Don_c55 - thanks for the reference. I continue to tell people that the human brain and nervous system are far more sensitive than is generally believed. Oohashi's result seem to confirm that. It was particular interesting that subjects said they heard nothing, yet there was brain activity associated with sound great than 20 KHz. Something is clearly going on above 20 KHz even if we can not "hear" it in the traditional sense. One question is whether modern electronics and speakers are taping into that high frequency information or whether somthing like Atmasphere's explanation is what is happening.
The more we know, the more we don't know.
The more we know, the more we don't know.