I read quite a few “experts” here with the absolute, unequivocal statement that nobody can hear about 20KHz. First, the human ear is an analog system. There is no 20KHz “cutoff” beyond which nobody can hear. Everyone has a curve of decreasing perception and some point where the curve falls below the level of perception. The 20Khz frequency cited was chosen for two reasons:
1) the vast majority of people cannot hear anything above that frequency (note that I did not say all) and
2) it’s a nice round number. …

A proper hearing test can help determine any hearing loss you may have by measuring your ability to differentiate and respond to a series of tones in a controlled sound environment. Take our new and still unscientific hearing test.

Matt here again. I took an mp3 listening test here a while ago. The shoot out was between an mp3 of 320kb/s vs. and mp3 of 128kb/s. A number of people commented that a test between a pure wav. file against a 320kb/s mp3 would be more useful.

Ever wondered about hearing voices in your head? I’m sure that’s exactly what the military was aiming for when they originally started project MEDUSA or Mob Excess Deterrent Using Silent Audio. The original “telepathic ray gun” works by sending a weak microwave into someone’s head, emitting noises only the person in the wave’s path can hear.

Many people worry about their computers being hacked. Very few worry about eavesdropping used on their printers. In the May 2009 issue of Scientific American, an article talks about potential office hacks using surveillance tactics, one being recording and analyzing the sounds made by a printer. So far, the research has been done on dot-matrix printers. These are the old noisy kind; however, they are also still in commercial use by industries like airlines, banks, and hospitals (each of which prints sensitive information).