Underwater locator beacon

An underwater locator beacon (ULB), underwater locating device (ULD), or underwater acoustic beacon is a device that emits an acoustic pulse intended to guide searchers equipped with a suitable receiver to the location of the beacon underwater. == Application == An underwater locator beacon is fitted to aviation flight recorders such as the cockpit voice recorder (CVR) and flight data recorder (FDR).

Source: Wikipedia — Underwater locator beacon (CC BY-SA 4.0)