Scientist's move levitating objects through space for first time.

[VWolf]

18. Thanks!

The original version had resonances in the audible range. Boy oh boy did that piss off the other grad students.

Eventually I got the thing to go purely ultrasonic. I even demonstrated a droplet freezing in mid-air by placing the entire apparatus in a thermal chamber. The trick there, of course, was to maintain the standing wave while the speed of sound changed due to the thermal excursions. I solved this by installing an "ultrasonic" microphone in the reflector and employing a phase-locked loop, locking the excitation signal transmitted to the piezos to the signal received by the microphone. A servo mechanism adjusted the transmitter/reflector separation accordingly to maintain lock.

It's probably the coolest project I've ever been involved in. Every time someone reminds me of it, I get happy.