Measuring and testing – Vibration
Patent
1997-04-18
1998-11-10
Williams, Hezron E.
Measuring and testing
Vibration
381 718, 381 7112, 381 731, G10K 1116
Patent
active
058346473
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a stationary device for attenuating noises generated by moving sources, and more precisely transport means such as aircraft or ground transportation.
BACKGROUND OF THE INVENTION
Transport noise is the main nuisance form of pollution affecting residential areas, indoors, when the windows are open and outdoors in gardens, parks, green spaces and terraces. This nuisance is increasing inexorably over time in most urban or suburban regions because of the increase in traffic, in particular resulting from the improvement in vehicle performance, this being in spite of the constant improvement in terms of their intrinsic noise level.
In view of the physical nature of sound, that is to say a pressure wave propagated "mechanically" in air, this implies that the only way of being protected from noise is to be sheltered by a screen interposed between the noise source and the region to be protected. This screen reflects or absorbs the sound which is received.
This solution is mostly used, either with walls of property or with "antinoise" walls, along traffic lanes. When it is economically and aesthetically acceptable, this solution is satisfactory. Unfortunately, its performance is limited by the effect of diffraction of the sound behind the screen, the effects of which are essentially connected with the span of the screen and the distance between the noise source (or the region to be protected) and the screen, these distances being taken relative to the wavelength of the sound to be attenuated.
However, for the noise produced by aircraft on take-off, a screen which allows a high level of protection is in general not architecturally acceptable.
In all cases, the optical opacity of the screens leads to an often unacceptable loss of view and light. Finally, the reflection of the incident noise increases the sound intensity nearby and often prohibits the use of a solution of this type.
These reasons create a need for acoustic screens which are more lightweight, open and transparent, and are designed with a view to being installed in an architectural assembly or an industrial plant.
With this in mind, it is possible to reduce the noise emitted by sources by adding to them a device which generates an opposite sound wave which, by interfering and combining with the initial wave, reduces the overall total sound intensity. This combination is possible by virtue of the fact that the auxiliary source, slaved to the main source, generates waves which have the same origin and therefore the same geometry. This device is, in particular, applied to car exhausts. A fortiori, it makes it possible to equip fixed installations of machines which emit noise, for example industrial gas outlets or air-conditioner blowers as described in patent EP-A-0,557,071. In order to be effective, these devices need to be attached to the main noise source, which cannot be envisaged for many transport means, in particular aircraft.
Stationary devices have also been proposed which are intended to be installed in a region which it is desired to protect. For example, document FR-A-1,494,967 describes a general electroacoustic device for absorbing sound and noise, comprising a plurality of secondary sound sources, each of which consists of a microphone, directed at the sound source to be neutralized, an amplifier and a loudspeaker directed towards the region to be protected, and is intended to emit a sound signal, with the same amplitude but opposite sign, in the direction of the said region.
However, this device presupposes that the secondary sources are distributed over a closed surface which covers all of the volume to be protected, which is not the case for a screen, the edge effects of which are a significant factor.
Furthermore, it presupposes that the acoustic centre of the microphones coincides with that of the associated loudspeakers, which is practically unachieveable for the following two reasons: of the incident noise wave and the pressure of the antinoise wave generated by the associated loudspea
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patent: 4208735 (1980-06-01), Suzuki et al.
patent: 4829590 (1989-05-01), Ghose
patent: 5224168 (1993-06-01), Martinez et al.
patent: 5363451 (1994-11-01), Martinez et al.
patent: 5438624 (1995-08-01), Lewiner et al.
"Active Control of the Sound Radiated by a Vibrating Body Using Only a Layer of Simple Sources", The Journal of the Acoustical Society of America, (Apr. 1995) No. 4.
Gaudriot Lionel
Martinat Jacques
Comptoir De La Technologie
Miller Rose M.
Williams Hezron E.
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