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.

BZ-ZKGRODND 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 connectedjto 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

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 within 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

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

According to the present invention there is provided a muffler for muffling noise in a fluid flow, comprising a chamber, means for injecting anti-noise into the chamber, a conduit for conveying a fluid stream, means for receiving fluid flowing from the conduit, and coupling means including a radial aperture between the conduit and the receiving means, for acoustically coupling the chamber to the fluid stream, wherein the coupling means is arranged such that the chamber becomes pressurised in use, such as to restrict flow of fluid from the fluid stream through the chamber.

Preferably, the muffler is arranged to act as an acoustical filter.

Conveniently, the conduit passes through the chamber, teiminating just short of an opening in a wall of the chamber, in which case the conduit teπnination 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.

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:

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 5 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 25 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.

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 5.

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 flow path does not become established through the chamber. Hence, the environment in the central chamber

10 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 5 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 5. Once the control loop including processing unit 12 has come into operation, the microphone 11 senses the

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 1000 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

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 placed in any convenient position e.g. within the central chamber 10 or at the discharge end of the outflow pipe 3.

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 in low 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 interport in the baffles 8 and 9 to modify the acoustic characteristics of the chamber 10.

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.