This list is starting to look like a rec.audio digital/analog battle... %)
So of course I can't resist throwing in $0.02:
1. The experiments that "showed" that people can detect (not consciously
hear) sounds up to 100 kHz involved very high volume and transducers
directly on the skull. In normal situations, very few people can hear
much over 20 or maybe 23 kHz. If you could, things like ultrasonic
motion detectors would drive people mad. Remember, the high pitched
whine from a US TV set is only 15.75 kHz. Try playing a clean LP on a
high-end setup and using an EQ to knock out everything below 16 kHz.
What's left will typically have tons of phase distortion, noise, etc.
2. Ultrasonics don't influence lower, audible frequencies "by interference."
This is a classic rec.audio claim, and you can verify that it doesn't
happen by feeding sine waves of appropriate frequencies through a speaker
with a good tweeter. You won't hear a beat frequency.
A standard experiment is to make a 44.1 kHz digital recording of an LP and
then compare the DAT of the LP to the original. Most reports are that no
difference can be heard in blind tests. I would expect that, at least for
idm type music, digital's ability to go right down to 0 Hz in the bass
would be more relevant... The French police were able to cause all sorts of
devastating (and personally embarassing) physical and emotional effects in
rioters with very low frequency riot horns (5 Hz?). Obviously this has
limited commercial application... "Fear Rave with DJ DC! Make sure to
bring mouthwash and a change of clothes!"... %)