The circuit in Fig 1 uses a pair of microphones to extract a voice from a noisy background. You must mount electret microphones X1 and X2 on the left and right sides of the user's chest.
Design Ideas: June 23, 1994
The circuit in Fig 1 uses a pair of microphones to extract a voice from a noisy background. You must mount electret microphones X1 and X2 on the left and right sides of the user's chest.
Sound sources not equidistant from the microphones experience a phase shift. Because sound travels at 1120 ft/sec, the maximum phase shift for this design's mounting scheme is about 0.5 msec. This phase shift corresponds to a 90 degrees phase shift at about 1000 Hz. Thus, all sounds in the vocal frequency range, except the user's voice, tend to cancel.
IC1A and IC1B buffer the signals from the microphones. R1 and R2 provide the needed bias current for the electret microphones. R3's and R4's values determine the conversion coefficients. Choose R3 and R4 so that IC1A's and IC1B's outputs never saturate. The bipolar 4-quadrant multiplier, IC2, separates the audio signal from the noisy background. R5 and R6 provide additional voltage gain after multiplication. R7, C1, R8, and C2 form a lowpass filter. (DI #1449)