US 2308886 A
Description (OCR text may contain errors)
1943a w. P. MASON 2,308,886
ACOUSTIC WAVE FILTER I Filed Nov. 29, 1940 Q INVENTOR m R MASON ATTORNEY Patented Jan. 19, 1943 ACOUSTIC WAVE FILTER- Warren P. Mason, West Orange, N. 1., assimor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 29, 1940, Serial No. 367,737
This invention relates to acoustic wave filters and more particularly to filters of the suppres sion type adapted to handle a large volume of all.
The principal object of the invention is to suppress a band of objectionable frequencies while permitting the free how of a large volume of all".
An air conditioning installation usually iii-- cludes a blower to provide a forced circulation of air in the room to be conditioned. The blowor may be arranged either to force air intothe room or to withdraw air from the room. In either case it is desirable to introduce an ocoustic wave filter between the blower and the air duct communicating with the room in order to suppress the objectionable audible frequencies set up by the blower. If a large room is being con ditioned the filter must be capable of effectively suppressing the noise while permitting the substontially unrestricted flow of a large volume of air per unit of time.
The acoustic wave filter of the present invention is a. simple and efllcient device suitable for the purpose indicated. The filter comprises o main conduit, preferably of rectangular crosssection, and one or more pairs of side branches arranged to form a symmetrical structure about a central longitudinal plane. For proper funotioning of the filter only plane waves the ireenemies to be suppressed are permitted to reach the openings into the side branches. 'Io accomplish this the width dimension of the main com duit is limited to a wave-length M corresponding to the highest frequency f0 to be suppressed in the sections of the filter with which the side branches are associated. For the same reason the main conduit is divided in the direction of its depth by one or more partitions which are parallel to the width dimension and are spaced apart by a distance not exceeding in. These par titions are preferably extended into and all the way across the side branches.
If the air entering the filter has an unsymmetrical particle velocity, as may be the case when the filter is fed by a blower, the cross-partitions at the input end of the main conduit preferably have a spacing which does not exceed a. halt that is, io. The half-wsve spac so be advantageously e resin con Cit of each pair is omitted, the cross-partitions should have the halt-wave spacing throughout the entire length of the main conduit, and also, preferably, in the side branches.
The different pairs of side branches preferably have diirerent lengths so that they will hove diflferent frequencies of resonance. These resonances determine the frequencies 0! maximum attenuation for the filter sections and they are so chosen that they all fall below In and are spaced'for maximum sustained attenuation in the band to be suppressed.
The spacing between the successive openings into the side branches and the longitudinal dimensions of the side branches determine the lower cut-off of the filter, that is, the lowest Irequency which will be attenuated. For the most efllcient use of space the side branches occupy the full length of the main conduit, that is, a preceding branch abuts against the next succeeding one.
All of the partitions in the filter parallel to the direction of the air flow are preferably made rounded on the ends toward the flow and sharp on the ends from which the air leaves. For some of the partitions, especially those serving as guides in curved sections of the main conduit, the strecmlimng may advantageously be carried further. For most efficient results the openings into the side branches preferably constitute approximately one-third of the cross-sectional area of the side branch. To improve the suppression of undesired frequencies higher than fo the main conduit may be extended at one end and lined with sound absorbing material such, for example, as felt or glass wool. To provide an impedance match between the filter and its load the discharge end of the main conduit may be flared by curving one or both pairs of sides outwardly.
The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying draw" ings, in which like reference characters represent like or similar parts and in which:
Fig. 1 is an elevation, partly in section, of an acoustic wave filter in accordance with the invention;
Fig. 21s a plan view of the filter c! Fig. 1, part ly in section, along the line 2 2: and
Fig. 3 is a sectional view along the line 3-4 l, with parts of the side branches broken 2 nd i the a main conduit 4 or rectangular cross-section extending from one end to the other and a number of pairs of side branches 5, 5', 6, 6' and I, 1. The two branches of each pair are arranged opposite to each other to form a balanced structure. The side branches are closed chambers communicating with the main conduit 4 by means of openings such as 8 and 8'.
At the intake end l 0! the filter the main conduit I is extended in the form of a curved section i I which describes a 90 degree angle. The section H has a number of partitions l2, l3 and H which extend from one side l to the other side I 8 and are parallel to the curved sides 29 and 30.
