US2100655A - Sound attenuating device - Google Patents

Description

Nov. 30, 1937. R. B. BOURNE SOUND ATTENUTING DEVICE Original Filed Aug. 9, 1935 INVENTOR BY ROLAND B gaap/v5 branches which are acoustically coupled to a main branch is seen to be in the saving in length. In

Patented Nov. 30, 1937 l T I* UNITED STATES PATENT OFFICE 2,100,655 SOUND ATTENUATING DEVICE Roland E. Bourne, Hartford, Conn., assignor to The Maxim Silencer Company, Hartford, Conn., a corporation of Connecticut Original application August 9, 1935, Serial No. 35,428. Divided and this application October 26, 1935, Serial No. 46,846

14 Claims. (Cl. 181-59) The present invention relates to sound attenubranch is acoustically coupled as are non-reflexed ating devices, such as are used for example in sidebranches of the same frequency charactersilencing noisy engine exhausts and the like. istics.

Such silencing devices make use of so-called side- One main advantage of a reiiexed linear sidesound conducting channel to produce attenuation cases where a limited space is available for inof sound waves therein, either by a relatively distallation purposes, the use of reflexed siderect interaction between the channel and the branches permits the attenuation of lower fresidebranch or by the somewhat more complex quencies than would otherwise be possible. For action of a wave filter where a plurality of sidea sidebranch reflexed once, the saving in length is branches are spaced apart along the main chansubstantially fifty percent. The cross sectional nel by distances bearing a definite relation to the area of the reflexed sidebranch is generally made major sound Waves to be attenuated. the same as that of a non-reilexed sidebranch in I have discovered that a very material saving order to secure the same acoustic properties, of space in commercial silencer construction can since the reiiexing is done coaxially this results l5 be obtained by folding the sidebranch back upon in a greater overall diameter for the silencer; but itself one or more times, the successively folded the diameters of silencers employing linear sideportions being generally coaxial and of progresbranches are usually relatively small so that this sively increasing or decreasing diameter. The offers no serious objection. Furthermore, the diinvention nds use primarily in so-called linear ameter is not increased in the same proportion as sidebranches, which by reason of their length the length is decreased, since the areas of the being material in proportion to the wave length passages are functions of the squares of their diof the particular sound waves vunder consideraameters. I have found that it is desirable to tion possess attenuating properties which are make the cross sectional area of the reliexed sidefunctions of the length of the sidebranch rather branch a function of the length of the sidebranch than its volume. In particular the utility of the in the same manner as in the case of a non-reinvention will be especially marked in the case of flexed sidebranch of the same acoustic properties. a linear sdebranch closed at its end and folded This applies to tapered sidebranches as well as to upon itself in such a manner as to retain subthose of uniform cross sectional area. Slight stantially the linear characteristics of a sidevariations of the form of the sidebranch from branch of the same total overall length; although What would be given by a strict adherence to some of the structural advantages of the inventheory are of relatively slight importance. tion may be applied to sidebranches of the com- Refiexed sidebranches may be used in conjuncpound resonator type, where two or more sepation with an associated main channel to form any rately acting resonators are connected in series type of selective acoustic device that can be by a restricted conducting passage. Acoustically formed with non-reflexed linear sidebranches. these two types of sidebranch are wholly distinct, They may be used in connection with other kinds but in the manner in which space in construction of sidebranches and with folded or reexed main is saved the invention presents features affecting channels. Two reilexed sidebranches may be both types. disposed in parallel or one non-reflexed sidesidebranches folded on themselves in such a branch may be disposed in parallel with a reway that they preserve the characteristics of a flexed sidebranch. ther advantages and apsingle resonator of the same total acoustic length plications are disclosed as the specification prowill be referred to throughout this specification ceeds or will be apparent to those skilled in the as reflexed sidebranches. Careful tests with art from the following description.

such sidebranches show that it is readily practi- Referring to the drawing, cable to calculate their acoustic properties in sub- Figs. 1 to 4 inclusive are diagrammatic views stantially the same manner as though they were showing the application of the invention to closed not reflexed. provided that proper care is given reilexed linear sidebranches of non-uniform cross to the design of the sidebranch at the point sectional area; no where reflexing occurs so that the device is not Fig. 5 is a similar view showing the applicaconverted into a compound resonator. Reflexed tion of the invention to combinations of closed linear acoustic sidebranches are substantially as reflexed linear sidebranches partly of uniform effective in attenuating pertinent sound frequenand partly of non-uniform cross sectional area; cies in a main acoustic channel to which the side- Figs. 6 and 7 are similar views showing further 55 applications of theinvention to combinations of reflexed linear sidebranches partly of uniform and partly oi non-uniform cross sectional area; and Y Fig. 8 is a graph showing the frequency vs. attenuation characteristics of the embodiment of the invention shown in Fig. 4.

