Science of Synthesizing Audible Sounds from Ultrasonic Intermodulation Distortion

How can inaudible ultrasonic signals lead to audible byproducts? When multiple ultrasonic tones pass through a nonlinear medium such as air or a microphone, the result is intermodulation distortion. In our experiment, we have two signals. One is a 180 Hz sine wave AM modulated over a 32 kHz ultrasonic carrier. The second is a simple 25 kHz ultrasonic sine wave. The smartphone displays the Fourier transform of repeated intermodulation distortion in the air and smartphone microphone circuitry. The 2nd order intermodulation distortion includes the difference between the two signals, which appears centered at 7 kHz and peppered with sidebands from the modulated 180 Hz. The higher order intermodulation distortion products create additional ripples in the spectrum at 7 kHz as well as several other frequencies. Matlab simulations predict the strong 7 kHz intermodulation distortion product, and we suspect the 4 kHz tones are the result of secondary intermodulation distortion in the microphone. Learn more at spqr.eecs.umich.edu and usslab.org.

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