,the physical ductance, using the language of the eleotrical analogies characteristics of the engines, pumps or compressors in employed in acoustic theory, so that their natural fre- the system. Additionally, the 'acoustic vibrations tend to 20 quencies 'are such that they will absorb the energy of the generate mechanical vibration in the system with conse­ acoustic waves at the desired location in the acoustic quent damage which frequently may be quite severe. spectrum and in the region of acoustic repsonse of the The frequency and wave shape of Vhe energy input into resonators, and do not introduce any substantial resistance the flowing system also modifies the nature of the acoustic to the flow of the fluid whose noise is to be suppressed. energy in the fluid stream and the nature of its acoustic 25 To accomplish these results! place in the flow path of 8pectrum. the fluid, an acoustic resonator in the form of a closed The acoustic problem of noise and energy loss due to chamber, having an entrance port or por,ts so positioned noise in such systems where this problem has been found with respect to the direction of flow of the fluid so that most aggravated has resulted in the application of various the acoustic wave pressure may be cOIll.!Ilunicated from muffler devices, which in principle depend on absorbing 30 the flowing stream directly through ,the port. The cham­ the sound by reducing the pressure peaks, i.e., the ampli­ ber being full of the fluid is thus subject to compression tude of the acoustic wave energy in the flowing stream and rarefaction induced by the propagation of the wave without substantial change in the acoustic spectrum. It energy through the port. The gas in the chamber acts is conventional in acoustic theory to consider acoustic like a compressible spring of frequency response deter- systems in terms analogous to electrical circuits. Viewed 35 mined by the free volume of the chamber and the area in this manner these devices in various forms· introduce and geometry of the port entrance. a resistance in the circuit to cause a damping of the The filter thus may act to reduce the magnitude of the wave energy by reducing the amplitude of the waves. positive and negative peak frequencies of acoustic waves This resistance may be a series resistance as when baffling within the range of frequency to which the filter responds of various sorts in introduced into the flowing stream, 40 without introducing any substantial back pressure into or a pamllel resistance as where the pressure pulses are the system. Since the noise level is determined by the permitted to escape in a direction perpendicular to the magnitude of ,the resonance peaks, the reduction in the flowing stream into a baffling chamber filled with sound energy in these peaks causes a reduotion in the noise absorbing material and pressure is reduced by the flow level of the stream of fluid without requiring reduction of the stream in and out of the baffling chamber. 45 in the amplitude of the waves across the entire acoustic Studies which I have made of the energy distribution spectrum as is the case in prior art damping known to in the acoustic spectrum of such flowing fluid streams applicant. Such filters are particularly useful when it is show that these streams resonate in certain frequency desired to selectively absorb relatively low frequencies. ranges. Where I desire to add to the aotion of the ,low frequency In all such previously mentioned prior art and other 50 response acoustic filter, one having a higher frequency similar systems, the sound absorption is not selective, absorption of the acoustic spectrum, I use as part of the since operating merely to introduce a resistance, the na­ aforesaid filter or as a separate element an acoustic filter ture of the acoustic frequency distribution in the aCOllS,tic having a relatively higher frequency response. In order spectrum is substantially unaltered but the energy in the 55 to obtain the proper frequency response characteristics various wave lengths are reduced by reason of the energy of ,the acoustic filter at the selected high frequencies, the loss in overcoming this resistance. In order to reduce ratio of the port area to the volume of the acoustic filter the pressure peaks of the acoustic waves in the region is preferably made greater than for the filter of lower of resonance, considerable resistance is required. frequency respouse. In order ,to reduce the volume of The acoustic energy loss thus results in a large pressure the filter I introduce a filter element so as to divide the drop in the system, introducing a substantial back pres­ 60 chamber into a plurality of small conduits which com­ sure and a drop in the efficiency of the system. Addi­ municate with the entrance ports, and if desired, also tionally, frequently these systems introduce reflective with each other. In this way I reduce the free volume surfaces, which because of interference phenomena gen­ of the acoustic filter chamber to obtain the proper ratio erate standing waves and thus also genemte localized of the port area to the free volume in communication areas of high pressure which add to the back pressure 65 therewith. in the system. I also plaoe the port area so that the acoustic wave pres­ Instead of relying solely or in the main on the sound sure may be communicated to and through the ports into absorbing effeot of acoustic resistance elements, I so the free volume of the filled acoustic filter. In a pre­ modify the acoustic characteristics of the pipe line at the 70 ferred embodiment, the flow of the fluid is made to pass point where the acoustic energy is to be modified so as over each port, for example, in a direction parallel to the to selectively absorb the acoustic energy from the entire plane of the port entrance. 