At the discharge end H the main conduit 4 is extended in a section I! lined internally with a layer of sound absorbing material I! to increase the suppression oi the higher frequencies. The section l8 terminates in an end portion 20 which has flaring sides 2| and 22 designed to match the impedance of the filter to that of the load. The partition 13 is extended through the main conduit 4, including the section l8, and all the way across the side branches 5, 5', 5, 6', 1 and 1. Each side of the partition I3 within the section i8 is also covered with a layer of sound absorbing material i9. The layer l9 may, for example, be felt, glass wool or other suitable material and may have a thickness of perhaps one inch. The walls and partitions of the filter may be made of any suitable metal and may be welded, soldered or otherwise secured at the joints.
Some design considerations to be observed in dimensioning the filter will now be taken up. It is known that the side branches of an acoustic iilter will function properly only when a plane wave alone is propagated down the main conduit. The conditions necessary to insure only a plane wave in a conducting tube of rectangular cross-section will now be considered. The first case of interest is the one in which the air has an unsymmetrical particle velocity.
The equation of motion for any type of sinusoidal air waves passing through a conduit of rectangular crcss-section is -o e Et 531 where e is the velocity potential, to is 21 times,
=0 when z= in.
For the next simplest case, when m l and 12:0.
In Equation 6 it is apparent that ii the value of the expression under each of the radicals is negative and therefore the square root is imaginary. This represents a wave which is attenuated as it is propagated through the conduit. The other values of m and 11, represent more complicated wave forms which will be atw 1' fat tenuated to a higher frequency in the conduit.
In expression. (7) if yr, the width of the conduit in the y direction, is substituted for 2b and 2140 is substituted for u: there is obtained where M is the wave-length corresponding to the frequency in. The rule to be deduced from expression (8) is that, for the unsymmetrical case, if the width 111 of the main conduit is kept less than then all waves of frequency Io or lower permitted to pass through the conduit will be plane waves. Since plane waves only are properly attenuated by the side branches of the filter it follows that In is the highest frequency which can be successfully suppressed by the filter in the sections including the side branches.
An unsymmetrical case would be presented if half of the side branches, for example branches 5, 6 and '7, were omitted. In this case the dimension yr should be half a wave-length. Another unsymmetrical case occurs where the direction of the air flow is changed, as in section 11. Here the partitions i2, i3 and H are spaced apart a distance at equal to /2)).
The symmetrical case will now be considered. The filter as shown represents such a case, since the side branches occur in oppositely disposed pairs. The half-wave spacing of the partitions i2, I3 and I4 assures only plane waves leaving the section II. The side branches, however, set up particle velocities in the 1/ direction, for the pressure wave causes air to flow into the side branches. However, due to the balanced construction of the side branches, the direction of flow has a plane of symmetry down the center and therefore any parasitic wave set up by the side branch must have an opposite flow on the two sides.
. waves involving 171:1 will have a particle 51:. 1. direction given by that this wave will be attenuated rapidly as it passes along the main branch provided the dimension m is less than a wave-length for the frequency Io, that is,
A comparison of expressions '(8) and (11) shows that for the symmetrical case the dimensioning may be twice that for the unsymmetrical case.
The depth or a: dimension of the main conduit may have any value and in practice is chosen to permit the passage of the required volume of air through the filter. However, if the dimension :1, as shown in Fig. 2, exceeds a wave-length M ii; is advisable to insert one or more partitions, such as l3, running the full length of the main conduit and preferably extending into the side branches. These partitions are spaced apart a distance p which does not exceed a wave-length M. In the example shown,
x1=2p=2Ao (12) However, it is to be understood that in general iti=hko (13) where h is any. integer, in which case there will be (h-1) partitions required.
The opening into the main branch, such as 8" for the side branch 1, preferably extends for approximately one-third of the width of the branch and is centrally located. Thus, as shown in Fig. 1, the dimensions e1, 82 and 8a are all equal. If the opening is smaller than this the air encounters too high a resistance in flowing into and out of the side branch, and if the opening is larger the pressure is not sufficiently uniform over the opening. In either case the efficiency of the filter is impaired. The portions, such as ll, of the main conduit forming the openings into the side branches are preferably made comparatively sharp on the side nearer the discharge end of the filter, as shown in Fig. 2 at 24, and rounded on the side nearer the intake end, as shown at 25.
