Hydrophones

The hydrophone is a specialized underwater microphone: it converts acoustic pressure waves in water into measurable electrical signals using piezoelectric ceramics.

This is a critical sensor for underwater acoustics, functioning by leveraging the piezoelectric effect in ceramics to transduce pressure changes into voltage signals. Unlike air microphones, hydrophones are engineered for high-pressure aquatic environments, capturing sound across a wide frequency range. Applications are diverse and specific: the U.S. Navy uses them in sonobuoys for anti-submarine warfare, while NOAA's Pacific Marine Environmental Laboratory (PMEL) deploys them to monitor global ocean acoustics, tracking marine mammal vocalizations (e.g., whale calls) and geophysical events like earthquakes or icequakes. Deploying multiple hydrophones in an array amplifies sensitivity and enables precise directional tracking of sound sources.

https://oceanservice.noaa.gov/facts/hydrophone.html