I need a far better coupling of energy to water than will be
accomplished by putting a gas interface between them. Maybe some kind of
linear actuator...
You would be surprised what an enclosed pocket of gas transfers to a
flat membrane in a closed spaced, fired directly from a transducer. That
bit of dampening is actually better than what you are after, but if that
is what you want, you would then want a sub woofer driver with a
super-light weight post glued in place of the coil cap that fires the
membrane directly. Then, you can have a lower frequency device you are
after.
The bullhorn works though. Think of how they can hear into office
buildings using laser interferometry. Under water, it isn't even needed.
One can hear the sound quite a distance away from very little input
power. Your voice at the back of the bullhorn would do, much less
(more?) electrical transducers.
Anyway,play a drum under water and you get sound projection at great
distances. Easy to blow out transducers and listening gear if peaks are
not kept chopped so that anomalies like small (or large) explosions do
not destroy some segment of your gear. (your monitor speakers and your
ears).
