1. A transponder or device that emits an acoustic signal upon being activated by sensing a coded electrical signal, or vice versa. Pingers placed on the sea bottom or in anchored buoys can be interrogated by a ship transmitting a coded acoustic (sonar) signal and the distance to the pinger determined by traveltime measurements. 2. A shallow-penetration, high-power transducer used in marine engineering studies in soft-bottom areas.