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Considerations in Applying Noise Cancellation Techniques to Telephones

Alan S. Nasar
Department of Mechanical Engineering
Cooper Union
51 Astor Place
New York, NY 10003

Popular version of paper 3aEA11
Presented Wednesday morning 15 May 1996
Acoustical Society of America, Indianapolis, Indiana
Embargoed until 15 May 1996

The goal of this project is to design a system that increases the intelligibility of a voice transmission through a telephone or similar device in noisy environments. The design incorporates an auxiliary microphone into the transmitter portion of an ordinary telephone handset. This microphone is located in such a manner that it receives the ambient noise without any speech from the user. The signal from this microphone is subtracted from that of the principal microphone to produce a final signal that contains significantly less background noise than the original signal. The corrected signal is then sent through the telephone in the same manner that the transmitter would in an ordinary telephone.

Active noise control is a noise reduction technique that uses "antinoise" to cancel unwanted sounds. An acitve noise control system duplicates offending sounds and causes them to be emitted out of phase with the original sound by electro-acoustical means. The basic principle used in this type of system is that of destructive acoustic interference. This system can be utilized for many applications in the use in noisy environments such as subway stations, city streets, aircraft cabins, truck cabins, construction sites, etc.

Figure 1 shows a schematic of the circuit for the noise-cancellation telephone. The function of the circuitry in Figure 1 is to reverse the noise signal and add it to the voice and noise signal.

Figure 1

Signal cancellation works only if the same signal is transmitted to both microphones, the principal and cancellation, at the same time and with equal intensity. For this to happen, both microphones must be located at the same relative distance from the ambient noise source. Also, in order to achieve optimal performance the voice signal must be prevented from being transmitted to the auxiliary microphone which would cause cancellation of the voice as well, thus decreasing the intensity of the voice signal.

Two equivalent microphones have been tested at various angles with respect to each other, achieving a cancellation of at least 15 dB throughout most of the frequency spectrum. Although these results show a substantial reduction in noise transmitted, one must keep in mind that the microphones were not mounted in a telephone - rather, they were free standing. The presence of the telephone handset will disturb the acoustic field in this region. Hence the results obtained are inconclusive regarding the optimal placement of the auxiliary microphone in a telephone handset. Further testing using actual telephone handsets with the auxiliary microphone at different positions will be conducted to establish the optimal positioning of the auxiliary microphone.

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