Oh I know about those as they are used for audio description in cinemas and
theatres. I was thinking more of exhibitions where an exhibit has a button
to press to hear a description, but if there are a lot of these the sound
when used normally is spilling over to other exhibits and hence makes it
hard to detect the one you want.
However if it has to be so loud as to make ears go into clipping even
though you cannot hear the actual frequency then its a non starter.
Brian

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This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please!
"Phil Allison" wrote in message
...
Brian Gaff wrote:

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If you amplitude modulated a 40Khz ultrasonic sound that you could
obviously
not hear with audio, could you hear it?


** Yes, but with many conditions and limitations. It's a fringe idea looking
for an application that others methods cannot do better.


I was thinking it might be a good way to beam audio.

** Really ?

A very effective way to "beam audio" is to frequency modulate pulsed,
Infra-Red light from an array of LEDs. Perfectly possible cover an entire
hall or auditorium this way, or use a much narrower beam if desired.

Listeners wear lightweight, battery powered headphones fitted with a
sensitive IR detector, FM demodulator and audio amplifier. Sennheiser made
and still make systems like this intended for "assisted hearing" or language
translation for audience members.

I once got a box of about 30 IR headsets plus master transmitters dropped on
me from a local Theatre & Performing Arts centre for their 5 yearly service
and battery change. NiCd button cells have a limited life when left on
continuous charge and most showed signs of corrosion too.

The carrier frequency used in the late 1980s was about 40kHz ( so ultrasonic
?) and audio modulation was extracted by a common IC found in FM tuners.
Sound quality was good with only a slight background hiss.

The "line of sight" only nature of IR audio systems is actually a big plus
in such environments.



..... Phil