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Create a device capable of directing sound like a flashlight, creating the ability to whisper into someone's ear from a distance of 20+ meters (similar to the LRAD: http://en.wikipedia.org/wiki/LRAD).

Create a device capable of directing sound like a flashlight, creating the ability to whisper into someone's ear from a distance of 20+ meters (similar to the LRAD: http://en.wikipedia.org/wiki/LRAD).

 
Conceptual ideas:
- Use a large emitting surface to bundle ultrasound waves (for example an old satellite dish).
- Use an ultrasonic power transducer (UPT) originally meant for use in ultrasound cleaning devices.
- Original audio signal is demodulated by the physical properties of air.
- Modulate carrier frequency with audio signal (FM modulation).
 
To discuss:
- What's the best way to use a UPT:
   - mount it on a larger surface
   - mount it in front of a satellite dish (in place of the receiver)
- Is it possible to modulate the carrier frequency with a UPT? Is it still efficient? (a UPT has a very large Q: >500)
- If a UPT is not suitable: How to create an affordable HF transducer?
- Electronics!
 
Some background info: