Acoustic attenuation device with active double wall

Details Acoustic attenuation device with active double wall Acoustic attenuation device with active double wall

1996-02-15

1998-03-03

Kuntz, Curtis

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Adjacent ear

381 94, H04B 1500, A61F 1106

Patent

active

057244324

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to an acoustic attenuation device, comprising two substantially parallel plates defining a rectangularly shaped space, noise detection means arranged between the two plates, inverse noise emission means arranged between the two plates, and control means for controlling the inverse noise emission means in such a way as to minimize a quantity supplied by the noise detection means.
Applications of the invention are, for example, in the field of sound insulation of premises, in particular with double glazing, in the production of cowlings for equipment that generates noise, or in the field of insulating the passenger compartments of means of transport.
A device of the type indicated above, termed active double wall, relies on the operating principle summarized below.
The mass-spring-mass resonant frequency of a double wall constituted by two parallel rectangular plates separated by an air sheet of thickness d is given by the equation: ##EQU1## with: p.sub.0 : density of the medium located between the plates (1.18 Kg/m.sup.3 in the case of air) in the case of air). ##EQU2## stiffness of the air sheet m.sub.1, m.sub.2 : mass per unit area of the plates (in kg/m.sup.2)
This resonant frequency generally lies between 50 and 250 Hz.
Overall, for a given frequency f, the acoustic behavior of a double wall is considered to be as follows: between the plates remains small. The double wall behaves as a single wall of equivalent mass. vibrate in phase opposition. This leads to large variations in volume of the air sheet (phenomenon of "breathing" of the plates) and to poor acoustic insulation by the double wall. sheet. The acoustic insulation of the wall then increases rapidly with frequency.
The attenuation device aims to compensate for the poor acoustic insulation provided by the double wall close to f.sub.mrm. The principle consists in preventing, by means of an electro-acoustic system, any variation in volume of the air sheet.
The acoustic pressure field in the air sheet can be written in the form of a modal series: ##EQU3## with: .alpha..sub.1mn : amplitude of mode 1,m,n the case of a parallelepipedally shaped air sheet: (n.pi.z/L.sub.z) (3)
The eigenfrequency f.sub.lmn of a mode with indices (l,m,n) of the air sheet is given by the equation: ##EQU4##
The variation in volume of the air sheet is directly proportional to the amplitude of the (0,0,0) mode, without the amplitude of the other modes close to the resonant frequency f.sub.mrm of the wall being affected. However, it is difficult to measure and excite only this mode by actions which, a priori, involve all the modes. Indeed, the expression given above (2) for the acoustic pressure shows that the measurement taken by a microphone will include the responses of modes other than the (0,0,0) mode.
It is desirable, in order to obtain efficient attenuation, to reduce the contribution, in the quantity to be minimized, of the low-frequency modes other than the (0,0,0) mode, and to operate so that the inverse noise emission means excite the (0,0,0) mode predominantly while exciting the other modes of the air sheet as little as possible.
One object of the invention is thus to improve the efficiency of the attenuation provided by an active double wall device.


SUMMARY OF THE INVENTION

To this end, the invention provides an acoustic attenuation device of the type indicated at the start, characterized in that the inverse noise emission means comprise four actuators whose respective positions parallel to the plates correspond approximately to the four points constituting the centers of the sides of the rectangular shape of said internal space, in that the noise detection means comprise four sensors whose respective positions parallel to the plates correspond approximately to the four points constituting the centers of the sides of a rhombus whose vertices are the centers of the sides of the rectangular shape of said internal space, in that the four actuators are controlled in phase, and in that the quantity to be mi

REFERENCES:
patent: 4947356 (1990-08-01), Elliott
patent: 5024288 (1991-06-01), Shepherd et al.
patent: 5245552 (1993-09-01), Andersson
patent: 5315661 (1994-05-01), Gossman et al.
patent: 5627897 (1997-05-01), Gagliardini et al.
Patent Abstracts of Japan, vol. 015, No. 276 (M-1135) 12 Jul. 1991 for: JP,A,03,095,349 (Hitachi Plant Eng & Constru Co Ltd).
Journal of the Acoustical Society of America, vol. 92, No. 3, Sep. 1992, New York US, pp. 1521-1533, Clark et al., `Optimal placement of piezoelectric actuators and polyvinylidene fluoride error sensors in active structural acoustic approaches`.
WO 94 05005, Noise Cancellation Technologies, 3 Mar. 1994.

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