In article , Don Pearce
wrote:
Actually, 30 and 33kHz are going to generate products at 27 and 36kHZ -
not much help.

Why are you assuming their'd be no f1 - f2 result?

I made a file with 20kHz and 38kHz. The 2kHz product was clearly
audible, and very level dependent. There was actually some kind of
threshold effect. I prevented distortion in the amplifier being a
confounding factor by recording them on separate channels.

2*f1 - f2 ?

Jim

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