Editor --

Last year I read with amazement the description of Elwood ("Woody") Norris' HyperSonic Sound system. This home-schooled and self-taught tinkerer has developed, of all things, a highly directional ultrasonic sound system that allows one to generate audible sound in a chosen location. For this innovation, he received the 2005 Lemelson-MIT Prize for invention.

http://en.wikinews.org/wiki/Elwood_Norris_receives_2005_Lemelson-MIT_Prize_for_invention

His system works in the ultrasonic range of about 200 to 300 kHz, well above 20 kHz, the typical upper limit of audible freequencies. In order to generate audible sound, the system produces two ultrasonic signals with slightly different frequencies. The signals are directed in narrow beams from separate locations so that they intersect at a target. The amazing thing is that the superimposition of these signals creates an audible sound in the place where the beams intersect.

I wanted to demonstrate the acoustic principles that allow Woody's system to work. I found a downloadable "Stereo Sound Frequency Generator" developed by Oleg Shmelyoff. This program allowed me to generate and play sounds made up of one or more sine waves, square waves or sawtooth waves. It was possible with my computer and speaker system to play, for example a sine wave at 10 kHz in one speaker and 11 KHz in the other speaker. Naturally, I could hear both of those frequencies. However, I also heard the difference frequency of 1000 Hz. This was true whether the two signals went into one speaker or one signal into each of the two speakers.

The next thing we did was to recruit my friend Bob, an inquisitive scientist who happens to be quite deaf above 5000 Hz. He was not able to hear either the 10 KHz or the 11 KHz tones, yet he heard the 1000 Hz difference frequency quite well.

After that, we generated sine waves just beyond the frequency limit of normal human hearing but apparently still within the limit of the speakers attached to the computer. While I was not able to hear either of the near-ultrasonic signals going into either speaker, I was clearly able to discern the difference frequency.

Now conceptually it is evident that one could modulate one or both of the beams to create whatever sound one wishes to create. In fact, a single sideband ultrasonic signal in one beam would be demodulated where it intersects another beam carrying the carrier frequency. Such a single sideband signal would be much like a single sideband signal output from an amateur radio transmitter, though it would be in the ultrasonic range rather than the usual radio frequencies.

This tinkering, with downloadable software and a personal computer, led me to read and learn more about several related concepts:

Nonlinearity in electrical and mechinical systems.
Intermodulation distortion.
Single sideband modulation.

Ronald Leemhuis

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