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Abstract:
A commercial analogue active noise reduction (ANR) communication headset, constructed using a circumaural hearing protector, has been modified to explore the performance of a digital ANR system for this application. The apparatus retains one ear cup and cushion, together with the earphones and attached microphone of the commercial device. The sound field outside the ear cup is sensed by a miniature microphone attached to the exterior of the ear cup, and forms the "reference" signal. This signal is fed forward to an adaptive controller, which drives the earphone within the ear cup. Control of the sound pressure within the volume enclosed by the hearing protector is maintained by an "error" microphone attached to the earphone, using the filtered-X LMS algorithm. The error-path impulse response is identified off-line. A Texas Instruments TMS320C31 floating point digital signal processor has been used to implement the control system in real-time. The noise reduction of the adaptive digital system will be discussed when the hearing protector is subjected to band-limited noise in the frequency range from 50 to 1000 Hz. (Work done in collaboration with the Defence and Civil Institute of Environmental Medicine, Toronto, Ontario).
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