Electrical audio signal processing systems and devices – Hearing protectors – electrical
Patent
1987-09-23
1989-05-23
Isen, Forester W.
Electrical audio signal processing systems and devices
Hearing protectors, electrical
381 94, A61F 1102
Patent
active
048337196
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to methods and apparatuses for attenuating external origin noise reaching the eardrum, and for improving the intelligibility of electro-acoustic communications.
The technical field of the invention is the construction of acoustical protectors for the ear.
Passive soundproofing means are known such as headsets or earmuffs placed over the ears to protect persons required to remain in very noisy surroundings. Such means are used, for example, by workers working in certain types of workshop, by the drivers of very noisy vehicles (airplanes, tanks, . . .), by ground personnel at airports, etc. . .
Such headsets generally comprise a muff of absorbent material which envelops the ear.
In this type of sound insulation, incident airbourne sound waves are attenuated by reflection and by absorption in the mass of the material which is acting as a passive screen.
Experience shows that passive soundproofing means are not very effective for low frequency sounds, and in particular in the frequency range below 500 Hz. In order to be effective at such frequencies, such headsets would require prohibitive densities or thicknesses of material.
Sound insulating headsets are known which additionally include a loudspeaker or an electro-acoustic transducer incorporated inside each muff in order to allow the user to hear messages which are transmitted by the electro-acoustic means.
The problem to be solved is to improve the efficiency of headsets and other passive soundproofing means by adding apparatus thereto for improving the attentuation of sounds of external origin lying in the frequency range where the passive means is not very effective.
Apparatus known as an active acoustical attenuator is also known which serves to attenuate some sounds by making them interfere with other sounds which are set up in phase opposition to the sounds to be attenuated.
First attempts took place around 1953-1956 and have been described by Olson and May.
The active acoustical attenuator proposed by these authors comprises a microphone connected to a loudspeaker via an electronic amplifier in such a manner that the loudspeaker produces pressure within the cavity which opposes the pressure due to the incident wave picked up by the microphone. If the incident wave is random noise, the attenuation obtained by this method is not very good, and in addition this method gives rise to instabilities due to reasonances at certain frequencies ("howlaround", or the Larsen effect).
In 1955-1956, Hawley and Simshauer published work inspired by Olson's work. In order to avoid resonances due to sound feedback, Hawley proposed making soundproofing headsets which attenuated only some kinds of unwanted noise, which was either in the form of pure harmonic sounds or else in the form of very narrowband noise. Such headsets can only attenuate noise of a form determined in advance and they do not attenuate noise lying in a wide frequency range.
At pages 399 and 403 of Inter Noise 1983, Chaplin and Smith describe an anti-noise device enabling harmonic sounds to be attenuated by making them interfere with a synchronous sound in phase opposition as generated by a synthesizer controlled by digital electronics. Such apparatus using a sound which is synchronous with the sounds to be attenuated can be used only for attenuating a noise made up of a pure frequency together with its harmonics. Any variations in the frequency of the sound must be slow in order to make it possible for the digital processing to correct the frequency of the feedback signal. The need for a synchronization signal means that a noise canceller must be provided for each source of noise. Digital electronics is complex and expensive to implement. This method is thus not usable for improving the efficiency of passive soundproofing means intended to be used in very noisy locations in sound fields essentially constituted by random noise.
French Pat. No. 75 34 024 (A.N.V.A.R.) describes active sound absorbing apparatus for attenuating plane sound waves propagating along a duct.
Frenc
REFERENCES:
patent: 4494074 (1985-01-01), Bose
Carme Christian E.
Roure Alain R.
Centre National de la Recherche Scientifique
Isen Forester W.
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