Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Adjacent ear
Patent
1989-10-23
1991-05-21
Isen, Forester W.
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Adjacent ear
381 96, G10K 1116
Patent
active
050182030
DESCRIPTION:
BRIEF SUMMARY
Conventional noise attenuation techniques rely either on the generation of anti-sound (i.e. the generation of sound waves in anti-phase to the sound to be attenuated) or on the use of heavy sound absorbing or sound reflecting barriers. These techniques are not suitable for all applications. For example, the attenuation of noise to which airline passengers are subjected is a special problem which the invention aims to solve.
According to one aspect of the invention apparatus for attenuating noise comprises a diaphragm one side of which is subjected to the noise, sensing means for sensing the actual displacement of the diaphragm in response to the noise, and deflecting means responsive to the sensing means for deflecting the diaphragm in order to cancel the displacement, to attenuate noise transmitted from one side of the diaphragm to the other side of the diaphragm.
The invention therefore relies on the use of a diaphragm which is driven by the deflecting means in order to simulate a structure with a very high stiffness and hence a very high attenuation of transmitted sound.
The diaphragm may be tensioned, e.g. in a surrounding frame, and the deflecting means are preferably driven from the sensing means through a negative feedback loop which acts to null the displacement of the diaphragm, with the result that little or no sound is transmitted from the one side of the diaphragm to the other side of the diaphragm.
The deflecting means may be electrostatic in nature, in which case the diaphragm is preferably a flexible metal or a metallised or otherwise conductively coated plastics film to which a D.C. polarising voltage is applied disposed between two fixed perforated electrodes between which a deflecting voltage is applied. The D.C. polarising voltage may also be supplied by coating the perforated fixed electrodes with an electret material. The deflecting means may alternatively be piezoelectric, in which case the diaphragm may be a polyvinylidene fluoride (PVDF) film with opposed metallised surfaces providing the metal electrodes to which the deflecting voltage is applied. The application of a differential voltage between the electrodes causes the PVDF film to distort so that the film elongates and thins and thereby deflects in the required manner. As an alternative to PVDF film, electro thermomechanical film (ETMF) may be used. This is a material which behaves in a similar fashion to PVDF in that it distorts on application of a deflecting voltage applied across metal electrodes. A further possibility for the deflecting means is to use an electro mechanical arrangement comprising a film printed with a wiring "coil" cooperating with magnets.
The sensing means may be any of the above forms of deflecting means, or may alternatively be optical in character, such as an interferometer, focus probe or optical triangulation. Alternatively, other methods for proximity detection can be employed such as capacitive or inductive techniques.
In order to cope with complex phases of the sound impinging on the diaphragm, the latter may be divided up into separate areas, each area having a corresponding sensing means cooperating with a deflecting means, so that each area of the diaphragm is separately driven and can respond to the variation of sound impinging on that area.
As previously mentioned, the invention was devised specifically for reducing the noise reaching the ears of airline passengers. Ambient noise in aircraft is caused largely by the flow of air past the aircraft fuselage and wings and, to a lesser extent, by the aircraft engines. The requirement is for a lightweight easily fitted noise attenuating means which will be accepted by the travelling public and over which public address messages, film soundtracks or other audio information can be transmitted. For this application the invention is conveniently made in the form of headphones with each earpiece having a tensioned diaphragm which covers the user's ear when worn. Public address messages, film soundtracks and other audio information can be fed to the head
REFERENCES:
patent: 2387845 (1945-10-01), Harry
patent: 2972018 (1961-02-01), Hawley et al.
patent: 4712247 (1987-12-01), Swarte
Patent Abstracts of Japan, vol. 6, No. 173, 09/07/1982.
Edge Gordon M.
Sawyers Craig G.
Isen Forester W.
Scientific Generics Limited
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