A reflective optical position or object detector, containing a light source (LED) and detector (phototransistor) is placed proximate to an acoustic membrane such that the output of the detector produces an electric signal corresponding to the motion of the membrane toward and away from the detector. Groups of these detectors can be placed at different locations under a single membrane to reproduce the frequency and harmonic content of the motion of the membrane at those locations, and the signals from each can be combined in variable proportion to a resultant electrical signal. These groups can be bounded by isolating frames and several bounded groups can be placed under a single membrane, or be covered by separate membranes. The groups with their bounding frames can be moved toward or away from the membrane, placing more or less tension upon the membrane, thereby altering the harmonic and frequency content of its vibration.
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