Here is an interesting paper that addresses the existence of seemingly inaudible music frequencies above 20kHz and discusses the potential effects of these frequencies on the listener:
http://www.cco.caltech.edu/%7Eboyk/spectra/spectra.htm
The paper raises the possibility that ultrasonic frequencies may be perceived in a different manner through a different part of the inner ear. I have no technical knowledge in this area, so I leave this to others to debate.
Having heard two different speaker systems with treble extension significantly above 20kHz, I have personally experienced a greater sense of hall ambience and air from these speakers than I have ever heard through speakers that use more conventional tweeters. The dimensions of the recording venue - and, particularly, the height of the ceilings in the venue - were more apparent than I have otherwise ever heard from an audio system. It has been my running hypothesis that the additional treble extension produces this enhanced sense of space, but this is just my own subjective impression.
http://www.cco.caltech.edu/%7Eboyk/spectra/spectra.htm
The paper raises the possibility that ultrasonic frequencies may be perceived in a different manner through a different part of the inner ear. I have no technical knowledge in this area, so I leave this to others to debate.
Having heard two different speaker systems with treble extension significantly above 20kHz, I have personally experienced a greater sense of hall ambience and air from these speakers than I have ever heard through speakers that use more conventional tweeters. The dimensions of the recording venue - and, particularly, the height of the ceilings in the venue - were more apparent than I have otherwise ever heard from an audio system. It has been my running hypothesis that the additional treble extension produces this enhanced sense of space, but this is just my own subjective impression.