David:
I have  to endorse the recomendation to use ..uh...'less than perfectly
harmonic' sounds when investigating tuning/wave interactions....pure
waves will not only audibly and often annoyingly 'beat', but 'subsonic'
beat/interference can produce some wildly varying amplitudes, as large
amounts of the signal energy in the two signals cancel or reinforce
each other. And instead of a lovely complex interaction of sound, one
gets wildly varying, sometimes nice, but often unlistenable results.
Probably not what you're looking for.But take heart! There are several
simple solutions.
In addition to having a somewhat "noisy" or inharmonic basic waveform,
one can also
1)play 2 or 3 copies of the same signal separated in frequency by a very
small amount
("chorusing" or "piano tuning solution": ask a piano tuner, if you don't
know about this
already)
2) add very small and fairly rapid random variation in the frquencies of
the two signals.
you'll be amazed how this can "enliven" and "thicken" a sound. This
type  of "chorusing" has the advantage of being less obviously
"colored", which seems to just
mean that we can't as easily recognise the chorus' addition to the
overal timbre.


Good luck, and I bet you'll end up with some nice engaging ambient
drones.


CharlieB