AU668100B2 - Muffler - Google Patents

Description

OPI DATE 05/04/93 APPLN. ID 24891/92 I AOJP DATE 10/06/93 PCT NUMBER PCT/GB92/01594 11111 11111111111111 1111111111111111 AU9224891 (51) International Patent Classification 5 FO1N 1/06 (11) International Publication Number: Al (43) International Publication Date:

S(PCT)

WO 93/05282 18 March 1993 (18.03.93) (21) International Application Number: (22) International Filing Date: Priority data: 9118779.9 29 Augus PCT/GB92/01594 28 August 1992 (28.08.92) t 1991 (29.08.91) GB (74) Agents: READ, Matthew, Charles et al.; Venner Shipley Co., 20 Little Britain, London ECIA 7DH (GB).

(81) Designated States: AU, CA, JP, KR, US, European patent (AT, BE, CH, DE, DK, ES, FR, GB, GR, IE, IT, LU, MC, NL, SE).

Published With international search report.

668100 (71) Appliant 6r a dsigated Sales ecept US). ACTIVE -NOISE AND VIBRATION TECHNOLOGIES INC.- 3i8 East Wier Avnue, FlutiliA, AZ 85040 (72) Inventors; and Inventors/Applicanis (for US only) JONES, Owen [GB/ GB]; Water Run Cottage, Mill Lane, Chattisham, Ipswich IP8 3PX TRINDER, Michael, Charles, John [GB/GB]; Rymer, Hadleigh Road, East Bergholt, Colchester, Essex C07 6QX (GB).

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CotMcc.-VtCu.A OloOIJ U .S .P (54) Title: MUFFLER (57) Abstract

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A muffler for muffling noise in a fluid flow, such as exhaust gases from an internal combustion engine comprises a chamber loudspeakers for injecting anti-noise into the chamber under the control of processing circuitry (12) in dependence upon residual noise detected by a microphone The gas flows from an inflow pipe to an outflow pipe past a radial aperture (13) that produces an acoustic coupling between the chamber (10) and the gas flow, to allow the anti-noise to mix with noise in the gas flow and produce noise cancellation. The coupling is so arranged that the chamber (16) becomes pressurised by the gas flow, in use, so as to restrict hot gas flow through the chamber and thereby provide an essentially benign environment for the loudspeaker.

Y WO 93/05282 PCTGB92/01594 1

MUFFLER

FIELD OF THE INVENTION The present invention relates to a muffler for muffling noise in a fluid flow, for example the exhaust flow from of an internal combustion engine.

BACKGROUND OF THE INVENTION Most noise from an internal combustion engine propagates from the engine's exhaust ports and has commonly been suppressed by means of mufflers, also known as silencers, located in an exhaust pipe connected,to the ports, Such mufflers are entirely passive devices.

It has been proposed to use active noise control techniques to control the noise propagating along the exhaust pipe. In one such system, the exhaust pipe is caused to pass through a chamber, in which a loudspeaker is mounted. The loudspeaker is used to produce anti-noise in the chamber, for cancelling the exhaust noise. It has been found that in order for L L WO 93/05282 PCT/GB92/01594 2 there to be sufficient acoustic coupling between the loudspeaker and the inside of the exhaust pipe, the portion of the exhaust pipe passing through the chamber should be perforated. However, a problem with this arrangement is that exhaust gases pass out through the perforations located towards the chamber inlet. This escape of exhaust gases results in a pressure differential along the length of the perforated portion of the pipe. As the pressure in the perforated portion of the pipe is lower towards the chamber outlet than towards the chamber inlet, exhaust gases in the chamber are drawn back into the exhaust pipe through the perforations located towards the chamber outlet. Thus, there is a constant flow of hot exhaust gases through the chamber during operation. These hot exhaust gases are deleterious to the loudspeaker mounted in the chamber.

In an alternative arrangement, an inflow pipe passes completely through the chamber and terminates wit-iin an outflow pipe. However, it has been found that when such an arrangement is used in a virtual earth noise cancellation system, the positioning of the system I'i sC- ll~ CI -CII microphone is extremely critical. Even with the microphone optimally placed, it is still not possible to achieve effective cancellation over a wide range of frequencies.

It is an object of the present invention to overcome the aforementioned disadvantages of the prior art and provide a muffler which enables satisfactory acoustic coupling to the interior of an exhaust pipe while providing a benign environment for a loudspeaker.

Summary of the Invention This invention is one broad form provides a muffler for muffling noise in a fluid flow, comprising an acoustic mixing chamber, means to inject anti-noise into the chamber, a conduit for conveying a fluid stream, receiving means coaxially aligned with the conduit for receiving fluid flowing therefrom, and an acoustic coupling means arranged to couple noise from the conduit into the chamber for mixing therein with said anti-noise, wherein the acoustic coupling means comprises a gap between an end of the conduit and the receiving means, the gap opening into the chamber.

o 0.: o T r t 0 4 o o a 4a "o00 u L i c~ WO 93/05282 PCT/GB92/01594 -4 Preferably, the muffler is arranged to act as an acoustical filter.