'I1he characteristics of this 2',990,907 3 4 acoustic filter are that it absorbs high frequency acoustic connected to the outer periphery thereof an annular energy and has a high frequency cut-off point. "pancake" type c6JisfriiCtion generally represented by the When the relatively low frequency filters are used in numeral 24, also preferably formed of sheet metal. Said conjunction with the high frequency filter described pancake is composed of an annular chamber 26 having a above, I may attenuate and suppress the low and medium 1) rounded or bumped annular wall- 28, the inner periphery frequencies of the flowing fluid and the high frequency of which is welded or otherwise suitablY connected at 30 acoustic waves, and thus obtain a large ;reduction in the to the wall of tube 12; The outer periphery of the noise level of the flowing stream without introducing any bumped wall 28 is bent outwardly for a short distance substantial back pressure into the system. forming a wall 32 parallel to the axis of tube 12, and The general form of the low frequency filter is a 10 said wall 32 is then bent inwardly normal to the axis of rel10nance chamber having a port entrance to which is tube 12 to form an annular wall 34, which is connected connected a conduit of low frictional resistance and low­ to the wall of tube 12, e.g., by welding, as at 36. Hence, reflectance in the form of a horn or a diffuser with a it is seen that the annular chamber 26 is bounded by take"off discharge outlet in a direction to one side of the walls 28, 32, 34, and the outer wall portion 3S of tube 12. port opening of the resonance chamber. In the preferred 15 The chamber wall 28 is rounded or bumped for stiffness embodiment the flow is discharged generally perpendicu­ and to increase the natuml frequency so as to prevent lar to the direction of flow and to the axis of the port vibration in the throat 22 at low frequencies. opening to the resonance chamber. The other element of the "pancake" construction 24 In the general form of the relatively higher frequency comprises an annular chanlber 40 which is spaced from filter the flow of gases is again directed through the dif­ 20 annular chamber 26 in a direction 'axially along tube 12. fuser and horn and the flow is similarly directed through The annular chamber 40 is formed at the inner end of a a passageway in a direction to one side of the aXis. The cylindrical resonance chamber 42 which is bumped out­ passageway is formed of a foraminated boundary wall wardly at its end 44 to provide stiffness. The resonance which forms also the wan of a chamber filled with mate­ chamber42 is mounted on the axis of the conical tube 12 rial to reduce the free volume of the chamber, as will be 25 and has a diameter about equal to the diameter of the further described. In a preferred' embodiment the pas­ outer wall 32 of chamber 26. Along the inside wall of sageway is made to surround the diffuser as, for example, chamber 42 near the inner end thereof is an annular in: the form of an annulus, so that the surface area of the ring 46, e.g., formed· of sheet metal and connected as by boundary walls may be great in relation to the volume 30 welding at 48 to said wall. Ring 46 extends inwardly of to permit a sufficient number of small holes to be placed chamber 42 in a plane normal to the axis of tube 12 to a so that the ratio of the total ports area, I.e., the holes, point axially opposite the flared end 22 of tube 12. Ring bear the desired relation to the volume of the chambers 46 is bent outwardly a short distance to form a short to give desired resonance characteristics. cylinddcal wall 50 whose aXis coincides with the axis of The passageway and chamber are adjusted in width 35 tube 12, said wall 50 having a diameter substantially and thickness along the flow path of the fluid so that the equal to or greater than the diameter of the flared end resonance characteristics of the chamber may be spread of tube 12. The opposite end of wall 50 is bent outward­ over the desired band of frequencies. ly to form an annular wall 52 which extends toward wall I may instead of using the filled chamber tuned to the 34 of chamber 26, at an acute angle to said wall 34, wall higher frequencies in combination with the free volume 40 52 being connected to the inner end of the cylindrical filter tuned to the lower frequencieS, use it alone when wall of resonance chamber 42. Chamber 40 is thus the acoustic spectrum of the fluid has s6 HttIe energy bounded by ring 46, walls 50 and 52, and the inner waIl in the lower frequencies as not to be disturbing~ portion 54 of the resonance chamber 42. Instead of the diffuser acting as the intake and the side The resonance chamber 42 with the chamber 40 formed take-off as the outlet, the fiO\v may be reversed. 45 at the inner end thereof is connected to the flared end 22 The invention will be more readily understood from of tube 12 by means of a plurality of lugs or arms 58 the following description of some preferred embodiments connected by suitable means, such as welding, at spaced thereof, taken in connection with the appended drawings inte,rvals about the outer periphery of walls 34 and 52 wherein: of chambers 26 and 40. In this manner the adjacent FIGURE 1 is a view in elevation, with the parts shown 50 walls 34 and 52 are spaced from each other forming an in Section, of one form of the invention device; annular passage 60 between chambers 26 and 40, said FIG. 2 is a section taken on Hne 2...:...2 of PIG. 1; passage communicating, via the large intermediate pas­ FIG. 3 is a view in elevation, with the parts shown in sageway 61 between tube 12 and resonance chamber 42, section, of another form of my device; with the open flared end 22 of tube 12 and the circular FIG. 4 is a section taken on Hne 4-4 of PIG. 3; 55 open end 62 of the resonance chamber 42, formed by the FIG. 5 is a section taken on Hne 5-5 of FIG. 3; annular wall 50 thereof. The annular passage 60 de­ FIG. 6 is an enlarged fragmentary section of a modified creases in depth or thickness from its inner end 64 to its detail of the device of PIG. 1 taken in the direction of the outer end 66 which communicates with the atmosphere. arrows 6-6 in FIG. 1; A series of concentric rows of round spaced holes or FIG. 7 is a fnigmentary section taken 6n line 7-7 of 60 'apertures 70 is formed in the annular wall 34 of chamber FIG. 6; 26, 'and ,a series of concentric rows of similar holes 72 is FIG. 8 shows the device of PIG. 1 employed on an formed in the adjacent annular wall 52 of chamber .:II). exhaust muffler on an internal combustion engine; and These holes form entrance ports for the fluid whose FIG. 9 is a graphic illustration of the attenuation char­ acoustic energy is to be selectively attenuated according acteristics of certain acoustic ,filters according to the 65 to the invention. The