Possibly https://en.wikipedia.org/wiki/Sonobuoy https://en.wikipedia.org/wiki/Sonobuoy: “A sonobuoy (a portmanteau of sonar and buoy) is a relatively small buo

Possibly https://en.wikipedia.org/wiki/Sonobuoy https://en.wikipedia.org/wiki/Sonobuoy: “A sonobuoy (a portmanteau of sonar and buoy) is a relatively small buoy – typically 13 cm (5 in) diameter and 91 cm (3 ft) long – expendable sonar system that is dropped/ejected from aircraft or ships conducting anti-submarine warfare or underwater acoustic research.” The microphone used would be a https://en.wikipedia.org/wiki/Hydrophone https://en.wikipedia.org/wiki/Hydrophone: “A hydrophone (Ancient Greek: ὕδωρ + φωνή, lit. 'water + sound') is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potential when subjected to a pressure change, such as a sound wave. A hydrophone can detect airborne sounds, but will be insensitive because it is designed to match the acoustic impedance of water, a denser fluid than air. Sound travels 4.3 times faster in water than in air, and a sound wave in water exerts a pressure 60 times that exerted by a wave of the same amplitude in air. Similarly, a standard microphone can be buried in the ground, or immersed in water if it is put in a waterproof container, but will give poor performance due to the similarly bad acoustic impedance match.”