Beats vs. Difference Tones

DGoncz wrote:

>My physics teacher told me about an article in a physics journal from back in
>the 60's or 70's about a pehnomenon called the "ghost flute".

rtomes wrote:

>Probably Scientific American. I don't think a physics journal would
>publish a paper on perception of a beat note.

I think we all should keep in mind the difference between beats and
difference tones. Beats are periodic reinforcement and cancellation of
a two-tone waveform in a *linear* medium (electronic or acoustical).
There is *no* physical energy created at the beat note frequency. One
can prove this with two oscillators and a spectrum analyzer. A
difference tone (a.k.a. a Tartini tone) is produced by mixing two tones
in a *non-linear* medium, such as the human ear (or electronically in a
fuzz-box), also called heterodyning. This process *does* create
physical energy at the difference frequency (and sum frequency, and many
other higher order products). It is this *real* tone that the human
subject hears, eg, the "ghost flute".