Conveniently, the conduit passes through the chamber, terminating just short of an opening in a wall of the chamber, in which case the conduit termination and the aperture can be coaxially aligned and the acoustic coupling means comprises the gap between the conduit termination and the opening. Alternatively, the conduit may run along the side of a chamber, the acoustic coupling means comprising a slot coupling the interior of the chamber with the gas flow in the conduit. The coupling means may also include bridging members for physically coupling the conduit to the receiving means.

Advantageously, the internal diameter of the conduit is less than the internal diameter of the receiving means. However, under certain circumstances, this may not be necessary to achieve satisfactory performance by the muffler.

WO 93/05282 PCT/GB92/01594 5 Preferably, the muffler will include a microphone.

Conveniently, the means for injecting anti-noise into the chamber comprises a loudspeaker. A plurality of loudspeakers may be employed.

Advantageously, the passive frequency response of the muffler may be tailored by resistively or capacitively loading the loudspeaker. Also, the drive circuit for the loudspeaker may include an amplifier circuit with a gain/frequency response configured to drive the loudspeaker so that its frequency response is modified to have a desired characteristic over a given frequency range.

The muffler may be employed in an active noise cancellation system, for example in the exhaust system of an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 1 11 1 11WO/ 28 1/ 1 ,IG. .2.

WO 93/05282 PP/GB92/01594 -6- Figure 1 is a side view of a muffler according to the present invention; and Figure 2 is a diagramatic representation of a muffler according to the present invention, employed in an active noise cancellation system.

DESCRIPTION OF EMBODIMENTS Referring to Figure 1 and 2, a muffler 1 comprises a rectangular box 2 formed from sheet metal. An outflow pipe 3 extends outwardly from a circular opening 4 located in a wall 2a of the box 2. An inflow pipe extends through a wall 2b of the box 2, opposite the opening 4, to a point just short of and in axial alignment with the opening 4. In a box 2 having a dimension A of 400 mm, the inflow pipe 5 terminates mm short of the aperture 4. The space between the end of the inflow pipe and the opening 4 defines a radial aperture 13 which in use produces an acoustical coupling between a chamber 10 and gas flowing from the inflow pipe 5 to the outflow pipe 3, as will be explained in more detail hereinafter.

WO 93/05282 PCT/GB92/01594 7 A pair of partitions 8 and 9 run parallel to the inflow pipe 5 and divide the box into three chambers. A central chamber 10 is defined between the partitions 8 and 9. Two moving coil loudspeakers 6 and 7 are mounted in the partitions 8 and 9 respectively, such that they are directed towards the inflow pipe A microphone 11 is mounted on the outflow pipe 3, adjacent to where it joins the box 2. An electronic processing unit 12, such as disclosed in our copending UK Patent Application No. 9116433.5 has an input which is electrically coupled to the microphone 11 and outputs which are coupled to respective speakers 6 and 7.

In operation, a portion of a gas stream flowing along the inflow pipe 5 will initially enter the central chamber 10. However, the central chamber 10 will quickly become pressurised thus restricting the further ingress of exhaust gases. Consequently, an exhaust gas Sflow path does not become established through the chamber. Hence, the environment in the central chamber Ii WO 93/05282 PCT/GB92/01594 -8 remains essentially benign for the speakers 6 and 7 as they are not exposed directly to a hot gas flow.

Accompanying the gas stream are sound waves, comprising engine noise, which propagate along the inflow pipe into the chamber 1 and out through the opening 4. The geometry of the muffler is designed so that higher frequencies, in the order of a few hundred hertz, are attenuated in the muffler.

Sound emerging from the muffler and propagating along the outflow pipe 3 is sensed by the microphone 11 which sends an electrical signal representative of this sound to the input of the processing unit 12. The processing unit 12 outputs electrical cancelling signals which drive the speakers 6 and 7 to produce anti-noise. The anti-noise mixes with the engine noise in the central chamber 10, which results in the sound propagating along the outflow pipe 3 having a much smaller amplitude than that propagating along the inflow pipe Once the control loop including processing unit 12 has come into operation, the microphone 1i senses the r WO 93/05282 PCT/GB92/01594 WO 93/05282 PCr/GB92/01594 9 result of the mixing of the engine noise and the anti-noise produced by the speakers 6 and 7.

