US2041767A - Silencer - Google Patents

Description

My 25, 1936 w. A. JACK, 3D 2,041,767

SILENCER Filed Jupe 20, 1931 2 Sheets-Sheet 1 INVENTOR /3 /V/W/'dm J2e /LZZ` ATTORNEY-5 May 26, 1936.

w. A. JACK, an

SILENCER Filed June 20, 1931 2 Sheets-Sheet 2 INVENTOR .50u/7d ATTORNEYS Patented May 26, 1936 UNITED STATES PATENT OFFICE srmNcEa Delaware Application June 20, 1931, Serial No. 54.5,776

2 Claims. (Cl. 181-41) This invention relates to improvements in silencers for noise-producing gases and especially silencers for the intakes and exhausts of internal combustion engines, compressors and the like.

It is an object of this invention to provide a silencer which is highly effective for low pitched sounds. It is the principal object of this invention, however, to provide a mumer or silencer which is equally effective in removing both low and high pitched sounds. Further objects will become apparent when the following description is read in conjunction with the accompanying drawings in which:

Figs. 1 to 10 show in cross section various types of silencers embodying the inventions of this application.

Silencers for noise-producing owing gases, such as are encountered in engine intakes and exhausts are based on various principles. The silencer most commonly encountered is based on the baille principle. This type of silencer, although it silences effectively, builds up a backpressure which is undesirable for many purposes. In a second type the duct through which the gases pass is lined with a sound-absorbing or a gas-pressure absorbing and sound-absorbing material along which the gases pass. There is practically no increase of back-pressure due to this method of sound absorption. If the duct is curved or bent the back-pressure increases due to the change in the direction of the gases. 'This second type of silencer is described in the Schnell Patent No. 1,811,762, granted June 23, 1931. This type of construction gives excellent results for most purposes but has the disadvantage of not being as effective on low-frequency sounds, especially those below 400 cycles per second, as on' high-frequency sounds. These sounds usually are not as objectionable as the high-frequency sounds. Although the absorbing materials used in the Schnell silencer vary in their effectiveness with sounds of different frequencies none have proved highly elective with the low notes. Absorbing materials having very small openings have proved most effective but these are undesirable for engine exhausts since the fine holes are easily clogged by the carbon deposited by the gases. Lengthening the silencer is effective but this is undesirable from the standpoint of cost and increased back-pressure due to increased surface friction.

` It is possible to remove low notes, as well as high notes, from moving gases 4by meansA of acoustic filters or neutralizers. These have been used in various forms for neutralizing sounds of various kinds. The difficulty which is encountered in the removal of sounds of a wide range of frequencies is that the apparatus becomes very cumbersome and costly since each neutralizer or 5 filter is effective over a narrow band of frequencies.` Sincethe absorbing type of silencer is effective over a Wide range of frequencies, I have found that I can supplement this absorber type of silencer with a neutralizing type effective over l0 a narrow band of loW frequencies and, thus effectively silence the entire range of frequencies.

'I 'he filters or neutralizers for low frequencies do not need long tubes but are effective with very short lengths so that the silencer may be kept l5 within reasonable dimensions. I have further discovered, as will be explained more in detail hereinafter, how to silence most effectively by the combination of absorber andd neutralizer either the low frequencies or higher frequencies 20 when both are present and it is desirable to remove both from the moving gas.

In the forms of the invention herein illustrated,

Fig. 1 shows a simple type of acoustic lter or 25 neutralizer combined with sound-absorbing material. A substantially imperforate casing I0 is employed. This casing has a sound inlet opening I I and a sound outlet opening I2. These may be reversed so that II is the outlet land I2 the 30 inlet, though the rst arrangement is preferred for some types of construction. A duct I3 extends from one of the openings toward the other opening. 'I'his duct has walls of sound-absorbing material or a combined gas-pressure absorbing 35 and sound-absorbing material I4 of the character described in the Schnell patent. vIf the absorbing wall is not made of a molded structure, a foraminous tub'e I5 is required to face the absorbing material I4. 'Ihis foraminous tube may 40 be of screen, wire mesh, perforated metal and the like as described in the Schnell patent, so that the sounds and gas-pressure waves will reach the absorber. For engine and compressor n intakes flamable absorbers such as cotton,'wool, i" hair-felt, Wood liber and the like may be used, Whereas for gas engine exhausts and other hot gases there may be used non-fiamable absorbers such as steel-Wool and other metallic wool, ex-

foliated vermiculite, asbestos fibers, rock wool, ff.

