US3396812A - Acoustic quarter wave tube - Google Patents

Description

Aug. 13, 1968 R, w|| 0x ET AL ACOUSTIC QUARTER WAVE TUBE Filed July 5, 1967 INVENTORS FREQUENCY XN Y A mm www mE im B wm DN T mo m n Hw mA R lv. M www no o n United States Patent O 3,396,812 ACOUSTIC QUARTER WAVE TUBE Richard Wilcox and J Alson Beaman, Columbus, Ind'., assignors to ArvinA Industries, Inc., Columbus, Ind., a corporation of Indiana Filed July 5, 1967, Ser. No. 651,236 2 Claims. (Cl. 181-48) ABSTRACT OF DISCLOSURE A sound attenuating system employing a tube connected to a gas carrying pipe adapted to be connected to a source of sound energy for attenuating the noise level of the sound waves discharged from thte pipe. The pipe has one or more sound pressure points along its length, and said tube connected to the pipe at one of said sound pressure points. The tube has a length equal toI one-fourth of the wave length of the frequency producing said one pressure point and has at least one opening in its end remote from its connection to said pipe for modulating its attenuating effect.

Background of the invention Quarter wave tubes have long been known and used in the acoustical art. Such tubes have one open end and one closed end and normally are uniform in their crosssectional extents along their lengths. They have lengths one-fourth as long as th wave length of the frequency which they are to attenuate. A sound wave entering one of these tubes from a gas carrying pipe will travel to the closed end of the tube and be reected back to the gas carrying pipe where it arrives opposite in phase to the oncoming wave in the pipe. This interference between the two waves results in attenuation.

Although such quarter wave tubes produce a high degree of attenuation, the attenuation is limited to a narrow band of frequencies. Thus, a sound attenuating system having a quarter wave tube reduces the noise level of the sound energy source to which it is connected, but the quarter wave tube produces an objectionable sharp drop in the overall noise level across a narrow band of frequencies which in most cases is perceptible to the human ear. The instant invention is directed to a sound attenuating system employing a modulated quarter wave tube which provides attenuation spread over a larger band of frequencies.

Summary of the invention In accordance with one form of the invention, a quarter wave tube having a closed end and an open end is connected at its open end to a gas carrying pipe which in turn is connected to a source of sound energy. Said gas carrying pipe has one or more pressure points disposed along its length, and the quarter wave tube is connected to said pipe at one of those pressure points. The quarter wave tube has a length equal to one-fourth of the wave length of the frequency producing the sound pressure point at which the tube is connected to the pipe. An opening having a cross-sectional extent in the range of from about 1% to about 7% of the cross-sectional extent of the tube is formed in the closed end of said tube to modulate the attenuation eifect of the tube and spread said attenuation over a larger band of frequencies.

Brief description of the drawings The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a side elevation of an automotive exhaust system employing a quarter wave tube embodying the invention;

FIG. 2 is a fragmentary enlarged longitudinal section of the quarter wave tube and tail pipe shown in FIG. l; and

FIG. 3 is a graph showing the improved attenuation results achieved vby the use of a quarter wave tube embodying the invention.

Detailed description In the exhaust system illustrated in FIG. l, an exhaust pipe 10 is provided with a pair of branches 11 at its upstream end adapted to be connected to the exhaust manifolds of an automotive engine, The downstream end of said exhaust pipe is connected to a muffler 12 which in turn is connected to a tail pipe 14 for discharging the exhaust gases to the atmosphere. The mutller will attenuate the noise level of the exhaust gases. However, in some instances the muler alone may not be able to effect the desired degree of attenuation, and it may be necessary to employ a quarter wave tube 16 in the system.

kAs will be understood from well known principles of acoustics, maximum sound pressure points are created by the exhaust gas noise along the lengths of the exhaust and tail pipes 10 and `14, the locations of said points being a function of the pipe lengths. To attenuate the sound wave frequencies producing these pressure points and thus reduce the noise level of the gases discharged from the tail pipe 14, the quarter wave tube 16 is connected to the exhaust or tail pipes at one of those pressure points. In the illustrated embodiment, the tube 16 is connected to the tail pipe 14 at one of its maximum sound pressure points. As shown in FIG. 2, the tube 16, which has a length equal to one quarter of the length of the sound wave of the frequency producing the pressure point at which it is connected to pipe 14, is open to the pipe 14 at its connection thereto but is closed at its opposite end by an end cap 18. One or more openings 20 are formed in the tube 16 along its length or in the end cap 18. Desirably, said openings are in spaced relation to the connection of the tube to the pipe 14 and provide an overall open area in the tube having a cross-sectional extent equal to from about 1% to about 7% of the cross-sectional extent of said tube.

