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A little though of this curious subject...
Maybe, depending on the model, it are outputting audible tones with ultrasonic hardware, when it emits 2 ultrasonic tones (2-tone mode, but I dunno about models using this scheme), and it intermodulate. 8kHz audible can for eg. significate a 30kHz and 38kHz making a IMD difference tone.
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So many projects, so little time...
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