The present application is a division ci my copending application Serial No. 35,428, ledAuguSt' 9, 1935, on which Patent No. 2,675,265 issued March 30, 1937.

Unless otherwise stated, the term casing as used herein includes an inlet opening and an outlet opening in the respective endsthereof.

The embodiments of the invention herein shown are largely schematic and all are shown as employing Ycircular cross sections. outlet connections may be pipe anges, sleeves or any other suitable arrangement for connecting the device to a conduit wherein sound waves may occur.

In a reexed sidebranch which is Vfunctioning linearly theV sound wave encounters no changes in acoustic imped-ance which are not functions of' the distance along the sidebranch from its point of coupling .to the main channel. In this way `the acoustic Voperation of the reexed side- Ybranches which form the subject of the present invention is distinguished from the action of compound resonators, in which two or more sidebranches are coupled in series to the main channel. The separate sidebranches of a compound resonator are connectedby an acoustic element of relatively low conductivity, the sound wave encountering at the connecting Zone a relatively sudden change or impedance which is no longer a function of the distance from the point ci coupling to the main channel. structurally some of the features of the present invention may iind use in theY application of compound resonators as silencing devices, but the acoustic operation of the structural features when arranged to func-Y i tion as a single relexed sidebranch is wholly distinct from what it would be when arranged to Yfunction as two or more separate sidebranches in series or, in other words, va comigioundY resonator. The diierence in action maybe produced by a relatively slight structural change, such as substituting a restricted conductivity for theY evenness of the cross sectional area preserved vthroughout the reexed portion of the 'relexed main channel; but acoustically it results 1n entirely different eiects.

"-The principle of reflexing is applicable toV closed linear sidebranches whose areas change as a function of distance along their lengths'. For instance, al complete closed circular cone withcpen base has many useful acoustic propertiesas has been described in my Patent No. 2,017,744, granted October 15,1935. One disadvantage in'using conical sidebranches comes from the fact that they are Vsubstantially half a wavelength long for their fundamental response frequency, as compared to a closed cylinder vwhich is only onefourth wave long for its fundamental frequency, and therefore Y require a longer physical structure to attenuate the same sound frequency. The factv that the conical side branch responds to a iull integral series of overtones is, however, of great acoustic importance.V I have found that the acoustic properties of such a sidebranch are in no wise materially altered by reeXing while the length can ,be shortened in the same way as a cylindrical sidebranch. Y A

Fig. 1 shows a simple'applcation of a reflexed stantially after the manner Y scribed reflexed Aconical The inlet andA open end. It will be seen conical sidebranch. It comprises a cylindrical casing H15, aniinteriorly and coaXially disposed conical member Ii'li closed at its small end by a transverse header itl and an inner complete cone iS. The v-arious members are so proportioned that the cross sectional area of the sidebranch so formed varies with distance from the coupling zone E99 at the base of the cone to the apex subfor a true cone. For example, the annular opening i lo, the'peripheral opening lll, and the'open end ll2 of the cone Hi8 are each substantially equal to one-fourth of the area of the opening liiS. The above dev sidebranch is coupled to the main Vconducting channel H3 at-the yannular opening leg. YThe acoustic length L of the reileXed sidebranch is approximately equal to the sum vci the slant heights of the two cones H16 and Hi8. The frequencies oi maximum attenuation are given by @L 7TC=1, 2, 3, 4, etc. (l)

A closed conical sidebranch may be reilexed more than once without jeopardizing its operation acoustically. Figure 2 shows an embodimentV of w.)