size of such holes or apertures is invention. determined in accordance with certain desjgn parameters Referring to FIGS. 1 and 2, there is shown an acoustic of the acoustic filter, 'as discussed more fully below. In filter or mute 10 having an elongated conical entry tube 12, chambers 26 and 40 is loosely packed a porous filler ma­ preferably formed of sheet metal, having a fluid entrance terial 74 for reducing the free volume of said chambers, port 14 and a flange 18 adjacent said port, said flange 70 and which may be a mineral fiber or wool such as fiber being adapted to be connected by means of bolts 20 to glass, asbestos, or similar material, or may be in the form the exhaust of a unit, e.g., an internal combustion engine, of a metallic wool or fibers such as steel wooL The filler to attenuate the undesirable frequencies of the acoustic matedal need not be a sound-absorbing material, so long energy of the fluid discharged from said unit. as it is of a foraminous or porous nature and has a plu- The enlarged flared end 22 of the conical tube 12 has 75 ramy of small passa~es or interstices of more or less uni- 2,990,90'i" 5 6 forni size dIstributed therethrough and which may be in wee volume of the resonator. The resonator will respond communication with each other. When loosely packed to a frequency band which is maximized at the theoretical into chambers 26 and 40, the small passages and inter­ response frequency, as shown more clearly hereinafter. stices of the filler material are also in communication with The fluid and 'acoustic wave energy then pass from the the entrance ports or holes 70 and 72. 5 passageway 61 into the inner circular entrance 64 of the While the use of fiber glass 74 as a filler in chambers annular passage 60, progressing outwardly therethrough 26 alld 40 constitutes a preferred embodiment, I do not in a radial direction between the adjacent walls 34 and 52 rely on the Use of this material or the other filler mate- of the "pancake" 24, to the circular exit port 66 of pas- rials noted above for their sound-absorbing properties, sage 60. Hence, it is seen that the take-off of the fluid and hence filler materials of low sotind-absorbing proper- 10 flow from the first filter element comprising the resonator ties can be used. Thus, for e~ample, a series of corrugated chamber 42 to the second filter element in the form of the sheets 76 (see FIGS. 6 and 7) can be connected across "pancake" construction 24 comprising the annular cham­ walls 28 and 34 of chamber 26 and across walls 46 and 52 bers 26 and 40, is located at an angle to the original di­ of chamber 40, the sheets being juxt,aposed adjacent each rection of fluid flow. The surfaces of the second filter other to form a large number of small passages 77 in 15 element can be made either parallel to, or at suchan angle communication with holes 70 and 72. These corrugated to the oJ1iginal direction of fluid flow so that no substantial sheets do not function essentially as a sound-absorbing degree of wave reflection and thus no substantial degree material, but rather to reduce the free volume of cham- of interference phenomena are encountered. It is seen in bers 26 and 40, ,and form the minute passages or conduits FIG. 1 that the filter surface 34 of the "pancake" 24 is 77, whereby the acoustic characteristics of the fluid are 20 positioned at about right angles to the original flow of modified to selectively absorb the acousNc energy of the fluid while the filter surface 52 of the "pancake" is at an desired portion of the frequency spectrum. acute angle to such flow. For example, the device of FIG. 1 can be designed It is to be observed that along the path of flow of the and employed, where the fluid has an acoustic spectrum fluid, at each point, the circumference at the annulus 34 with a resonance peak at about 50 c.p.s., to attenuate 25 is many times greater than the linear dimension of each of the low frequencies of the acoustic spectrum of a fluid, the chambers 26 and 40 in a direction transverse to the e.g., frequencies between 20 and 150 cycles per second circumference. (c.p.s.), with a maximum response at 50 c.p.s., the funda­ The flow of fluid through annular passage 60 over ports mental disturbing frequency of the particular fluid. The 70 and 72 in the passage walls 34 and 52 is in a direction fluid whose acoustic energy is to be attenuated passes into 30 substantially parallel to the plane of the port entrances the entrance port 14 of the conical tube 12, and passes or to such walls. The acoustic pressure waves generated axially through the tube, the passage 61 adjacent the by the fluid passing through passage 60 travel through the flated mouth 22 of the tube, and into the closed resonance ports 70 and 72 into the interstices or conduits, e.g., 76, chamber 42, the pressure of the fluid decreasing as:it passes formed by the filler material in the annular chambers 26 along conical tube 12 to the flared end 22 thereof. Since 35 and 40, and the fluid in said conduits is subjected to com­ resonance chamber 42 is full of fluid, the chamber is sub­ pressions and rarefications,causing attenuation of the jected to compressions and rarefications caused by the acoustic energy in the frequency range for which the sec­ propagation of wave energy. ond filter element comprising the "pancake" construction The conical tube 12 need not be 'a hom, and thus may 24 is designed. The frequency response of said second ~e designed to have no low frequency cut-off point. It 40 filter element is at higher frequencies the greater is the thus may act entirely as a diffuser to drop the entry or ratio of the areas of the port entrances 70 and 72, to the exhaust pressure. It is noted that the diffuser cone 12 free volume of the chambers 26 and 40, and also to the has a shape such that there are no reflective surfaces. annular area of passageway 60 in a direction transverse The percentage of the energy reflected from the diffuser to the annular chambers 26 and 40. It is thus seen that surface may, in fact, be held down to 15% or less, and 4;') by filling the chambers with a loose porous material, such essentially no back pressure is developed. The conical as fiber glass, so as to reduce the free volume of said entry tube, may, however, be designed as a hom to at­ chambers, for a given port area