Noise Cancellation

Abstract

A pipe for an engine exhaust is joined with a cancellation tube in a V-joint and beyond this joint a single pipe portion extends to an exhaust opening. The cancellation tube provides a shunt path for sound waves produced by a sound transducer disposed at the outside end of the cancellation tube. The sound transducer is driven by an amplifier and phase inverting network which in turn responds to a sensor transducer in the first pipe. This transducer establishes a signal in accordance with the noise in the first pipe and the sound transducer thus produces sound waves corresponding to and about 180.degree. out of phase with the engine noise. At the V-joint, the noise of the engine exhaust is largely cancelled by the waves in the cancellation tube.

Description

BACKGROUND OF THE INVENTION

This invention relates to active noise cancellation and more particularly to active noise cancellation in sound transmission channels that carry sound waves from a noise source.

Various noise sources transmit sound through confined sound transmission channels. For instance, combustion engines cause pulsating, high volume noise which is most noticeably emitted through their exhaust channels. Previously, noise suppression for this exhaust has involved passive devices such as mufflers, that are effective silencers only at the expense of obstructing the free flow of the exhaust, resulting in undesirable back pressure on the engine. Other sound transmission channels present similar difficulties in noise reduction. It has been recognized theoretically that active noise reduction concepts could be employed to cancel noise waves. However, the prior art has not revealed a successful active device for use in confined channels.

For example, noise is very often random in character, and for effective cancellation, the active device has to nearly duplicate the noise waves. In the prior art, active noise cancellation has been provided only for single or regular tones, and therefore is not useful for such applications as engine exhaust noise cancellation. As well, prior art devices are dependent on the noise source, requiring tuning to the source in order to duplicate the waves. Further problems are experienced in attempting to obtain a mechanical arrangement of the active device in order to provide cancelling waves which are in the same wave plane as the noise waves. Without this matching of the wave planes, cancellation occurs only at points, or only for a small part of the noise waves.

These and other problems in the art have not previously been solved. The theory has not developed into successful practice.

SUMMARY OF THE INVENTION

The invention is directed to an active noise cancellation concept which is successfully employed in confined sound transmission channels and even with such severe noise sources as combustion engine exhaust systems. Other uses and advantages of the invention will be apparent from the drawings and description.

The practice of the invention provides effective noise cancellation in any confined noise channel, such as pipes, solid bars, and the like, regardless of the noise source and without obstructing the use of the channel. The noise source is at one end of a first transmission channel so that sound waves are generally confined within the channel. A second transmission channel, or a cancellation channel, is joined to the first at a joint.

Cancelling sound waves are provided in the cancellation channel, and are channeled to the joint. At the joint, the cancelling waves are generally oppositely out of phase with, but effectively in the same plane as the noise waves in the first channel. The amplitudes and shape of the noise waves and cancelling waves are also similar, so that the two largely cancel beyond the joint.

To provide the cancelling waves, a sensor transducer is disposed at a predetermined position on the first channel. Being in communication with the pressure pulses which comprise the noise in the channel, the sensor transducer establishes a corresponding signal to drive a driver transducer that is disposed on the cancellation channel at a predetermined position such that the distance between the sensor and the joint is related to the distance between the driver and the joint, and the sensed sound wave and the cancelling wave meet at the joint. Phase inversion is provided for the sensor signal to cause the cancellation wave to be generally oppositely phased from the noise wave at the joint.

This structure operates in accordance with the invention to provide effective noise cancellation, which works regardless of the noise source, is effective for practically any noise frequency or complexity, and avoids obstruction of the use of the sound transmission channel.

The sensor is positioned on the first channel so as to minimize feedback from the cancellation wave, which otherwise could cause instability in the system. This feedback will also be effected by the angle of the joint, and the optimum form of the invention, as limited by practical considerations of design, would be to have a relatively small angle inclined toward the noise source; although, a less perfect practice of the invention could be accomplished at any angle for channels whose thickness are small relative to the wave lengths involved.

The drawings illustrate the best mode presently contemplated by the inventor for carrying out the invention.

In the drawings:

FIG. 1 is a perspective view of an example of the invention with parts shown diagrammatically; and

FIG. 2 is a diagrammatic illustration of the invention.

DESCRIPTION

In the drawings, a noise source 1 is diagrammatically shown as a four cylinder combustion engine in FIG. 1. The engine has four exhaust ports which emit exhaust into a manifold 2 connected to an exhaust pipe 3. Pipe 3 extends to a V-joint 4 where it is interconnected with a cancellation tube 5 in a "V" configuration. Added to the V-joint is a rear exhaust pipe 6, to add a bottom leg to the V-joint. Exhaust from source 1 passes through pipes 3 and 6 and out an exhaust opening 7 in the latter.

Cancellation tube 5 and exhaust pipe 3 are shown intersecting at a small angle, of the order of 30.degree. or less for optimum operation in this example, and both are preferably at an angle relative to rear pipe 6. More complete cancellation of the noise will result at these small angles. But good cancellation can occur with any angle at the joint, if the diameter of pipe 3 is considerably less than the noise wave length. In that case, the pressure waves will be generally oppositely phased across the joint.

To produce cancelling sound waves in cancellation tube 5, an active system 8 couples to the exhaust pipe and cancellation tube. Included in this system is a sensor 9, which is a pressure responsive transducer with a diaphragm 10 in communication with the inside of pipe 3 through a hole 11. A seal 12 is disposed between the body of sensor 9 and pipe 3 to avoid leakage of exhaust from hole 11 into the atmosphere.

