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See here: https://www.youtube.com/watch?v=PD3Y1l8XyUw

Another route would be to mix the ultrasound with another sound closer to the ear, then there is no need for an electronic ear at any point. The interference between sound can cause the inaudible frequencies to become audible.

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Just on this topic, would it be possible to make a whistle to do the same thing? I.e. crafted so it emits both ultrasound, and the audible counterpart which interferes with it to make the return audible?

Perhaps it could be such that the ultrasound warbles whilst interfering sound does not (or vice versa), which would make the sources easier to distinguish also.

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