"Kalman Rubinson" wrote in message

On Wed, 16 Jul 2003 16:34:34 +0100, "Graham Holloway"
wrote:

I seem to remember reading about a Korean(?) company experimenting
with a public address system that generated a carrier around 20KHz
plus an audio signal shifted up by 20KHz, the assumption being that
the non-linearity of the ear would make it audible. The transducers
were placed in a matrix several feet above head height, and because
of beaming at these frequencies the sound(!) would appear generally
from above. But the last I heard was that they were experimenting
with a membrane above the listener to create additional
non-linearity.

Or this: http://www.atcsd.com/tl_hss.html

Right, except AFAIK this one is based on the nonlinearity of air. That's a
whole 'nuther thing with SPLs to match.