In order to impede the air flow as little as possible the guides l2, l3 and H are preferably streamlined to a certain extent. As shown in Fig. 2 the outer end 26 is made rounded and the inner end 21 is comparatively thin. Each guide may be made from a single sheet of metal doubled back upon itself and formed in the shape shown.
If a number ofv pairs of side branches are used, as shown in Fig. 1, each pair is preferably given a different length from the other pairs so that the various pairs will have different frequencies of resonance, all falling below In. Fbr example, the branch 1 may have a length 91 and a resonance at f1, branch 6 a length g2 and a resonance at I: and branch a length as and a resonance at is. The frequences f1, f2 and is are chosen to provide the maximum sustained attenuation over the frequency range below fa which is to be suppressed. Since the side branches occupy all available space along the sides of the main conduit it the distances qr, (12 and Q3 which the branches 1, 8 and 5, respectively, extend along the main conduit 1 determine the lowest frequency which will be suppressed by the filter. The longer these dimensions are made the lower will be the lower cut-off of the filter. The branches 5', 0' and I are similar, respectively, to the branches I, 6 and l. The principles to be followed in designing the side branches are more fully discussed in Patent 1,692,317 to G. W. Stewart issued November 20,
1928, and in my prior Patent 1,874,326 issued August 30, 1932.
The frequencies above f0 will not be attenuated by the sections of the filter which include the side branches but they will be attenuated to a considerable extent in the extension 18 of the main conduit which is divided by the partition I3 and lined throughout with a layer of sound absorbing material H as already mentioned. The longer the dimension .9 is made the greater will, be the suppression of the frequencies above In.
As a specific example suitable dimensioning for a filter meet a certain set of requirements will be given. It is assumed that the filter is to carry 3430 cubic feet of air per minute and that the principal attenuation band is to extend from 200 to 1760 cycles per second. The velocity of sound in air is taken as 1100 feet per second. The widthdimension w of the main conduit 4 is equal to one wave-length at the frequency 10:1760, and from expression (11) is found to be The depth dimension m1 of the main conduit is chosen as two wave-lengths and therefore x1=27w=15 inches The spacing p between the central partition I! and each of the side walls of the main conduit 4 is equal to a wave-length and therefore =M=7.5 inches In accordance with expression (8 the guides ll,
13 and H in section II have a half wave-length spacing, and therefore d=='/Xo==3.75 inches The frequencies of resonance for the side branches 1, 0 and I are chosen, respectively, as
f1=250 cycles per second f2=360 cycles per second fs=455 cycles per second giving the following lengths for the side branches:
of: 10 inches 9r= 6 inches 0:: 4 inches Since the lowest frequency to be attenuated is 200 cycles per second the following are suitable longitudinal dimensions for the side branches:
q1=18 inches (12:9 inches 13: 6 inches The side branch 1' has an opening 8' equal to one-third of its longitudinal dimension qi and therefore e1==e2=es= /3q1=6 inches A suitable length r for the high frequency section it of the main conduit is 42 inches.
Although the sections of the filter which include the side branches are designed to provide an attenuation band extending only from 200 to 17cc cycles per second, the high frequency section l8 will furnish enough attenuation at frequencies above 1760 cycles so that all frequencies between 200 and 8000 cycles are effectively suppressed by the filter structure as a whole.
If a filter with greater air carrying capacity is required it is only necessary to extend the depth dimension :n in increments equal to p and to provide the required numberof partitions such as H with a spacing not exceeding 1). It is to be understood that, in practice, any one or all of the dimensions may vary somewhat from those given. Also, if a different frequency band is to be suppressed, the dimensions should be changed accordingly. The dimensions given for m, p and d are the maximum permissible for the conditions assumed, and provide the most eflicient use of material. These dimensions may, of course, be smaller and the filter will still function to suppress the required band, but the air capacity will be correspondingly reduced.
-Iibat is claimed is: I
1. An acoustic wave filter for suppressing a bond frequencies comprising a main conduit of rectangular crosssection and a pair of cppositely disposed side branches connected to said main conduit, said side branches having a fre quency of resonance which falls within the band 01'' frequencies to be suppressed, the depth 01' said main conduit being greater than X0, said main conduit being divided by longitudinal partitions parallel the width dimension and the spacing of said partitions not exceeding X0.
