The coolest idea I have seen at this ATC was shown in a poster session given by Stephen Tarzia from Northwestern. He and others are generating ultrasound from a PC and picking up the echoes to determine user presence and attention. Neilsen for PCs.

About half the PCs he's tested can generate and detect ultrasound, generally up to about 30KHz. He hasn't run many tests to see if the non-functioning ones have non- functioning mikes, speakers, or both.

Their software emits a continuous 20 KHz sine wave, and listens for changes in the echos. The idea of sonar leaps to mind, but he says the ultrasonic chirps make an annoying audible clicking sound.

Of course, I immediately thought of using ultrasound as a covert channel between computers. Turns out Madhavapeddy et al have already done this. But this is a channel worth remembering: I bet one could use it in a number of situations where one assumes computers are not connected.

Do cell phones have frequency ranges for these games? I suspect so.

Home security may be an easy application for this. But I also think there is a nice, easy interactive science museum exhibit in this idea. Have the exhibit respond to your presence in some way, and let the user dial the frequency, seeing the computer response even in sounds too high to hear. It would also be a good way to test the high end of the user's hearing range.