In the operation of a conventional quarter wave tube without any openings 20, a sound wave moving through pipe 14 will travel into the tube 16 and be reilected back through the tube from the end cap 18 to arrive back at the tail pipe out of phase with the oncoming wave in said pipe. This produces an interference between the two waves resulting in an attenuation of the narrow band of frequencies lying on either side of the frequency having a wave length equal to four times the length of the tube.

The dotted line curve A in FIG. 3 shows an attenuation curve for an automotive exhaust system to which no quarter wave tube has been connected. As shown, there is an objectionable peak A at the frequency between c.p.s. and 130 c.p.s. Curve B in chain link lines in FIG. 3 shows the attenuation curve for the same exhaust system connected to the same sound energy source, but provided with a quarter wave tube not having any of the openings 20. The tube has a frequency response of c.p.s. and is connected to the tail pipe at a pressure point established by the sound waves having a 110 c.p.s. frequency. As shown by curve B, the quarter wave tube reduces the objectionable peak A between 90 c.p.s. and c.p.s. However, the quarter wave tube produces an objectionable slope at B. The slope B', because of its steepness resulting from the narrow band of attenuation of the quarter wave tube, produces an objectionable noise. Curve C of FIG. 3 shows the attenuation curve for the same exhaust system connected to the same sound energy source shown in curve B with the exception that the quarter wave tube has an Opening in its end cap. As shown, the peak A is eliminated as well as the sharp narrow band of attenuation shown at B in curve B. Thus, the opening 20 produces a modulating effect 0n the attenuation of the quarter wave tube Without sacrificing any attenuation efficiency. Indeed, it is noted that the area under curve C in the range from 40 to 180 c.p.s. is less than the corresponding areas under curves A and B. Thus, the total noise suppression elected by the system is increased even though the objectionable slope in the unmodulated quarter wave curve B have been eliminated.

Although the tube 16 has been shown as being mounted externally of the pipe 14, it can, of course, be mounted internally of either pipe 10 or 14. It is only necessary that it be mounted on or in one of said pipes in a location such that its open end is located adjacent the sound pressure point created by the frequency Whose wave length is four times as long as said tube.

While the instant invention has been described and illustrated in connection with an automotive exhaust system, it is to be understood, that the modulated quarter wave tube can be used in combination With any acoustical system employing a pipe connected to a source of sound energy. It is to be further understood, of course, that any number of said modulated quarter wave tubes can be connected to a pipe connected to a source of sound energy.

We claim:

1. In a sound attenuating system, a gas carrying pipe adapted to be connected to a source of sound energy and having a plurality of sound pressure points spaced therealong, an elongated tube connected to said pipe having an open end in communication with said pipe at one of said pressure points and Ia closed end remote from its connection to said pipe, said tube having a length equal to about one fourth of the wave length producing said one pressure point, and at least one opening formed adjacent the closed end of said tube.

2. In a sound attenuating system, a gas carrying pipe adapted to be connected to a source of sound energy and having a plurality `of sound pressure points spaced therealong, an elongated tube connected to said pipe having an open end in communication with said pipe at one of said pressure poi-nts and a closed end remote from its connection to said pipe, said tube having a length equal to about one fourth of the Wave length producing said one pressure point, and at least one opening formed adjacent the closed end of said tube, the open area dened by said at least one opening having an overall cross-sectional extent equal to from about 1% to about 7% of the cross-sectional extent of the tube.

References Cited UNITED STATES PATENTS 2,027,359 1/1936 Wood et al. 181-59 XR 2,122,447 7/ 1938 Zand 1811-48 XR 2,189,425 2/ 1940 Leadbetter 181-54 2,297,046 9/ 1942 Bourne 181-48 3,209,858 10/196'5 Jettinghoff 181-54 3,263,772 8/1966 Irwin et al. 181-59 FOREIGN PATENTS 1,374,264 8/1964 France.

ROBERT S. WARD, IR., Primary Examiner.

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