Electrical audio signal processing systems and devices – Hearing protectors – electrical
Patent
1989-04-07
1990-08-28
Isen, Forester W.
Electrical audio signal processing systems and devices
Hearing protectors, electrical
381 71, 381 96, A61F 1102
Patent
active
049532170
DESCRIPTION:
BRIEF SUMMARY
This invention relates to systems for reducing the level of acoustic noise fields within ear-defenders or earphone structures worn by personnel (e.g., pilots, vehicle drivers, military personnel) in high noise environments.
Known active noise reduction (ANR) systems for reducing the acoustic noise filed in ear-defenders comprise noise pick-up microphones and noise-cancelling sound generators (usually known as loudspeakers) mounted within the internal cavities or enclosures of the respective ear-defenders. The noise pick-up microphones produce electrical signal outputs in response to the acoustic noise fields within the cavities and these signal outputs are phase inverted, filtered and amplified in a feedback loop and fed to the noise-cancelling sound generators which produce noise-cancelling acoustic signals of substantially the same amplitude but of opposite phase to the acoustic noise field waveforms. The design considerations underlying such ANR systems are described in "Some transducer design considerations for earphone active noise reduction systems", Twiney et al., Vol. 7, part 2, pp. 95-102, Proc. Spring Conference, 1985, York, Institute of Acoustics.
Problems arise through inherent imperfections in the pick-up microphones and sound generators, by way of unwanted phase changes producing signal enhancement or by way of failure to cope with large amplitude signals in certain frequency regions.
One problem which occurs is that of large pressure pulses (buffets) which occur inside an ear-defender or earphone structure due to relative movement between the human head and the earphone, or propagate to the earphone from a device that causes a rapid pressure change, e.g. a gun, helicopter, vehicle, explosive device. These pulses are very high in amplitude, and create large signals in the feedback loop as a result of high system loop gain. Due to the inadequacy of the sound generator to produce enough sound output, drive voltages appear at the sound generators which are higher than the maximum input voltage, and may overdrive the sound generator and cause permanent failure.
Another problem which arises is that of signal enhancement at certain frequencies within the bandwidth of the feedback loop wherein due to imperfect transfer functions of the noise pickup microphone and sound generator the ANR will, at certain frequencies be feeding in-phase (i.e. positive feedback) signals rather than anti-phase (i.e. negative feedback) signals to the sound generator.
A further problem which occurs is that due to the imperfect transfer functions of both the microphone and generator, the total bandwidth for feedback signals having an appropriate phase is limited, being bounded by regions in which positive feedback occurs. It is usual to employ in feedback systems in general a lowpass first order filter operating at a high frequency in order to stabilize the loop. However such first order low pass filters are not appropriate for filtering out sound energy frequencies in ANR systems because of the large phase changes which occur in the cut-off regions which give rise to problems of positive feedback and signal enhancement.
It was previously thought, as appears from the article referred to above, that electronic processing to overcome problems in ANR systems had limited application because of the causal relation between amplitude and phase response of electronic filters.
Nevertheless it has now been found as a result of careful investigation into the problems arising in feedback loops of ANR systems, that electronic processing may be used to advantage.
It is an object of the present invention to overcome one or more of the above problems.
Accordingly the present invention provides in a first aspect an active noise reduction system comprising:
a noise-cancelling sound generator, a microphone acoustically coupled to said generator, a feedback loop connected between the microphone and the generator, the feedback loop including loop stabilisation means for filtering and inverting the phase of the microphone signal and means for a
REFERENCES:
patent: 4061875 (1977-12-01), Freifeld et al.
patent: 4455675 (1984-06-01), Bose et al.
patent: 4480333 (1984-10-01), Ross
patent: 4494074 (1985-01-01), Bose
patent: 4566118 (1986-01-01), Chaplin et al.
"Active Attenuating Noise--the State of the Art" Glenn E. Warnaka, Noise Control Engineering, May-Jun. 1982 pp. 100-109.
"Some Transducer Design Considerations for Earphone Active Noise Reduction Systems" R. C. Twiney et al., vol. 7, part 2 pp. 95-102, Proc. Spring Conference 1985, York, England, Institute of Acoustics.
"Handbook of Filter Synthesis", A. Zerev, p. 74.
Lancaster Active Filter Cookbook, 1979, p. 146, FIG. 6-24.
Salloway Anthony J.
Twiney Robert C.
Isen Forester W.
Plessey Overseas Limited
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