the transverse header l i9, and a conical member i253 nested within the member H8. These three members are so proportioned thatr the acoustic passage alongthe sidebrancn suiers a progressive change in cross sectional area 4after the manner of a simple true circular cone but with no abrupt changes due to the folded construction. The cross l section areas at the points of reflexure ll, E22l are made according to the area a true cone would have at the corresponding distance from the that the sidebranch'is coupled to the main channel |23 through an annular 'opening lZli. Ihe acousticY performance of such a sidebranch is very closely equivalent to that of a corresponding simple non-reexed cone. Fig. 3 shows an end-pipe silencer suitable for intakes where no iurther beyond theY silencer is used and is particularly adaptedto attcnuating simple noises largely made up of a se 'es oifharmonically related overtones. It comor s a. pipe 525 to which is adjustably affixed a transverse header if carrying a short` cylin` drical memberY i'vafri-Xed at one end to the header, and having attached to its other end the single reiexed conical sidebranch 25. The Vcylinc'ier l2? extends a short distance beyond the inner endof the centrally disposed conduit 25 andthe reilexed conical sid-ebranch properly begins at the region denoted by the junction Zone iid between said cylinder @2i and the conical section t28. The main channel comprises the interior of the conduit 25, a portion of the interior'oi member l2? between the end of the pipe 25 and tho Zone iZQ, the annular space between the members 25' and i'i, and a suitable opening cr series ci openings itil in said cylinder i2? and located adjacent the header 126. This sidebranch is coupled to the aorementioned main channel at a point where the lat- YterY is reflexed after the manner described in my copending application Serial No. 14,370. The openings 53o might be placed in the header |'2'6 without changing the action in any way. By making the header |26 adjustable along the length of the pipe |25 it is possible to tune this sidebranch to existing noises within certain limits by varying the length of the interior of the cylinder |21 adjacent the zone |29 which acts as a part of the sidebranch.

Fig. 4 shows an embodiment of the invention wherein use is made of two releXed conical sidebranches acoustically coupled to a main sound conducting channel at spaced points along its length. It comprises a, cylindrical casing |35, and two coaxially and interiorly disposed reeXed conical sidebranches |36, |31 respectively, each having a common transverse header |38. By making the respective acoustic lengths L2 and L3 equal, there results an acoustic wave filter which operates in accordance with the following:

Cosh fy=Cos C -m (2) wherein L1=the length of the main channel between sections S2=the cross sectional area at the entrance to the sidebranches S1=the cross sectional area of the main channel.

rIhe transverse header |38, being of less diameter than the open bases of the cones, produces a contracted space around the cones which, in this embodiment, I have utilized for the introduction of a certain amount of acoustic resistance distributed along the main channel |39. This resistance is in the form of suitable sound absorbing material shown at |49 and held in place by a perforated cylindrical member |4|, giving sound waves access to the absorbent material within. The amount of sound absorbing material used depends upon the size of the device. 'Ihe major function of the sound absorbing material is to prevent series resonance in the mainv channel |39. Series resonance in sound attenuating devices is likely to result in reduced attenuation peaks especially if the resonant frequencies occur at ornear the resonant frequencies of the sidebranches. The actual acoustic performance of a device built in accordance with Fig. 4 is shown in Fig. 8. The amount of sound absorbing material used was small although its eiect .becomes increasingly important with higher frequencies as is shown by the increased minima in the dips of the curve. The important uationpeaks occur at values of vrC equal substantially to l, 2, 3, etc. A solution of Equation (2) shows that maximum attenuation occurs at points slightly below those above reployed in connection with a reiiexed Sidebranch of non-uniform cross sectional area to obtain desirable mechanical and acoustic characteristics. A cylindrical casing contains a reflexed conical sidebranch |45 and a reiiexed cylindrical sidebranch |41. The transverse header |48 common to both sidebranches is of such a diameter that the necessary area relations as previously pointed out are preserved in both the conical sidebranch |45 and the cylindrical sidebranch .551. The maximum cross sectional area of the conical sidebranch, for this disposition, is twice the constant cross sectional area of the cylindrical sidebranch. Attenuation peaks occur at values of wL Td equal to .5, 1.0, 1.5, 2.0, etc., L being the acoustic length, identical for each sidebranch. It will be seen that the acoustic coupling to the main sound conducting channel |43 is very favorable especially for sound waves entering the device in the direction shown by the arrow |59. Each sidebranch provides attenuation in such a manner that their combination produces no pass bands having theoretically aero attenuation.

