US2801705A - Silencer - Google Patents

Description

srLENcER Jacob K. Brixius, Cleveland, and Richard E. Brown,

p Chagrin. Falls, Ohio, assignors to Air-Maze Corporation, Cleveland, Ohio, a corporation of Delaware Application January 26, 1955, Serial No. 484,204

2 Claims. (Cl. 181-50) This'invention relates to improvements in an air intake Silencer for use with a stream of air or other gas flowing vthrough a compressor or the like.

.An object of this invention is to provide a silencing device adapted to silence sound waves having a frequency of the order of 800 cycles per second or less.

I Another object of this invention is to provide a silencing v.device adapted to eliminate hissing and whistling caused lby lharmonicsset up in the silencer itself.

Another object of this invention is to provide a smaller ratio between the inlet and outlet areas of a silencing xdevice which produces a drop in the static pressure in the device and has a silencing effect.

A further object is to provide a silencing device composed of parts which need not be constructed to extremely clos-e tolerances and one which does not require Other United States Patent features of this invention reside in the arrangement and desig'n of the parts for carrying out their appropriate functions.

a Other objects and advantages of this invention will be apparenti from the accompanying drawings and the following description and the essential features will be set forth' in the appended claims. Iii the drawings,

Fig. 1 is av central sectional view of our improved silencing device, taken along thevline 1-1 of Fig. 2.

Fig. Y2 'isa cross sectional view taken along the plane of line 2-2 of Fig. -1.

I iig.- 3 show s a graph which illustrates an increase in pressure drop as the volume of air fiow becomes greater, lfor several different types of silencer construction.

p ''lh'e presentn device is adapted to be connected to the airintake manifold of a compresser or other noise-producirig mechanism. VWhile my `device may take a number of forms, I have chosen to disclose an embodiment comipising a cylindrical casing 10 having an outwardly convex rounded bottom portion 11 which contains an orifice 12 whose axis is concentric with the axis of the cylindrical casing 10. vAttached to the periphery of the orifice 12 and projecting through the bottom of the casing is a collar for outlet pipe 13, used to secure or attach the device to the intake manifold of the noise-producing mechanism, air flow being in the direction of the arrow in Fig. 1. The upper periphery of the cylindrical casing 10 is connected in any suitable manner to a top member 15. The connection shown is constructed by extending the upper edge of .the cylindrical casing outwardly and then inwardly in substantially U-shape to engage and retain the outer peripheral edge of the top member 15. This joint may be brazed, soldered or welded for securing the members together.

Air flow into the interior of the casing 10 is by means of a pair of spaced inlet openings through the top member Patented Aug. 6, 1957 2 15. A hollow cylindrical tube 18 is secured into each of the openings and extends downwardly to a point near the bottom of the housing 10. The tubes are of such length as to cause entering air to be directed in a vertically downward stream. As viewed in Fig. l, the tubes 18 are substantially equally spaced from the axis of the cylindrical casing and have axes which are ou-t of alignment or eccentric to the axis of the casing 10 and orifice 12. An annular filter element 20 preferably formed of a large number of layers of wire mesh screen, is provided above top member 15. The bottom of the filter element 20 is seated just inside the peripheral edge of the casing and extends circumferentially all around the top of the casing. A cover 21 secures the filter element to the casing 10 by means of wing nut 23 threaded on bolt 22, which in turn is fastened to the top member 15 by welding or other'suitable means. All air entering the casing 10 Vmust first pass through and be filtered by the filter element The present device is known as a low frequency silencer or one which is designed to silence sound waves in the range of 10 to 800 cycles per second. The size here illustrated supplies air at approximately 40 cubic feet per minute. Previous silencers of this general type employed a single inlet Vtube whose position within the silencer had to be located with extreme precision. The axis of the single tube had to be exactly concentric with that of the outlet orifice. The spacing between the end of the tube and the outlet orifice was extremely critical and had to be very accurately adjusted. Great care had to be employed in the manufacture of the tube itself since any burrs or Vrough spots present on the tube would cause annoying sounds. The one-tube construction was limited to a cross sectional area which was approximately 40% of the cross sectional area of the outlet orifice. It was found that if the area of the single inlet tube was greater than 40% of the outlet orifice, there was a substantial loss in silencing efficiency.

In the present invention, the total fiow area or cross sectional area of the inlet tubes 18 is approximately 70% of the cross sectional area of the outlet orifice 12. It has been found that a total inlet tube area of between 60% and 80% of -the outlet orifice will work efiiciently. In a practical sense, this means that the present invention will supply the Vsame amount of air with less pressure drop across the silencer, thus increasing the efficiency of the air moving device connected at 13. These results are readily illus-trated in the graph of Fig. 3, wherein the abscissa of the graph illustrates the air iiow in cubic feet per minute and the ordinate represents the static pressure drop in inches of water. Curve A represents a single tube silencer having a cross sectional inlet tube area equal to approximately 40% of the outlet orifice area. Curve B represents the present invention wherein the total cross Vsectional area of the inlet -tubes 18 is equal to approximately of the cross sectional area of the outlet orifice 12. Curve C represents the present invention wherein the total cross sectional area of .the inlet tubes was increased to equal approximately of the cross sectional area of the outlet orifice.

