The nonlinear properties of air with ultrasonic sound allows for the creation of a “virtual microphone,” which is the analog of the ultrasonic narrow-beam loudspeaker. When an ultrasonic wave (pump wave) mixes with a baseband audio sound, sidebands are created around the ultrasonic carrier, and these can be demodulated at the receiver. A preliminary investigation showed that the following technical requirements must be achieved: (a) generation of an ultrasonic wave with small phase noise; (b) reception of the wave over a wide dynamic range to allow for real-time demodulation; (c) a composite dynamic of 120 dB within the region around the 40-kHz carrier in order to achieve a microphone comparable to conventional microphones.
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