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