The project “Virtual Microphones” aims to replace conventional audio microphones by a system consisting of ultrasonic sensors. The fundamental physical principle utilized is that ultrasonic waves are phase-modulated when superimposed by audio waves. Due to the physical relationships the phase changes of the ultrasonic carrier are very small. The precise demodulation of the audio signal was in the focus of the investigation among other measures, like providing a very stable carrier frequency or the focusing of the ultrasound beam on the receivers. Several methods of demodulation for phase-modulated signals are compared. The comparatively low resulting signal-to-noise ratio after the demodulation required further signal processing in form of noise reduction to be investigated and its performance accessed.
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