Active control of noise

Details Active control of noise Active control of noise

1992-09-30

1994-11-08

Isen, Forester W.

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Adjacent ear

G10K 1116

Patent

active

053634500

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method and apparatus for the active control of noise in exhaust systems.
2. Discussion of Prior Art
Conventional exhaust systems consist of a length of pipe with silencers connected to an engine. The effect is to dampen the sound and vibration produced by the exhaust gases.
As there is no perfect exhaust system there is a requirement to improve the level of sound or vibration reduction which can be achieved.
Consequently the present invention seeks to reduce the sound and vibration in an exhaust system by a method of active noise control.


SUMMARY OF THE INVENTION

Accordingly there is provided a method for reducing the noise produced by an engine's exhaust system which comprises the introduction of extra fluid to the exhaust system in addition to that produced by the exhaust output of the engine characterized in that the introduction of fluid into the system is suitably timed such that the noise source (exhaust) and noise produced by the fluid introduction are of different phase and ideally opposite.
The term exhaust system is applicable to any hot gas flow where noise is present.
It is thought that by the suitably timed addition of a fluid such as air or water into an exhaust system that the sound and vibration which this produces can be in antiphase to the originating exhaust sound and vibration. This results in a reduction or cancelling of the sound and vibration. The total noise output of the system will actually be increased if the noise produced by fluid injection is in phase with the original exhaust noise.
To produce sound an effluent gas must contract or expand to create a volume velocity, that is, there must be a net volume change. Thus volume changes in the exhaust lead to noise.
By adding a fluid to the exhaust there is an initial increase in volume which may be offset by cooling or other effects which result in an overall decrease in volume. These volume changes can oppose those occurring naturally in the exhaust. The effects of volume increase and contraction cooling can be experienced with various fluids but addition of liquid such as water has the further effect of vaporisation thus resulting in a volume increase. The latent heat of vaporisation for a liquid introduced to the exhaust has the effect of cooling the pressure peaks. Therefore if cold water is added to the hot gases of an exhaust system the effects of heat extraction and vaporisation compete. In this case the cold water and subsequent vaporisation cool the gas causing a contraction while water vapor produced by the vaporisation causes expansion.
Preferably the fluid introduction takes place in the proximity of the outlet manifold of the exhaust.
Preferably the fluid introduced is a liquid. The boiling point of any liquid introduced should be lower than the temperature of the exhaust gases to ensure vaporisation of the liquid.
Preferably the fluid introduced is water.
Whilst there are various ways of introducing the fluid it is preferably injected into the exhaust system. In the case where turbo chargers or super chargers are fitted the fluid may be pressurized into the exhaust via a bleed from the turbo charger or super charger. Alternatively, fuel injectors can be used for fluid introduction to the exhaust system.
Preferably the fluid, when a liquid is introduced into the exhaust system, is a spray.
The effective operation of the invention's active noise control depends to some extent on adopting the appropriate procedure for a particular noise source. For example where there are large source strengths it is necessary to have low exhaust temperatures together with a fine spray of liquid, preferably water, injected into the exhaust to achieve good active noise control.
Alternatively for noise sources of high bandwidth, high Mach numbers for exhaust velocity and high exhaust temperatures should be used together with a fine spray of liquid and turbulent mixing of liquid and exhaust gases.
Bandwidth of the active noise reduction system can b

REFERENCES:
patent: 2692024 (1954-10-01), Burdett, Jr. et al.
patent: 3936606 (1976-02-01), Wanke
patent: 4194095 (1980-03-01), Doi et al.
patent: 4859345 (1989-08-01), Inagaki
patent: 5131229 (1992-07-01), Kriegler et al.
Japanese Abstract No. 57-140425 dated Aug. 1983.
Russian Abstract No. 1545-000-A dated Jun. 1987.
European Search Report.

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