With the arrangement described above, it has been found that a certain degree of enhancement can occur at noise frequencies in the 10 to 50 hertz region. The passive performance of the muffler in this region can be modified by reducing the electrical damping of the loudspeaker. This may be achieved by either connecting a low value resistor, for example 5 ohms, or a capacitance, for example 10C0 microfarrads, in parallel with each loudspeaker 6 and 7. The exact values for these additional components will depend on the geometry of the muffler employed and the degree of modification of the muffler response which is required.

The loudspeaker may be driven by circuits (within element 12) including amplifiers with frequency/gain characteristics selected to modify the natural characteristics of the loudspeakers. For example, the loudspeakers 6 and 7 may be made to appear acoustically as simple masses over a given frequency range. This is WO 93/05282 PCT/GB92/01594 10 achieved by arranging for the circuits driving the loudspeakers 6 and 7, to have output impedances which are the complex conjugates of the loudspeaker impedances over the given frequency range.

Whilst in the embodiment described hereinbefore, moving coil loudspeakers have been employed, other forms of transducer may usefully be employed such as piezo electric devices.

Although the inflow pipe 5 has been shown entering the chamber 1 through a wall opposite the aperture 4, alternative arrangements may be employed so long as the inflow pipe 5 terminates in alignment with an adjacent to the opening 4 so as to define a radial aperture such as the aperture 13.

Since, mixing of the engine noise and the anti-noise occurs within the central chamber 10 the position of the microphone 11 is not critical. Consequently, the microphone 11 may be pl .d in any convenient position e.g. within the centra.L chamber 10 or at the discharge end of the outflow pipe 3.

L in .I WO 93/05282 PCT/GB92/01594 11 The passive attenuation of higher frequency noise means that the processing unit 12 may be of a simple design since it need only function over a restricted frequency range.

In a modification, the muffler includes a compliant membrane (not shown) to isolate the region of the chamber 10 containing the loudspeakers 6 and 7 from the pipe 5, the membrane being substantially acoustically transparent for transmission of the anti-noise, but mechanically isolating gas from the exhaust from reaching the loudspeakers. As another modification, the inflow pipe 5 may be connected to the outflow pipe 3 on its lower side, thereby extending across the lower side of the aperture 13 in order to provide a flow path for liquid condensate in the pipe 5 to the outflow pipe 3. In this way an accumulation of liquid condensates in the chamber 10 is avoided.

It may also be desirable to include at least one inteiport in the baffles 8 and 9 to modify the acoustic characteristics of the chamber I, PCT/GB92/01594 WO 93/05282 12 In the aforegoing description the term anti-noise is used to mean acoustic signals controlled in phase and amplitude so as to tend to cancel an unwanted noise.

L I 1

Claims (9)

1. A muffler for muffling noise in a fluid flow, comprising an acoustic mixing chamber, means to inject anti-noise into the chamber, a conduit for conveying a fluid stream, receiving means coaxially aligned with the conduit for receiving fluid flowing therefrom, and an acoustic coupling means arranged to couple noise from the conduit into the chamber for mixing therein with said anti-noise, wherein the acoustic coupling means comprises a gap between an end of the conduit and the receiving means, the gap opening into the chamber.

2. A muffler according to claim 1, wherein the muffler acts as an acoustical filter. 0 T: 3. A muffler according to claim 1 or 2, wherein the 4 acoustic coupling means is within the chamber. ai o 4

4. A muffler according to claim 1, 2 or 3, wherein the o 0° a conduit extends into the chamber and the receiving means o004 *I includes an opening in a wall of the chamber, the conduit terminating adjacent to and in axial alignment 4 with the opening. A muffler according to any preceding claim, wherein the internal diameter of the conduit is less than that of the opening of the receiving means. i _U( I I A C) w7o 14

6. A muffler according to any preceding claim including a microphone for detecting residual noise to produce an electrical error signal.

7. A muffler according to any preceding claim, wherein the means for injecting anti-noise comprises a loudspeaker.

8. A muffler according to claim 7, wherein the means for injecting anti-noise comprises a plurality of loudspeakers.

9. A muffler according to claim 7 or 8, wherein the or each sai;d loudspeaker is resistively loaded. A muffler according to claim,' 7, 8 or 9, wherein the or each loudspeaker is capacitively loaded. C>11. A muffler acrigto any one of claims 7 to 0: 20 wherein the or each said loudspeaker is caused to appear acoustically as a simple mass over a given frequency CO range.

12. A muffler according to any preceding claim, wherein the conduit is an exhaust pipe connecte9 to an internal combust3.)n engine. TR13. A muf fler according to any precering claim l- including a membrane for isolating said fluid flow from said anti-noise injecting means.

14. An active noise cancellation system including a muffler according to any preceding claim. A muffler substantially as hereinbefore described with reference to the accompanying drawings. Dated 19 January, 1996 Noise Cancellation Technologies Inc. Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON 6 o a 9e 4 4 1 4 RA /V Tr- 0