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Sum and difference signals

Let's say you have two coincident pure sine wave tones generated simultaneously - a 30,000 Hz signal and a 31,000 Hz signal. There will actually be four tones produced - the original two, plus (at greatly reduced volume) a tone that is the sum of the two (30,000 Hz + 31,000 Hz = 61,000 Hz), plus (at greatly reduced volume) a tone that is the difference of the two (21,000 Hz - 20,000 Hz = 1000 Hz). That difference signal is called a "beat".

This is at least one way in which harmonics above the normal frequency range of human hearing can affect the audible range.

Whether or not this translates into a requirement for playback systems with response well in to the ultrasonic range I cannot say. Theoretically the audible beat frequencies would have been captured in the original recording, and so would not have to be re-created by the physical interaction of ultrasonic frequencies. Personally I think bandwidth well into the ultrasonic range is desirable, but probably not cost-effective to pursue.

Below is a link to a site that explains beats much better than I did.

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