A place I worked at in the early 2000's used these in custom-designed acoustic telemetry systems deployed in the ocean (I don't remember which company we bought them from - the website I linked to is just an example of the type of device, but it is very similar). The system needed a way of communicating with devices that were dropped into the ocean and typical radio waves don't travel well in sea water. Acoustic modems were used instead, as sound travels well in water.
http://www.aquasent.com/acoustic-modems/
A place I worked at in the early 2000's used these in custom-designed acoustic telemetry systems deployed in the ocean (I don't remember which company we bought them from - the website I linked to is just an example of the type of device, but it is very similar). The system needed a way of communicating with devices that were dropped into the ocean and typical radio waves don't travel well in sea water. Acoustic modems were used instead, as sound travels well in water.