the ratio is increased, and tenuate acoustic wave energy of low frequencies at or the frequency response of the second filter element or below the resonance frequency of the resonance chamber "pancake" 24 can be made substantially higher than the 42, where the low frequency energy to be attenuated is of 50 frequency response of the first filter element comprising wave lengths to permit horns of reasonable dimensions the resonator 42, since the area of the port entrance 62 to be employed, for example, where acoustic energy fre­ to the unfilled free volume of the resonator chamber 42 quencies below about 300 c.p.S. are desired to be attenu­ is substantially less than the ratio of the total port areas ated;It is noted that there is no restriction between the 70 and 72 to the free volume of chambers 26 and 40. discharge end 22 of the diffuser cone 12 and the entrance 55 Hence, it is seen that the acoustic filter 10 comprises port 62 of the resonator chamber, said end 22 and en­ a low frequency response free volume resonator cham­ trance POlt 62 being of substantially the same area and in ber in series with a high frequency response filled volume alignment with each other. The entrance port 62 to the annular "pancake" element, these elements having a resonator is normal to the axis of fluid flow through the beneficial combined effect -as illustrated by curve A in conical tube 12, resulting in little or no energy loss due 60 the plot shown in FIG. 9. Curve A represents the acous­ to resistance to flow into and out of the resonator. Thus, tic attenuation or absorption characteristics of a par- all of the energy of the fluid is recovered except that ticular filter constructed as illustrated in FIG. 1. The amount which is used up in the entropy of the compres­ filter was designed to produce maximum noise suppres­ sion-rarefication cycle of the acoustic filter. This amount sion at 50 c.p.s., the greatest noise producing frequency of energy loss is an insignificant fmction of the total 65 of the particular fluid involved, with intermediate noise energy. suppression of relatively constant value over a relatively The resonator 42 is preferably tuned as a band rejec­ broad frequency band ranging from about 200 c.p.s. to tion filter which may have a fairly sharp absorption peak about 700 c.p.S. and a cut-off at about 7000 c.p.s. at about the fundamental disturbing frequency of the fluid Illustrating the principles of my invention, and re­ passing through the filter. Thus, the resonator is, for 70 ferring to FIG. 9, by properly proportioning the ratio of example, designed to have a resonance point at the fre­ the port area 62 to the volume of resonator chamber 42, quency resonance point of the fluid stream or at the fre­ the device may be built so as to produce maximum fre­ quency at which maximum attenuation is desired, e.g., quency response of the resonator at 50 c.p.s., and it is in the above example 50 c.p.s., by properly proportioning seen that the filter 10 will absorb a maximum of 60 the area of the port entrance 62 of the resonator to the 75 decibels of acoustic energy at this peak noise disturbing 2,9-90,907 7 8 frequency with an -attenuation of between 25.5 and 60 a direction perpendicular to the plates Or chamber walls decibels. at fre@encies between 20 and 50 c.p.s., and 34 and.52. about 25.5 to 60 decibels between 200 and 50 c.p.s. Thus, for a sharply tuned resonator, the plates should Hence, the. noise .level at· the peak disturbing' frequency be made parallel and the thickness of the chambers 26 of, 50 c.p.s. is reduced 60 decibels from the original 5 and 40 uniform along the entire radius R of the annular noise level. passageway 60. The resonance frequency f of the cham­ While the resonator element 42 of the acoustic filter ber at any cross section perpendicular to the radius, such 10. attenuates the acoustic energy at low frequencies, the as represented by the dotted line X in FIG. 1, is pro- "pancake" element 24 of the filter is designed to co­ portional to KyTIH2 where K is a constant depending operate with the resonator to attenuate the acoustic en­ 10 on the velocity of sound and the area and spacing of ergy at high frequencies, of the same fluid previously the ports, T is the average value of the thickness of the treated in the resonator. For example, if the peak dis­ chambers 26 and 40 measured at any point along the turbing frequency at high frequency levels of the fluid radius, and H is the avemge height or thickness para1lel is 400 c.p.s., and it is desired to obtain maximum at­ to the axis of the annular passage measured at such point tenuation at this frequency in the "pancake" element 24, 15 and at the passage inlet 64. Where the values of T and employing the equation H are maintained constant at each cross section along Al!4 the radius, the frequency to which the chamber is tuned TlI!2=Kf at each cross section will be the same and will be directly proportional to the ratio ydlp2, where d is the diameter where A is. the total port area,. V is the free volume, f 20 of the hole and p is the distance between the holes. is the frequency and K is a constant, the total volume If, however, Tor H, or both T and H are varied along. of chambers 26 and 40 would have to be the radius of the annular passageway 60, then the fre­ ( ~)2 quency response of the elemental sections of the annular 400 passage taken transverse thereto, for example, at the sec- 25 tion located at the place marked X in FIG. 1, without or 1Al4 of the volume of the resonator 42 for equal port changing the diameter or spacing of holes 70 and 72, areas; or for equal volumes of chambers 26 and 40, and will vary, becoming greater in the direction of flow as resonator 42, the port areas 70 and 72 of chambers 26 the value of yTI H2 increases and decreasing in the di- and 40 would have to be 4096 times that of the port area 62. of the resonator. To meet this, the sum of the 30 rection of flow as the value of yTIH2 becomes smaller. port areas formed by apertures 70 and 72 are made as Thus, the frequency band may be broadened to the side of the higher frequencies by increasing the ratio of the large as is practical by employing the "pancake" con~ struction and the free volume of chambers 26 and 40 is yTIH2 as for constant values of d and p or by increas­ substantially reduced by loosely packing said chambers ing the. ratio of ydlp2 for constant values of yTIH2, as with the filler material described above. 