Good ole' Servodrive. I've read some threads with Tom Danley on a few message boards and he is one incredibly fascinating individual.

Wouldn't the efficiencies between input/output transducers be inverse? A servo drive transducer would use very little energy to produce a lot of acoustic output, so wouldn't it require a large amount of acoustic input to generate any signal at all?

AC motors as well. Lots of brushless quadcopter motor controllers buzz out diagnostic/startup tones from the motors as they power up.