[notthetup]

Not very high power usually. They're usually made of piezoceramics.

I think they're expensive because they're niche. Narrow band piezoceramics which are used in fish finders and other commercial equipment are pretty cheap. These wider band transducers are niche to underwater comms.

> Also what frequency does an acoustic modem typically use?

Usually in the 10s of kHz. The NATO JANUS standard is around the 12kHz center frequency.