35 will be understood by those ski1led in the art. In the Based onthe aforementioned criteria, it has been found form illustrated in the 'figure I prefer to vary the that for most applications the total open area represented value of yTIH2 rather than vary the ratio of dip, since by the apertures 70 in wall 34 or apertures 72 in wall this ratio ydlp2 is determined. in part by the requirement 52 will not exceed 20% of the overall area of the wall '1() that the values of d and p be such as to retain the surface. In fact, in most applications the open area as packing in place and particularly since d and p are dec represented~ by the apertures in the particular wall will pendent variables, whereas T and H are independently represent considerably less than 20% of the overall area variable. of the wall. For example, on lower frequency applica­ tions from 200 to 400 cycles per second, the percentage In the form as illustrated in FIG. 1 the "alue of ...jTIH2 figure is approximately 5%. Only in exceptional ap­ .15 increases in the direction of. flow from the entrance to plications, for example in the case of gas jet silencing, the exit of the annular passageway and the resonant fre­ where frequencies approach 9600 cycles per second is it quency is thus a broad band of frequencies with the necessary or feasible to have the percentage of open area lowest frequency corresponding to the geometry at the exceed 20%. entrance to the annular chamber. It is essential, however, that the open area, that is the 50 Thus, as seen in curve A of FIG. 9, the acoustic energy spacing. and size of the apertures 70 and 72, be pre­ is attenuated by the "pancake" structure 24 over an ap­ selected according to the resonant frequency to be ab­ proximate frequency band ranging. from about 200 to sorbed. Thus with the foregoing criteria in mind, with about 700 c.p.s., with an acoustic absorption of about 25.5 higher frequency noises, more apertures are used in order decibels over this range. From about 700 to about 7000 to achieve a smaller effective volume behind each aper­ 56 c.p.s. the attenuation decreases until at 7000 c.p.s. and ture. On the other hand, fewer apertures are employed higher there is no longer any attenuation of acoustic with lower frequencies to obtain a larger effective volume energy, so that 7000 c;p.s. is the approximate so-caIled behind each aperture. cut-off of the acoustic filter 10. If the "pancake" element While the above criteria enable the "pancake" element 24 of the filter were omitted, the noise attenuation would 24 of the filter 10 to absorb in the region of resonance GO decrease rapidly above about 100 c.p.s., with a cut-off at 400 c.p.s., in the illustrative example used in connec­ point at a much lower frequency than when the "pan­ tion with FIG. 9, plot A, in order to broaden the fre­ cake" element is used. Hence, this shows the marked ad­ quency band of frequencies which are absorbed so that vantage of employJng the "pancake" element 24 in con­ the filter will absorb not only in the region of resonance junction with the resonance chamber 42 where the acous- but also on both sides thereof, i.e., so that it will also G5 tic spectrum has considerable energy in the frequencies absorb frequencies higher than resonance, or frequencies above the low resonance peaks. This is readily apparent lower, than resonance in substantial proportion, having from FIG. 9, in that attenuation of both low and high chosen the volume ,and port areas, the thickness of the noise producing frequencies can be attained, with a broad­ chambers· 26 and 40 at any cross section of the "pan­ ening of attenuation at the high frequencies, but with cake" 24 may be adjusted according to the following 70 little or no absorption of energy in the frequencies above principles. the audible range, I.e., 15,000 to 20,000 cycles or more. The. absorption characteristics of the chambers 26 and One application of the acoustic filter or mute 10 de­ 40 depend on the diameter of the ports and. number of scribed above is in -an internal combustion engine, as il­ porfs, i.e;, their spacing,the height of the. annular passage lustrated in FIG. 8. Numeral 80 represents the engine, and. the -linear dimension, of the chambers 26' and' 40 in 75 with the short pipes 82 representing the exhaust outlets 2,990,907 9 10 from the individual cylinders to the exhaust manifold 84. rngher range of the acoustic speCtrum. Iii this modifica­ The acoustic filter 10 is connected to the end of manifold tion the acoustic filter 90 comprises a conical tube 92 84, with the small end 14 of the conical tube 12 of the similar to tube 12 of FIG. 1, except that it is of shorter mute connected into the manifold. length. Tube 92 has at its reduced end a fluid port 94 As an illustrative example of results which have been 5 and a flange 96 adjacent said port, said flange being obtained, when operating the above engine with a con­ adapted to be connected by means of bolts 98 to the in­ ventional muffler, 7Vz inches of water pressure were gen­ let of a unit such as a compressor (not shown) to attenu­ erated at point 86 just ahead of the prior art muffler and ate the undesirable frequencies at the noise peaks of the' 9Vz inches of water manifold pressure at the opposite fluid entering said unit. end 88 of the exhaust manifold, making a 2-inch drop in 10 The enlarged flared open throat 100 at the opposite pressure. end of conical tube 92 is connected to a "pancake" con­ With the mute 10 of the design described above, the struction 102 similar to "pancake" 24, except as to the pressures were 1Vz inches of water at the discharge end outer "pancake" element 104. "Pancake" 102 is com­ 86 of the manifold ahead of the mute and 2Vz inches at posed of an annular chamber 106 having a rounded or' the opposite end 88 of the exhaust manifold. The drop 15 bumped annular wall 108, the inner periphery of which. in back pressure in the manifold was equal to an increase is suitably connected as by welding to the outer periphery of 30% in the volume of scavenging air in the cylinders of the wall of tube 92. The outer periphery of bumped: and a drop of 17 0 in the exhaust temperature, equivalent wall 108 is bent to form a