Fig. 6 shows how the principle of the invention may be applied to a spark catcher type or" exhaust silencer. The device cal casing 55 having a top header i with a suitable outlet connection $1 and a bottom header |58 with an eccentrically disposed inlet connection |69. An intermediate header |10 having a centrally disposed opening |11 therein supports an upwardly extending centrally disposed tubular conduit |12. Said conduit i12 extends to a point in adjacency to the header |55 leaving a slot-like opening E13 which gives access to the reflexed acoustic sidebranch i 'it oi acoustic length L. Depending rom the interior transverse header |13 is a cynder |15 at the lower end oi which and mounted exterior thereof is a spinner' assembly |15, the outside or" which supports a short cylindrical member l'i! extending down a short distance below said spinner assembly. A refleXed conical sidebranch l extends from the bottom of the cylinder E11 to the bottom header |58, the latter in this case forming the releXing point. The acoustic length of the sidebranch |18 is generally made equal to that of sidebranch |14. The operation of the device is as follows. The exhaust gas bearing objectionable sound waves enters the device through the bottom inlet |59, passes upward around the outside of the conical sidebranch 515, is deflected downward by the transverse partition. |19 through the spinner assembly thence passes upward through the enlarged channel 219 formed by the cylinder |15, thence through the opening i1! into the channel |89 formed by the tubular conduit |12 and so on out through the outlet opening |81. The sound waves suffer attenuation due to the reflexted conical sidebranch E18 and the reflexe'd cylindrical sidebranch |14 which, it will be seen, are spaced apart along the main conducting channel. The entrained dirt, sparks, etc. are thrown out oi the gas stream by being downwardly directed by the spinner assembly l1@ into the sidebranch |19` from whence they may be removed through a suitable handhole Uli.

Fig. 7 shows a further modification of the invention employing threeI refiexed sidebranches. This embodiment is particularly useful in cases Where the length is of importance. Attenuation comprises a cylindriis providedfor three sets of integrally'related sound frequencies and no sound frequency can pass through the device Vwithout suiering substantial attenuation. It comprises a casing |85 having end' headers |85, |87, and suitable inlet and outlet connections. Aixed to the header |85 is an interiorly disposed cylinder |88 which extends to a point in adjacency with Ythe header |81. Nested within the cylinder |88 and aixed to the header |81 is a cylinder |89, extending therefrom to a point in adjacency to the header |85. There is formed thereby the reexed cylindrical sidebranch |9 of acoustic length 2L acoustically coupled tothe main sound conducting channel ESI through the slot-like opening |92. Coaxially mounted within thisstructure is a unit similar to that shownV in Fig. 5 comprising'the reilexed cylindrical sidebranch H33 and the reflexed Vconical sidebranch ld. Each of these two sidebranches has an acoustic length L. Attenuation maxima occur at values of f WC Y equal to .25, .50, .'75, 1.0 etc. iThe main conducting channel |69 is largely annularrin form and has aco'ustically coupled to it the three reflexedY sidebranches as shown.

I claim: 1. An acoustic sidebranch in the form of a com.-

plete closed cone with open base nested within a truncated conical m-ember Vclosed at its smaller end, the apex of the cone being positioned toward the large Vend of said truncated conical member and the open base of said cone being in adj acency to the small end of said truncated conical memcomplete closed cone with open base'nested within VYa truncated cone having one open end and one closedend, with the open end of the complete coneV positioned near the closed end ofV the truncated cone, thereby forming an outer tapered annular passage in series with an inner tapered circular passage, the cross sectional area of the passage at the point of coupling between the two passagesV being substantially onequarter of the cross sectional area of the open end' of Vsaid. truncated conical member.

4. An acoustic silencing device comprising a casing having inlet and outlet openings at its ends, a pair ofV frusto-conical members located within the casing, and converging away from the ends thereof, the inner ends of said members being closed, and the outer ends being spaced from the inside or the casing to provide an annu- Y lar main sound conducting channel, and a complete cone located within each frusto-conical member with its open largerV end adjacent the smaller end of the frusto-conical member, there being a passageway between the inside of each complete cone and the annular space between said cone and the inside of the corresponding frusto-conical member of substantially the same cross sectional area as that of the inside of the open end of the complete cone.