For an air flow of approximately 50 cubic feet per minute, the single tube silencer, as illustrated by curve A, had a static pressure drop of approximately 16 inches of water; as compared to approximately ll inches of water for curve B; and 71/2 inches of water for curve C. The static pressure drop indicated by curve A was substantially 100% greater than that indicated by curve C, and approximately 50% greater than that of curve B. Under the conditions of the test, as illustrated by the graph in Fig. 3, a constant background noise of 56 decibels was maintained. A noise-making mechanism comprising a compressor was then operated without the benefit of a to 70 decibels.

orifice, as illustrated by curve C, reduced the total noise to 66 decibels, thus indicating that a more efficient silencer is provided when the total cross sectional area of the inlet tubes is equal to approximately 70% of the total cross sectional area of the outlet orifice than when 100% of the area of the outlet orifice is used.

It is interesting to note that the two tube silencer of the present invention, as illustrated by curve B, reduced the total noise as compared with the single tube silencer illustrated by curve A, by live decibels. As a practical matter, the noise produced by the single tube silencer was approximately four times greater than that of the silencer of the present invention illustrated by curve C, since it is a well-known scientific fact that the intensity of a sound doubles with each change of three decibels. Thus, a sound which is six decibels higher than another is perceived by the human ear as being four times as loud as the other. It follows, then that in the present test a difference of five decibels indicates the noise of the compressor, when operating with the single tube silencer, as being not quite four times as loud as the compressor operating under the same conditions but provided with our present invention having a total cross sectional inlet -tube area equal to approximately 70% of the cross sectional outlet area of the orifice 12. And this is .accomplished with a lower pressure drop through our improved silencer.

It was found that if the total cross sectional inlet area of the tubes in the present device was increased beyond 80% of the total cross sectional area of the outlet orifice, there was a loss in silencing efiiciency.

The inner surface of the top member is preferably lined with a sound-proofing material 24 of felt, cotton or the like, which has a tendency to reduce the highpitched sounds which are at times transmitted back along the air stream through the interior of the casing 10.

Fourteen different arrangements of the old single tube silencer showed a whistle at one or two points in the range of 800 to 2000 cycles per second. This new twotube silencer eliminates any whistle in this range.

Although I have shown a preferred construction of the present invention, I do not wish to limit myself to this specific construction. We have found that the number and location of tubes 18 is not critical, nor is the spacing between the lower end of the tubes and the bottom of the casing 10.

The exact theory under which the present phenomena occurs is not fully understood; it may be because of the elimination of a certain resonant frequency characteristic of the specific type of enclosure or for some other reason which is not herein necessary to explain. It appears that the prior art silencer with a single tube in the center of the chamber sets up harmonics, while the present two-tube silencer eliminates such harmonics. The fact remains that the present invention does produce the results heretofore described most efficiently and effectively.

In view of the foregoing description, taken in conjunction with the accompanying drawings, it is believed that a clear understanding of the construction, operation, and advantages of the device will be quite apparent to those skilled in this art.

It is to be understood, however, that even though there is herein shown and described a preferred embodiment of the invention the same is susceptible to certain changes fully comprehended by the spirit of the invention.

What we claim is:

l. A silencer consisting of a hollow housing having in a closed end thereof an outlet orifice, said housing having a plurality of spaced inlet tubes entering said housing through a wall opposite said closed end, said inlet tubes terminating in a zone spaced from said outlet orifice and having their tubular outlet openings completely out of axial registration with said outlet orifice,

said inlet tubes being of sufficient extent to cause entering fluid to be directed toward said closed end, said housing having imperforate walls save for said outlet orifice and said inlet tubes, said housing forming a resonance chamber communicating with the inner end of said tubes, the total cross sectional area of said inlet tubes being approximately sixty to eighty percent of the area of said outlet orice.

2. A silencer consisting of a hollow cylindrical housing having in a closed end thereof an outlet orifice concentric with the axis of said cylindrical housing, an opposite wall of said housing having a pair of spaced cylindrical inlet tubes entering said housing through said opposite end, said inlet tubes terminating short of said orifice and having their tubular outlet openings completely out of axial registration with said orifice, said inlet tubes being of suiiicent extent to cause entering fluid to be directed in streams toward said closed end, said housing having imperforate walls save for said outlet orifice and said inlet tubes, said housing forming a resonance chamber communicating with the inner end of said tubes, the total cross sectional area of said inlet tubes being approximately sixty to eighty percent of the cross sectional area of said orifice, the inner surface of said opposite end being lined with a sound absorbing material.

References Cited in the file of this patent v UNITED STATES PATENTS 728,200 Butler May 19, 1903 734,868 Hill July 28, 1903 2,553,326 Manning May 15, 1951 2,564,814 Perrin Aug 21, 1951 2,624,418 Bourne Ian. 6, 1953 FOREIGN PATENTS 391,180 Great Britain Apr. 18, 1933

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