Noise-cancelling microphones

J. F. Sear and R. Catpeniet, Noise cancelling microphone using a piezoelectric plastics transducing dement. EUxu Lett. 77 532 (1975). 

The performance of a head-mounted two-microphone adaptive noise-cancellation system was investigated by Weiss [Weiss, 1987] and Schwander and Levitt [Schwander and Levitt, 1987]. In this system, an omnidirectional microphone was used for the speech signal and a rear-facing hypercardioid microphone mounted directly above the speech microphone was used for the noise reference. In a room having a reverberation time of 0.4 sec, this system improved the speech recognition score to 74 percent from 34 percent correct for the unprocessed condition for normal-hearing listeners given... 

Berghe, J.V. and Wouters, J., An adaptive noise canceller for hearing aids using two nearby microphones, The Journal of the Acoustical Society of America, 103,1998, 3621. 

Microphones in this group include hand-held noise-canceling units for mobile communications and industrial plant operations, handset and headset-boom microphones for intercom and order wire circuits, and public address microphones. Each user group has its own particular set of needs, met through variations on a few basic designs. 

Adaptive noise canceling makes use of an auxiliary or reference input in addition to the noisy signal from the primary sensor. If a reference for the noise is desired, the auxiliary microphone is placed at a position in the noise field where the signal is weak or undetectable. This scheme operates on the basis of two main assumptions ... 

In active noise cancellation, the noisy input signal is adaptively filtered to produce a signal that is supplied to a canceling microphone. The output from the canceling microphone destructively interferes with the noise (Oppenheim et al, 1994). A block diagram appears in Fig. 12.99. 

Apparatus. The experiment was conducted on a Dell Inspiron 630m laptop with 1.6 GHz Intel Centrino processor and 1 GB of RAM running the Windows XP Home operating system. The resolution of the 14.1 LCD was 1280 by 800 pixels. The experiment was conducted in full screen mode, with the user s head situated about two feet from the screen. A noise-canceling headset microphone (Altec Lansing AHS322) was used for audio input. 

The purpose of the error microphone is to measure the unwanted noise field after destructive interference. The adaptive filter takes this measurement and produces an estimate of the noise process, which is then used to adjust the filter parameters so as to improve the cancellation effect. In general, there may be a multiple error microphones producing an improved noise estimate however, Oppenheim et al. (1994) presents a scheme using only a single input sensor. The approach is evaluated using both computer generated noise and with recordings of aircraft noise. 

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