pumice, Haydite and otherloose ceramic aggregates. 'Ihe aggregates may, however, be bonded together to leave intercommunicating absorbing channels. An exterior retaining wall I6 for the absorber is used with loose absorbers or f )r those that may disintegrate. However, this retaining wall is not required and may be omitted where the absorbing material is such that this may be done. 'Ihe absorbing material may be as thin as 1,/4 inch but preferably should be at least V2 inch thick to be most effective in absorbing sounds and gas-pressure peaks. A chamber I1 is left between the casing I0 and the absorber I4. The low pitched sounds are neutralized in this chamber. A substantial opening I9 is left between the absorbing wall I4 and the end wall I8 of the outer casing. The opening I3 permits the sounds in the gas, and especially the lowfrequency sounds passing through duct I3, to pass into annular chamber I1 where they are neutralized. Opening I9 need not be entirely open as shown. By extending the foraminous tube AI5 to the end wall I8, as shown in Fig, 2, the same effect may be obtained. l

'I'he diameter of duct I3 may vary with respectl to opening Il. If it is the same size as that of opening II the length of absorbing duct I3 is less than that of casing I0 as shown in Figs. 1 and 2. 'I'he length of duct I3 with respect to casing I0, the volume of annular chamber I1 and the other dimensions determine the neu.

tralizing characteristics of the device and these may be adjusted so that it will neutralize sounds of a certain band of low frequencies. As an example, in an acoustic filter or neutralizar in which absorber I4 was omitted, tube I5 (Fig. l) being unperforated, in which casing I?! was 3% inches in diameter and 6 inches long, increasing the length of tube I5 from 41/2 inches to 'I inches, 35

opening II being larger than the outside diameter of tube I5, greatly increased the neutralization of sounds in the 175-3'15 frequency band but decreased those Aof 375-1500 cycles per second. Increasing the diameter of the casing III increases further the efficiency of neutralization of sounds in the lower frequency band. When the tube I5 is smaller than opening I I, reversal of the sound l direction, so that it enters at I2, decreases the absorption of sound in this range of frequencies. Varying the shape of either the duct or casing or both from round to oval or some other shape also varies the characteristics.

Instead of having the opening I9 to neutralizing chamber I1 at one end of duct I3 it may be provided at the center, or some other point, as is shown in Fig. 3. This provides neutralizing chambers of two different lengths in one device. By placing the duct I3 oil center or making duct I3 and casing III of different cross sectional contours the dimensions of chamber I1 may be varied within the same apparatus and the absorbing characteristics thereof may be thereby varied.

In Fig. 4 the sound absorbing wall I4 or duct I3 is longer than the enlarged portion of casing III and the casing is provided with a co-axial reduced portion 50 into which the end of the duct extends. End wall I8 may be stepped as shown to leave annular opening 2li to allow the sounds to reach neutralizing chamber I1. 'Ihe sound should preferably enter at opening I I in this type of silencer although gases conveying the sounds may pass either way without apparently affecting the silencing characteristics of the device.

In Fig. 5 the absorber I4 or duct I3 does not extend to end wall I8 but stops short thereof. If a foraminous tube I5 interiorly lines duct I3 it Iextends only throughout the length of the absorbent lining I4. 'I'he end 2I, which is optional I8 and is blank or imperforate.

-In Fig. 6 the absorbing material 23 is arranged interorly of casing I0 to exteriorly line the neutralizing chamber I1. Imperforate tube 24 passes through the casing and may be varied in length, that is, it may extend beyond end Wall I8 as shown or it may not extend to it, as in Figs. 1 and 2. Opening 25 allows the sound passing through tube 24 to reach neutralizing chamber I1 and absorbing material 23. This construction, though more effective for low frequencies, is not as effective for absorbing high frequencies as those in which the central duct has an vabsorbing wall. The two types of constructions may be combined so that both the duct and casing'have absorbing walls.

In Fig. '1 the construction of Figs. 1 or 2 is modified by the insertion of a baille 26 extending inwardly from end wall I8 to form a more tor- -tuous or restricted and llonger opening 21 Ior the passage of sound into neutralizing chamber I1. A similar baille may be used in connection with opening I3 of Fig. 3, opening 20 of Fig. 4, opening 22 of Fig. 5 and opening 25 of Fig. 6. The length and spacing of baille'26 may be varied to change the neutralizing characteristics of the device. Tests indicate that the longer the overlap of baille 26 the lower the frequency band which is attenuated.

