Thanks for that Paul.

So given the non linearity of the ear, lets assume that a particular beat results in the subject perceiving a tone quite clearly at say 1khz whilst listening in room to their system.

Would I be right in thinking that it would not be possible to measure this phenomenon unless the equipment used were calibrated to mimic the exact non linearity of the subjects ear?

Christ you're quick Graham.! So from your explanation, is it impossible to show the affect of a beat in the frequency domain using conventional measurements?