July 4, 1961 W. S. EVERETT 2,990,907 ACOUSTIC FILTER Filed June 11, 1959 3 Sheets-Sheet 1 74 40 0.---.>2 .58 to 70 "1l!I.~a--34 22. /8 20 , 30 28 _~X 70 24 ~~~~~~~f;:;26 \:l Z 58 ~2B.6. 32 =- 76 y-r~.z. 77 J~ 00 7- rf ~8 /' oX. / 54 / 0 I ~' 40 7~- 58 70 8'1 July 4, 1961 W. S. EVERETT 2,990,907 ACOUSTIC FILTER Filed June 11, 1959 3 Sheets-Sheet 2 120 104~~--;18~ INVE IV TOR. 140 ILH£L~. ErR.£TT :By ~~ ATTCRN£'t, July 4, 1961 W. S. EVERETT 2,990,907 ACOUSTIC FILTER Filed June 11, 1959 3 Sheets-Sheet 3 o o No o (\,J o , .... 0 In° '\ 0;)2 I""'" I\. t- ID ....... ~ '\ \Cl ~ Q , I\. \ Z "- ", 0 I\. ~ tJ ~ ~ ~ I\. ru U) ~ " 1\ IL1 -r-J 1'\ ll. c:r 1\ r\ V'J '\ -OLiJ 1110,.J C()~v t- ~ lD 1..1 U) Z -t >- v II) Z UJ 1/ ::> (\j C!J IIJ \ J Q: I \ IJ.. I '" 1'0... I ........ II . I--.. I--~ ~ 1....- .... ~ 1/ IJ INVENTOR WILHELI'1 S. EVERETT B)/ ~t1~ ~ 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.