short outer wall 110, wal1110. to 10% increase in horsepower output of the engine. then being bent inwardly normal to the axis of tube 92 to The temperature at the exhaust 82 from each cylinder into 20 form an annular wall 112, which is connected to the the manifold was more uniform in the case of the in­ flared end of the conical tube, e.g., as by welding at 114. vention mute 10 than for the conventional muffler. Thus, annular chamber 106 is bounded by walls 108, 110,. While I do not wish to be bound to my theory of the 112 and the wall portion 116 of tube 92. reasons for the above advantages of operation employ­ The other element 104 of the "pancake" 102 is a cylin,- ing the invention mute 10, I believe that in the case of 25 drical chamber 118 which is spaced from annular cham­ the mufflers of the prior art, the baffling therein introduces ber 106 in a direction axially along tube 92. Chamber frictional resistance to flow and also introduces reflective 118 is formed of an inner conical wall 120, the apex 122', surfaces which generate wave interferences and thus of which is on the axis of tube 92. At the outer periph­ standing waves in the discharge conduit, which act to ery of conical wall 120 is connected a dome shaped head throttle the flow of the exhaust gases and create back 30 124. The chamber 118 has a diameter about equal to­ pressure. In the structure of my invention these are the diameter of the outer wall 110 of chamber 106. The: largely eliminated since I rely not on the damping result­ element 104 forming the cylindrical chamber 118 is con-· ing from friction or resistance to flow, but on the acous­ nected to the flared end 100 of tube 92 by means of a tic resonance properties of the acoustic filter or mute of series of arms or lugs 128 connected as by welding at the invention, which does not rely on the introduction 35 spaced intervals about the outer wall 110 of chamber of friction and resistance to flow. With a conventional 106, the opposite ends of said lugs being connected to muffler the cylinders of engine 80, discharging against the outer wall 129 of chamber 118, to thereby connect the standing wave produced in the manifold 84, would said walls and maintain them in spaced apart relation. also have a higher temperature than those cylinders of the Thus, the adjacent walls 112 and 120 are also spaced engine not discharging against such standing wave, re­ 40 from each other forming an annular passage 130 be­ sulting in non-uniform cylinder temperatures. Since my tween chambers 106 and 118, communicating with the mute acts to decrease the exhaust back pressures against open flared end 100 of conical tube 92. The annular the engine cylinders, it also improves the operation of the passage 130 decreases in depth from the inner end 132 engine. thereof to its outer end 134, which communicates with Assuming a wave length of 20 feet in the embodiment 45 the atmosphere. of FIG. 8 employing a conventional muffler, any reflec­ A series of concentric rows of round spaced holes 136 tions at 10 feet or any odd multiples thereof create is formed in the annular wall 112 of chamber 106 and a standing waves in the manifold 84, e.g., at the dotted line series of concentric rows of similar holes 138 is formed position 89, depending on the characteristics of the pipe, in the adjacent conical wall 120 of chamber 118, the size and the cylinder discharging into that area sees a high 50 of said holes being made in accordance with the design pressure area and does not scavenge properly, thus reduc­ criteria previously explained. In chambers 106 and 118 ing efficiency. Such standing waves are not produced em­ is placed a filler material of the type described above, ploying my acoustic filter. In my filter 10 the "pancake" e.g., fiber glass, and represented by numeral 140, to form element interposes minimum surface transverse to the di­ numerous small conduits in communication with the ports rection of propagation of the acoustic wave, thus prevent­ 55 or holes 136 and 138, and to substantially decrease the ing the system from creating standing waves. free volume of chambers 106 and 118. In this manner It is to be emphasized that in order to properly build the ratio of the total area of ports 136 and 138 to the up a resonant frequency, the opposing walls 52 and 34 of total free volume of chambers 106 and 118 is increased the "pancake" sections or chambers 26 and 40 must be in accordance with the invention principles as previously formed of relatively rigid matel1ial, preferably sheet metal. 60 explained. Such a construction, as previously mentioned, contrasts J:t will be observed that the annular passageway flares the prior art in which fabric walls, for example, are em­ in the direction of flow so that the value of yT/H2 ployed to absorb sound by flow resistance rather than by increases in the direction of flow, T remaining substan­ resonating or reactive surfaces. Thus, construction of tially constant. Thus, the resonant frequency of the the present invention is directed towards creating an 65 chamber is in a broad band with the lower frequency acoustic filter with minimum fluttering, paneling, and/or determined by the geometry of the intake entrance to vibration of the sidewalls defining the flow path. As a the annular chamber. For example, these characteristics consequence, of such a stiff or relatively rigid structure, are illustrated by curve B in the plot of FIG. 9, which the pressure drop and flow resistance through the unit curve represents the operational characteristics of filter may be held to a minimum. 