Many modifications of the structures described may be made. As an example, in Fig. 8 the construction of the Schnell mufller is modified by the insertion of a neutralizing chamber 28. 'I'he casing II) with the duct I3 having absorbing wall I4 passes from end II to end I2. The neutralizing chamber 28 is inserted in the absorbing wall I4 and is connected with duct I3 by opening 29. 'I'he characteristics of the neutralizer may be varied by the size and shape of the chamber and by the insertion of bafile 30 to provide a more tortuous or longer passage 3| for the sounds to reach the chamber 28. Portions of the walls or all of the walls of the chamber 28 may be of absorbing material. This is accomplished readily by making the walls of foraminous material 32 as shown.

In the modication of Fig. 9 the neutralizing chamber I1 is combined with the absorbing material I4 in end-to-end relation instead of in annular relation. The neutralizing chamber preferablyis also lined with an absorber 33.

i In the application of my invention to a special situation in which it was necessary to change the direction of gas flow due to limited space, the device of Fig. 10 was found satisfactory. The dlrection of gas flow and sound propagation is in opposite directions as indicated by the labeled arrows. With duct I3 having, absorbing wall I4 epen at both ends the lowdrequency sounds were not silenced, though thehigh-frequency sounds were removed satisfactorily. Duct I3 was then closed at the left end with a piece of sheet steel and the low frequency notes were then neutralized. By shutting oil.' the duct at 34, as shown by the dotted line, the low-frequency sounds were again in evidence. An absorbing wall at the left end instead of sheet steel did not decrease the low-frequency neutralization. It appears therefore that a sound-absorber for high frequencies (above about 500 cycles per second) when used to line a neutralizing chamber, although it presents. an absorbing surface where the sound wave 'apparently must be reflected, does not interfere with the neutralizing action of the chamber, to which it also adds an absorbing eil'ect.

As will be apparent to those skilled in the art,

many variations of the constructions described in the specific disclosures may be made.

Where the interior diameter of duct I3 is equal to that of the end opening Il the noise may be passed in either direction with equal silencing but where the duct diameter is less than the opening, as in Figs. 4 and 5, the operation is more effective when the noise entrance is at II.

Although the construction of most of the forms of my device shows the channel from duct I3 to the neutralizing chamber I1 to be adjacent to the end wall I8, it is understood that this channel may be placed at either end or at any predetermined point along the duct as in Fig. 3. 'I'he casings and ducts may be of varying shapes and the ducts need not be placed concentric with the casings. The casings maf also vary in ysize from end to end. These varif ions may be similar to those shown in Fig. 3 ai Figs. 8 to 11 of United States Patent No. 1,811, 32 to F. H. Schnell.

In instances where it is known that harmonics of low-frequency fundamentals exist in the source of sound to be mufiied, then the application of acoustic lters to remove first the fundamental may cause certain harmonics to build up excessively, these harmonics being of high frequency. Partial muiliing by absorption following the acoustic filter o'r neutralizar may still leave nearly as much intensity in the harmonic 'as originally existed with no muffler. In this case it is preferable to allow the absorption muflier to act iirst on the sound to be silenced. Conversely, however, it is possible for the absorption mufller to build up at low frequencies and thus make the apparent effect of the following acoustic filter less than it actually is. Therefore, it is possible for one effect to cancel the other and the order of muillers makes no diierence on the intensity of the overall sound, although it makes a difference in pitch, the rst case giving a resultant high pitch, and the second a low pitch. 'I'he relation between the periods of the two types of mufllers and their muiiling efficiency determines the resultant overall sound (which is equal, regardless of order, for one special case only). The proper order has to be determined for the case at hand. In the case of non-existence of harmonics, or in other words the existence of a low frequency fundamental and non-harmonic high frequencies, it is preferable to place the acoustic lter first to prevent the absorber munier from building up the low frequency due to the before-mentioned resonance effects.

Since gas-engine exhausts consist of a rapid succession of ,gas pulses, such exhausts are not unlike sounds which are a rapid succession of gas pressure waves. However, the gas pulses in the exhaust are of comparatively low frequency and therefore may be compared to low-frequency sounds. Data show however that these are not entirely alike, especially in their action on sound absorbers. Because of this relationship between Sound pulses and gas-pressure pulses the principles of my invention may be applied to both but due regard must be given to the differences that have been mentioned when applying these principles in practice.

I claim:

1. A silencer comprising a substantially imperforate casing having two openings therein, a duct for the sound to be silenced having a wall and a surrounding layer of sound-absorbing material, a space between said sound absorbing material and said casing, said wall extending from 25 one of said openings into the second of said openings, the cross sectional-area of said duct excluding said absorbing material being sufficiently smaller than the area of at least a portion of said second opening to provide a passage between said 30 duct and the wall of said second opening, said passage providing communication between said space and said second opening.

2. A silencer comprising a substantially imperforate casing having two openings therein, a $5 sorbing material and said casing.

WILLIAM A. JACK, 3RD.

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