The human hearing range is not restricted to between 20hz -20khz. Everyone has
a different hearing range, in fact Goodmans ran a speaker campaign based on the fact that women tested the speakers because women commonly have a greater
hearing range than men. Sub woofers produce sound that you can hear and lower
frequencies that you can feel. They are designed to produce bass when your main
speakers can't get any lower. Sub woofers from REL have adjustable characteristics to allow them to act as an extension to your speakers.
When Phillips started doing demos for DCC they claimed that the compression
used removed sounds that you could not hear either frequency cut off or removing
quiet sounds in loud passages. The only problem was that a lot of people could
hear that the higher frequencies had been cut off and could hear the quiet sounds in the loud passages and noticed when they were removed.
Some companys are know producing CD players which add very high frequency
sounds which are beyond the range of the CD recording frequency range. Legato-
link is an example.
Can you hear a faint very high frequency sound when watching the TV? Many
people can but is supposed to be above the human hearing range.