70 90. It is noted that the acoustic filter 90 illustrated Referring now to FIGS. 3, 4 and 5, there is shown a by curve B is designed to have a resonance frequency modification of the acoustic filter 10 of FIG. 1, and over a broad range maximizing in the range of about wherein the low frequency free volume resonator 42 is 300 to 800 c.p.s. with strong absorption in the range eliminated. Thus, the filter of the instant modification of 20 to 1500 cycles, falling off above 1500. Above is applicable where the resonance frequencies are in the 75 1500 cycles, absorption takes place in smaller degree 11 1-2 thrQughout the audiple range up to 20,000 cycles, with cave lookingfrom.the flared end 100 of tube 92, or wall a practical cut-off just above the audible range. This 120 may be parabolic, or even in the form of a plane filter unit would accordingly act efficiently on fluid gen­ parallel to the adjacent wall 112 of chamber 106. erating an acoustic spectrum resonant in the range of Further, it is noted that element 104' of the "pancake" 300 to 800 cycles and having considerable energy in 5 structure .10.2. is in. the· form of a cylindrical chamber the frequencies above and ,below such resonant fre­ rather than an annular chamber, such as 106. The quencies. Passage of the fluid, e.g. by suction, through central portion 150 of cylindrical chamber 118, located the annular duct 130 of the "pancake" between the ad­ opposite the flared throat 100 of tube 92, functions to a jacent walls 112 and 120 of chambers 106 and 118, causes degree as a resonator chamber which is filled with a the wave energy of the fluid to be propagated through 10 loose filler material. Also, the central portion 152 of the ports 136 and 133 of the "pancake" into the small conical wall 120, located opposite the flared throat 100 conduits formed in the fiber glass or other filler material of the conical tube 92, serves to reflect acoustic waves 140 to attenuate the acoustic energy of the fluid at the striking said central wall portion 152 back into the conical peak frequencies of noise level noted above and for diffuser tube 92, and aids in dissipation of such acoustic which the filter 96 is designed. The fluid then passes 15 wave energy by multiple reflections. through conical tube 92 and out the exit port 94 thereof, While I have described the small holes or ports 70, tube 92 again functioning essentially as a diffuser. nand 136, 138 as being formed in concentric rows in As illustrated in ,FIG. 9, curve B, filter 90 is designed the adjacent chamber walls, these apertures need not for maximum attenuation at 300-800 c.p.s. with the fre­ be in concentric rows, and may, for example, form quency band for maximum response of the filter broad- 20 squares with the holes approximately equally spaced from ened to give an attenuation of 35 to 36 decibels and each other. an attenuation varying down to about 27 decibels in the I may cover the outer peripheral annular opening of lower frequencies and down to about 10 decibels at the annular passage 60 or 130 with a screen, if desired, 10,000 cycles. It is thus seen that the "pancake" con­ to permit flow of fluid therethrough, wIJile preventing struction 102 of filter 90, with the concentric rows of 25 the ingress of foreign materials, such as bits of paper ports 136 and 138 communicating with the annular pas­ and the like from outside the unit, into said annular pas­ sage 130 and with the interiors of chambers 106 and sages. The use of such a screen does not affect the lUI, which have their free volume reduced by the porous functioning of my acoustic filter in the manner previously filler material l'<}O, results in a broadening of the fre­ described. quency range at which maximum absorption takes place. 30 While the "pancake" elements of the "pancake" struc­ Because of the increasing width of duct 130 in the direc­ ture 24 or 102 have been shown to be round or circular, tion of fluid flow from the fluid entrance 134 of the duct these "pancake" elements can be square or polygonal in at the outer periphery of the "pancake" structure 102 outer peripheral form. The term "annular chamber" to the inner duct exit 132 adjacent the flared end 100 and "annular passage" as employed in the specification of the conical tube 92, the range of maximum absorp- 35 8J,ld claims is intended to denote a ring-shaped chamber tion is broadened mostly at frequencies above the or passage, and said ring may be of circular, square, or resonance frequency of 400 c.p.s., i.e., from 300 to 800 polygonal shape, so long as it is continuous. c.p.s. From the foregoing, it is seen that I have designed an Again, as heretofore mentioned in conjunction with acoustic filter or mute which absorbs selected frequencies FIG. 1, it is essential to the present invention that the 40 of the acoustic spectrum, which may be low or high fre­ apertures 136 and 138 be of pre-selected size and spacing quencies, or both low and high frequencies. The filter according to the resonant frequency to be absorbed and can be designed for a particular cut-off frequency, that is, in accordance with the criteria heretofore explained. a frequency below which or above which, the filter no Also, except for applications of unusually high frequency, longer absorbs or attenuates the acoustic energy in sub­ the percentage open area of the walls 112 and 120, as stantial amounts. These advantages are obtained with­ represented by the ports 136 and 138, respectively, will 45 out introducing any substantial amount of resistance to not exceed 20%. Also, of course, it is necessary as here­ the flow of fluid whose undesirable noise peaks are being tofore mentioned that the walls 112 and 120 be formed attenuated by my device, and hence without development of relatively rigid, stiff material in order to properly of any substantial amount of back pressure. build up resonant frequencies. In consequence, it is The principles of my invention as embodied in my preferred to form these walls, as well as the entire unit 50 mute aT filter are applicable to any unit in which a cyl­ of FIG. 3, of metal. inder and piston employs a movement of a compressible While in the filter structures shown in FIGS. 1 and 3, fluid which discharges under pressure from the cylinder the radial passages 60 and 130 become narrower from into a lower pressure area, and including compressors, the inner to the outer periphery thereof, such passages hot air engines, steam engines, and internal combustion can be made narrower in the opposite direction, Le., they 55 engines, such as diesel engines. My filter can also be may become narrower proceeding from the outer to the employed in the suction and discharge of a fan, a rotary inner periphery of such annular passages. This does not compressor, jet engine, gas turbine, relief and safety change the function of the "pancake," but tends to valves, etc. broaden the range of frequency response thereof in a While I have described a particular embodiment of direction above or below the resonance response fre- 60 my invention for the purpose of illustration, it