Edited by Place Holder - 6/26/15 at 9:36pm
An ultrasound machine never made anyone deaf did it? And I'm sure I read that big thunderstorms can produce sounds around 1hz and that never hurt anyone.
I'm not an expert, and this is probably a grossly simplified and perhaps inaccurate explanation... But as I understand it the frequency range is not determined just by the ear drums, but by the ear canal and outer ear as well. Soundwaves reach our ear drums having been reflected off the outer ear and funneled down an irregular tunnel, and there are limits to the frequencies that can make it through.
If that is accurate then its not that sound above 20khz is hitting your ear drums and they aren't able to sense it, but that the inner ear is not letting it arrive at your ear drum by virtue of its shape/natural damping of soft materials.
I'll let the experts here tear that apart and point out how wrong I am now, but that was how I thought it worked.
Man the human body is amazing. I'm just speculating here but from a biological point of view, a child a can hear these subsonic and ultrasonic sounds, but loses them as they age from 8 and above. And I'm thinking there maybe some evolutionary thing going on here. Thunder creates 1 hz as you mentioned, a Tiger (or is it Lion?) can go sub sonic as well. And so from an evolutionary perspective, children were equipped to hear these to detect danger, but loses that ability once they age but become more physically able to defend themselves (spearing a big cat, running out for cover from thunderstorms).
I don't think it's because of this, I would say remains of evolution.
We were once mice/rats after all.
As an example that I happen to know of, dolphins actually grow legs while they're in the womb, yet they retract their legs (while still inside of the womb) inside a bit later, completely useless. I guess this kind of thing is basically the same, although here the "retraction" (starts and) finishes a bit later.
From another source:
"Ultra sonic frequencies however have too high a frequency and so can't pass through our basilar membrane. Therefore they don't cause it to vibrate therefore there are no hair cells activated, therefore it can't stretch and break any hair cells, there for it can't cause hearing damage."
Ultra sonic is defined at everything > 20kHz.
But I have read about audible low-frequency sounds that do indeed damage your ears, the example was the low frequency that trains and such transmit. You can't really hear it (you feel the shaking (and see it in some cases)) but it apparently does damage your hearing.
I found something interesting:
"Not true: Loud sound can change your heart rate, vision and reaction time. It may make you more aggressive and in general, negatively affect you."
Play back a inaudible sound in a mall really loud, and watch crime rate go up! Haha!