Sensor 9 may be a standard microphone for producing an electrical signal of a frequency corresponding to the noise waves of the exhaust at the position of sensor 9 and of an amplitude proportional to such noise waves. This signal is supplied via leads 13 to a phase changer network 14 and from there to an amplifier 15. Network 14 and amplifier 15 are individually known and network 14 has an active network which causes phase shift in the signal so as to be opposed to the amplitude of the sensor signal. Optimum phase shift would be 180.degree..

Amplifier 15 drives a driver transducer 16 to create and propagate cancelling pressure waves through cancelling tube 5. Driver 16 may also be a conventional device and is shown attached over the outer end of the tube to couple the output of driver 16 to the cavity and close the end of the tube. In this embodiment, the driver should be a high intensity transducer to match the noise source.

The components of active system 8 drive driver 16 to produce sound waves of the same amplitude and shape as the noise picked up by sensor 9. These components are interdependent parameters which, by testing, may be chosen in accordance with the particular devices employed. The distance between sensor 9 and joint 4, or the intersection point of the two wave planes, and the distance between that point and driver 16 is another important parameter. With the 180.degree. phase shift and with the same transmission media, these distances can be nearly equal. The intersecting waves are then out of phase after having traveled through the same distance. The phase changer network could include a suitable delay timer, not shown, so that the sensor could be moved closer to the noise source. The delay would account for the greater distance of travel for a wave from the sensor to the joint. Except where the wave lengths are very short, such as one-fourth the length of tube 3, slight variations in these distances will not unduly hamper noise cancellation. However, if sensor 9 is too far forward of driver 16, the cancellation can be effective only with more complex and exact phase changer networks, and the noise can be cancelled only if the noise waves are predictable.

The angle of intersection of the waves is a less important factor for effective noise cancellation. Wave pulses 17 are illustrated in FIG. 2 to demonstrate this. Where the pulses from the source and from the cancellation are as nearly in the same plane as practicable when they intersect, most effective and full cancellation will occur at joint 4. With usual wave lengths in engine exhaust noise, however, effective pressure wave cancellation will occur across the full diameter of the joint even where the angle of intersection is 90.degree. or more. For this reason, the angle between the axes of pipe 3 and cancellation tube 5 should be made as small as practical, but the matter is not critical unless the pipe diameter is large compared to noise wave length.

Feedback from the joint to sensor 9 will also have to be considered in deciding the exact arrangement of the sensor relative to the joint. The sensor should be, in effect, shielded from feedback of the cancellation waves for optimum stability. The arrangement of the sensor is best determined by trial and error testing with a particular system, and the drawings show a simple example of a successful arrangement.

The invention thus may be employed to effectively cancel noise in a tubular channel such as an exhaust pipe. The concept may be employed in any confined channel, however. The operation of the invention in this example could be initiated in any suitable manner, such as by a relay starter switch, not shown, interlocking with the ignition system for the engine. Sensor 9 automatically drives driver 16 whenever exhaust noise is experienced, and as described previously, the cancelling waves are produced in cancelling tube 5.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims which particularly point out and distinctly claim the subject matter which is regarded as the invention.

Claims

We claim:

1. An active noise reduction system for a noise source, comprising:

a first sound transmission member defining a sound channel for the noise source;

a second sound transmission member defining a second sound channel joined to said first sound channel at a common junction and extending outwardly from said first sound channel;

a sound sensor transducer coupled to the first sound transmission member and generating an electrical signal corresponding in frequency with and proportional to the sound in the first sound channel at the point of coupling, a driver member connected to and actuated by said transducer and generating and sending cancelling sound waves through said second sound channel to the first sound transmission member to meet the sound waves from the noise source at said common junction, and

said first and second sound transmission members being straight tubes and joined with an angle of less than 90.degree. and generally in a V-configuration and with an exhaust opening located at the apex of the V-configuration.

2. An active noise reduction system for a noise source, comprising:

a first sound transmission member defining a sound channel for the noise source;

a second sound transmission member defining a second sound channel joined to said first sound channel at a common junction and extending outwardly from said first sound channel;

a sound sensor transducer coupled to the first sound transmission member and generating an electrical signal corresponding in frequency with and proportional to the sound in the first sound channel at the point of coupling, a driver member connected to and actuated by said transducer and generating and sending cancelling sound waves through said second sound channel to the first sound transmission member to meet the sound waves from the noise source at said common junction, and

a phase change and amplification stage connected to said transducer to receive said electrical signal and to said driver member and producing generally a 180.degree. phase inversion,

said sensor transducer and said driver member are spaced generally equidistant from the intersection of said first and second sound channels in the direction toward the noise source.

3. An active noise reduction system for a noise source, comprising:

a first sound transmission member defining a confining sound channel having an input end for transmission of first sound waves from the noise source;

a second sound transmission member, said member being straight and substantially shorter than the first member and defining a second confining sound channel joined to said first sound channel at a common joint and extending outwardly from said first sound channel and rearwardly toward said input end at a selected angle;

sound transducer means connected to said first sound transmission member and to the input portion of said second sound transmission member in spaced relation to the connection to the first transmission member and sending cancelling sound waves through said second sound channel to the first sound transmission member to meet the sound waves from the noise source with the cancelling sound waves being of corresponding amplitude and of an opposite phase from the first sound waves; and

said selected angle being less than 90.degree. to define a V-configuration and selected to establish essentially coincident planes for the first sound waves and the cancelling sound waves at the common joint.

4. The apparatus of claim 3 wherein said selected angle is of the order of 30.degree..

5. The apparatus of claim 3 wherein the diameter of said first sound transmission member is substantially less than the noise wave length.