Noise induced hearing loss from frequencies > 22kHz?

[AnthonyMF]

There are some devices that keeps kids out of shopping centres at night called mosquito, these things have gone from "safe" now to "most probably unsafe" as it is believed they damage kids hearing, especially really young kids. They can cause nausea, headache and a bunch of other effects depending on SPL. Freq they emit is about 18k, some use 18k plus I think a second higher tone to cause beating and make it even more annoying (dangerous).

Can we perceive the beating caused by two tones beyond our range of hearing? ie, if a device produced 20kHz and 21kHz tones simultaneously, would we hear a 1kHz tone?

If we can't hear 20kHz or 21kHz then we won't hear the 1kHz difference of those two tones. The result is 1kHz amplitude modulation due to addition (or subtraction) of the two tones, not another tone per se.

This is assuming zero distortion transmission. If those two tones were at such high level to cause distortion in the transmission through air itself then you will get side band tones produced, just
like audio intermod products through say an amplifier or whatever.

There appears to be some developement in this area with a few products already out there using ultrasonic transducers to create sound that can have extreme directivity properties.
An American inventor, Woody Norris has developed some pretty amazing stuff that is being used for military (and other) applications.

http://www.lradx.com/site/
http://www.ted.com/talks/woody_norris_i ... hings.html

cheers

Really? I figured that beating was an acoustic phenomenon, or at least, physically manifest in whatever medium "sound" is in, as opposed to being psychological.

/strokes chin

[zenelectro]

There are some devices that keeps kids out of shopping centres at night called mosquito, these things have gone from "safe" now to "most probably unsafe" as it is believed they damage kids hearing, especially really young kids. They can cause nausea, headache and a bunch of other effects depending on SPL. Freq they emit is about 18k, some use 18k plus I think a second higher tone to cause beating and make it even more annoying (dangerous).

Can we perceive the beating caused by two tones beyond our range of hearing? ie, if a device produced 20kHz and 21kHz tones simultaneously, would we hear a 1kHz tone?

If we can't hear 20kHz or 21kHz then we won't hear the 1kHz difference of those two tones. The result is 1kHz amplitude modulation due to addition (or subtraction) of the two tones, not another tone per se.

This is assuming zero distortion transmission. If those two tones were at such high level to cause distortion in the transmission through air itself then you will get side band tones produced, just
like audio intermod products through say an amplifier or whatever.

There appears to be some developement in this area with a few products already out there using ultrasonic transducers to create sound that can have extreme directivity properties.
An American inventor, Woody Norris has developed some pretty amazing stuff that is being used for military (and other) applications.

http://www.lradx.com/site/
http://www.ted.com/talks/woody_norris_i ... hings.html

cheers

Really? I figured that beating was an acoustic phenomenon, or at least, physically manifest in whatever medium "sound" is in, as opposed to being psychological.

/strokes chin

Ref pic below (done in Ltspice) of the resultant when 20kHz + 22kHz waves are summed.

As you can see there is no actual 2kHz wave but a 2kHz amplitude modulation due to the phase adding
and subtracting of the 20k and 22k waves.

So it is physical not psycological, but you won't hear the 2k modulation (if you can't hear 20k)

Clear as mud?

cheers

[AnthonyMF]

Ahhh!! I get it... at least a little, I think now it just needs some 'toilet time' for me to really ponder