Looked at the designs and figured out how they do it. A neat idea really.
http://en.wikipedia.org/wiki/Cavitation
*Edit sorry, not exactly cavitation per se, as I don't think there are any liquid in there aside from the water vapors in the air.
More of a controlled vacuum colapse/sonic boom.
Think of the sound a syringe does when you put your finger on the hole and pull the plunger all the way out.