5. AnV acoustic silencing device comprising a casing having inlet andV outlet openings at its ends, a pair of frusto-conical members vlocated within the casing and converging away from the l o endsthereo-f, the inner ends of said members beingY closed and abutting each other, and the outer ends being spaced from the inside of the casing to provide an annular main sound conducting channel, a complete cone located within each irustofconical member with its open larger Vend adjacent the smaller end of the frusto-conical member, there being a passageway between the inside of each complete'cone and the annular space between said cone andthe inside of the corresponding frusto-conical member of substantially the same cross sectional area as that ofthe inside of the open end of the complete cone, a perforated member joining the larger ends of thek two frusto-conical members and bridging the contracted area between them, and a mass of sound absorbing material located between said perforated member and the exterior of the two frustoconical members.

6. An acoustic silencing device comprising a main sound conducting channel, and a reexed sidebranch of uniform cross sectional area and a reflexed sidebranch of constantly decreasing cross sectional area acoustically coupled to the main channel at points spaced along the length thereof.

'7. An Vacoustic silencing device comprising a main sound conducting channel and a reexed sidebranch of Vuniform cross sectional area and a reiiexedsidebranch of constantly decreasing cross sectional area each having the same acoustic length and acoustically coupled to the main channel at points spaced along the length thereof.

8. A sound attenuating device comprising three nested shells, the space within the innermost shell constituting a main sound conducting channel,

end closures extending from the outermost shell Y;

to the inner shell, an intermediate shell extending from. one of said end closures to a point in adjacency with the other of said end closuresV whereby is formed a closed acoustic sidebranch of uniform area throughout its length, the space be-V tween said outer shell and said intermediateshell constituting one section of said sidebranch, the space between said intermediate shell and said inner shell constituting another section of said sidebranch, said innerV shell being provided with an opening whereby said main channel is acoustically coupled to said sidebranch.

9. An acoustic silencing device comprising Va Vcasing, a frusto-conical memberolocated within the casing so as tol form an annular sound conducting channel and having its smaller end adjacent one end of the casing and itslarger end in communication with the annular channel, a

complete cone located within the frusto-conicalY member in such a manner asto form therewith a.

reflexed conical sidebranch, a sound conducting channel disposed coaxially within the casing and.

in communication both with the annular sound r conducting channel and with the open end of the reflexed'conical sidebranch, and a relexed annular cylindrical sidebranch located within the casing and around the coaxial channel, said annular sidebranch being acoustically coupled to Y the coaxial portion Yof the main sound conducting channel in a` region spaced from the coupling of said portion of the channel to the conical sidebranch.

10. An acoustic silencing device comprising a reflexed main channel and a reflexed sidebranch acoustically coupled to the main channel at the point of reiiexure of the channel.

11. An Vacoustic silencingY device comprising a casing having inlet and outlet openings at the ends thereof, a cylindrical and a frusto-conical member joined end to end with the small end Vof the frusto-conical member abutting one end of the cylindrical member, a partition located Within and dividing said members at their junction, said members being located coaxially with the casing, a second cylindrical member with one closed end located within the casing so as to form an annular sound conducting channel between it and the casing and so as to be telescoped over the rst cylindrical member, the annular space betweenA the two telescoped members, the gap between the open end of the iirst cylindrical member and the inside of the closed end of the second cylindrical member, and the interior of the rst cylindrical member being of substantially the same cross sectional area, and a complete cone located within the frusto-conical member with its open end directed towards the partition, there being a passage connecting the interior of the cone with the annular space between the cone and the frustoconical member located adjacent the open end of the cone and of substantially the same cross sectional area.

header to form a plurality of coaxial chambers connected together in reflexed fashion, the spacing of the shells from each other and from the headers being such that the several chambers and the passages connecting them all have substantially the same cross sectional area.

13. Ari acoustic silencing device comprising a casing, a reflexed annular sidebranch structure located within the casing and forming the outer boundary of an annular main sound conducting channel, and a reflexed cylindrical sidebranch structure and a reflexed conical sidebranch structure mounted coaxially within the casing and forming the inner boundary of the main channel, said several sidebranches being acoustically coupled to the main channel at points spaced along the length thereof.

14. A closed acoustic sidebranch comprising a truncated conical shell closed at its small end, and a second conical shell closed at its small end and located within the first shell spaced from the closed end thereof, the two shells tapering in opposite directions and having their dimensions so proportioned as to attenuate frequencies related to each other as 1:2:3:4; etc.

ROLAND B. BOURNE.

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