noun
sound locator; acoustic detector; hydrophone
A device for detecting and locating sound sources, used in military, maritime, and scientific contexts. 聴音機 is the more common spelling for equipment.
潜水艦を探知するために聴音機が使われた。
A sound locator was used to detect the submarine.
この聴音器は水中の音を集音できる。
This hydrophone can pick up underwater sounds.
Compound of 聴音 (listening to sound) + 機/器 (machine/device). The exact historical origin is uncertain, but it likely emerged in early 20th-century military and maritime technology contexts.