2. An acoustic wave filter for suppressing a f equencies comprising a main conduit ular cross section and a pair of op pcsitely csed side branches connected to said main conduit, said side branches having a frequency of resonance which falls within the band of frequencies to be suppressed, said main conduit having a curved section which includes therein guide partitions disposed parallel to the curved sides of said section and the spacing of said partitions not exceeding an.
3. An acoustic wave filter for suppressing a band of frequencies comprising a main conduit oi rectangular cross-section and a pair of oppositely disposed side branches connected to said main conduit, said side branches having a frequency of resonance which falls within the band of frequencies to be suppressed, and a portion of said main conduit being lined with a layer of sound absorbing material.
4. An acoustic wave filter for suppressing a band of frequencies comprising a main conduit of rectangular cross-section and a pair of oppositely disposed side branches connected to said main conduit, said side branches having a fre-' of resonance which falls within the band w. to be suppressed, and each of said nunicating with said main the area or which is third of the crosson, a pair of oppositely onnected to said main partitions extending rallel to the width 1 of said main con- :i partitions exceeding ending to the highest in accordance with claim 5 in which said partitions extend into and divide said side branches.
7. An acoustic wave filter in accordance with claim 5 in which said main conduit has a curved section and guide partitions disposed parallel to the curved sides of said section are inserted therein, the spacing of said partitions not exceed ng M.
8. An acoustic wave filter in accordance with claim 5 in which a portion of said main conduit is lined with a layer of sound absorbing material.
9. An acoustic wave filter in accordance with claim 5 in which each of said side branches communicates with said main conduit through an opening the area oi which is equal to approximately one-third of the cross-sectional area 0! the side branch.
10. An acoustic wave filter in accordance with claim 5 which includes a second pair of oppositely disposed side branches connected to said main conduit, said two pairs of branches having frequencies of resonance which differ from each other but fall within the band of frequencies to be suppressed.
11. An acoustic wave filter ior suppressing a band of frequencies comprising a main conduit of rectangular cross-section, a plurality of pairs of oppositely disposed side branches connected to said main conduit, longitudinal partitions extending across said main conduit and into said side branches, neither the width of said main conduit nor the spacing of said partitions exceeding the wave-length in corresponding to the highest frequency to be suppressed, said pairs of branches having frequencies of resonance which are all different but all of which fall within the band of frequencies to be suppressed, and a portion of said main conduit being lined with a layer or sound absorbing material.
12. An acoustic wave filter in accordance with claim 11 in which said main conduit has a curved section and guide partitions disposed parallel to the curved sides of said section are inserted therein, the spacing of said partitions not exneeding lo.
13. An acoustic wave filter in accordance with claim 11 in which each of said side branches co'mmunicates with said main conduit through an opening the area of which is equal to approximately one-third of the cross-sectional area of the side branch.
14. An acoustic wave filter in accordance with claim 11 in which said main conduit at one end gradually flares to a larger cross-sectional area.
15. An acoustic filter for suppressing a band of frequencies comprising a main conduit of rectangular cross-section and a pair of oppositely disposed side branches connected to said main conduit, the width of said main conduit being approximately equal to the wave-length in corresponding to the highest frequency to be suppressed.
16. An acoustic wave filter in accordance with claim 15 in which the depth of said main conduit is greater than M and said main conduit is divided by longitudinal partitions parallel to the width dimension, the spacing of said partitions being approximately equal to 17. An acoustic filter in accordance with claim 15 in which the depth oi said main conduit is equal approximately to 1m, where h is any integer and said main conduit is divided by (In-1) equally spaced longitudinal partitions disposed parallel to the width dimension,
18. An acoustic wave filter for suppressing a band of frequencies comprising a main conduit of rectangular cross-section having a straight section and a curved section, a pair of oppositely disposed side branches connected to said main conduit and longitudinal partitions extending across said main conduit parallel to the width dimension, the width of said main conduit being approximately equal to the wave-length M corresponding to the highest frequency to be suppressed, the spacing of said partitions in said straight section being equal approximately to M and the spacing of said partitions in said 5 curved section being equal approximately to WARREN P. MASON.