I really don't know what you all are wondering about.
If I mix two signals (300Hz and 303Hz) together I get a
sum with an overall amplitude envelope of 3 Hz. Everybody can
see that on an oscilloscope.
Therefore the brain does nothing more than just a dumb mixing (adding)
of both signals and the sum is perceived as a beating envelope. There is
really no mystic with "computing the difference" or so ...
Andre "Czesanne" Ch. Schnoor
Composer & Vocalist, Audioland International
czesanne at mail.hh.provi.deczesanne at proaudio.de