should be quency at the entrance to the passages, to thereby understood that various modifications and adaptations broaden the frequency range within whioh disturbing fre­ thereof may be made within the spirit of the invention as quencies of the acoustic spectrum are absorbed. set forth in the appended claims. The above specific examples and values were given What is claimed is: merely to illustrate the principles of my invention and 65 1. An acoustic filter, comprising in combination: first are not to be taken as limiting my invention. Those substantially rigid wall means defining an annular cham­ skilled in this art will known how to apply the above ber about a given axis; second substantially rigid wall principles to give the desired degree of attenuation at means defining another chamber axially spaced in one various frequencies and to obtain the desired position of direction from said first chamber; a conduit coupled to the resonance frequencies of the filters in the acoustic 70 the inner side walls of said annular chamber and extend­ spectrum and the width of the band of frequencies within ing axially therefrom in an opposite direction; a filler ma­ the resonance range of the filters. terial in said chambers, said filler material forming a In FIG. 3, it is noted that chamber wall 120 has been plurality of small conduits defining the free volume of shown as conical. However, such wall may have other said chambers; and, means defining a plurality of ports in geometric configurations, and may, for example, be con- 75 each of the opposing sidewalls of said chambers, the area 2,990,907 13 14 of said ports and said free volume being pre-determined to the inner sidewalls of said annular chamber and ex- according to the resonant frequency to be absorbed in tending a,'tially therefrom in an opposite direction; a filler accordance with the equation: material in said chambers; said filler material forming a Al/4 plurality of small conduits defining the free volume of Kf= Vl/2 5 said chambers; and, means defining a plurality of ports in each of the opposing sidewalls of said chambers, the in which A equals the area of said ports, V equals said area of said ports and said free volume being pre-deter- free volume, K equals a ,constant and f equals said reso. mined according to the resonant frequency to be absorbed nant frequency. in accordance with the equation: 2. An acoustic filter, comprising, in combination: first 10 Ail4 metallic wall means defining an annular chamber about a Kf= Viii given axis; second metallic wall means defining another chamber axially spaced in one direction from said first in which A equals the area of said ports, V equals said chamber; a conduit coupled to the inner sidewalls of said free volume, K equals a constant and f equals said reso- annular chamber 'and extending axially therefrom in an 15 nant frequency. opposite direction; a filler material in said chambers; 5. An acoustic filter, according to claim 4, in which said filler material forming a plurality of small conduits said means supporting said chambers in spaced apart rela- defining the free volume of said chambers; and, means tionship comprise a plurality of lug members rigidly in- defining a plurality of ports in each of the opposing side- terconnected between said chambers. walls of said chambers, the area of said ports and said 20 6. An acoustic filter, comprising in combination: first free volume being pre-determined according to the reso- substantially rigid wall means defining an annular cham- nant frequency to be absorbed in accordance with the ber about a given axis; second substantially rigid wall equation: means defining another chamber, said another chamber Al/4 being of pancake shape and axially spaced in one direc- Kf= Vil2 25 tion from said first 'chamber; a conduit coupled to the inner sidewalls of said annular chamber and extending in which A equals the 'area of said ports, V equals said axially therefrom in an opposite direction; a filler mate­ free volume, K equals a constant and f equals said reso­ rial in said chambers; said filler material forming a plu­ nant frequency. rality of small conduits defining the free volume of said 3. An acoustic filter, comprising, in combination: first 30 chambers; and, means defining a plurality of ports in each substantially 'rigid wall means defining an annular cham­ of the opposing sidewalls of said chambers, the area of ber about a given axis; second substantially rigid wall said ports and said free volume being pre-determined ac­ means defining another chamber axially spaced in one cording to the resonant frequency to be absorbed in ac- direction from said first chamber; a conduit of truncated cordance with the equation: conical shape; said 'conduit having its larger end coupled 35 Al/4 to the inner sidewalls of said annular chamber and ex­ Kf=ViI~ tending axially therefrom in an opposite direction; a filler material in said chambers; said filler material forming a in which A equals the area of said ports, V equals said plurality of small conduits defining the free volume of free volume, K equals a constant and f equals said reso­ said chambers; and, means defining a plurality of ports 40 nant frequency. in each of the opposing sidewalls of said chambers, the area of said ports and said free volume being pre-deter­ References Cited in the file of this patent mined according to the resonant frequency to be absorbed UNITED STATES PATENTS in accordance with the equation: 1,796,441 Compo Mar. 17, 1931 Ail4 1,934,463 Hartsock Nov. 7, 1933 Kf=Vl/2 2,020,903 Nickelsen Nov. 12, 1935 2,037,884 Day Apr. 21, 1936 in which A equals the area of said ports, V equals said 2,058,932 Wilson Oct. 27, 1936 free volume, k equals a constant ,and f equals said :reso- 2,164,365 Wilson July 4, 1939 nant frequency. ISO 2,323,955 Wilson July 13, 1943 An in 4. acoustic filter, comprising, combination: first 2,869,671 Schlachter et al. Jan. 20, 1959 substantially rigid wall means defining an annular cham- ber about a given axis; second substantially rigid wall FOREIGN PATENTS means defining another chamber axially spaced in one 664,331 France Apr. 22, 1929 direction from said first chamber; means supporting said M 275,495 !Italy June 24, 1930 chambers in spaced apart relationshipia conduit coupled 493,538 Great Britain Nov. 10, 1938