Acoustic "beat tones" aren't "real" tones— you hear them because of non-linearies in the ear-brain system, but you have to hear the initial tones first. (Well, unless you're talking >>130dB SPL levels where the air starts becoming non-linear, but then lower frequency recording would capture it fine)
If you could hear subharmonic beats from ultrasonics then it would be _very_ easy to demonstrate, alas.
IIRC, linearity is when you put a sound wave frequency into the medium (air) a some point, you can predict the frequency of the sound wave at some other place using a linear function - meaning that there is no distortion. Non-linear is when the physics of the medium starts screwing with that function.
I believe that it means that the superposition principle (http://en.wikipedia.org/wiki/Superposition_principle) doesn't hold: the net response at a point is not just the weighted sum of the individual responses.
so without very high power sounds and the nonlinearity business the whole "Sound from Ultrasound" wouldn't work? Huh, I guess all this time I misunderstood it.
Well, what I can think of is that of course you need to sample at > 2*max frequency if you do uniform sampling to avoid aliasing (by Nyquist), but that's not the same as playback.
Yes, there will be inter-modulations from higher frequencies. There are also from the audible spectrum but if the amp is linear enough they will be low.
If you could hear subharmonic beats from ultrasonics then it would be _very_ easy to demonstrate, alas.