Hydrophones: The Microphones That Listen to a Noisy Ocean

The ocean is not silent. Many marine animals use sound to communicate, to find mates and to hunt. Hydrophones are how we hear it: underwater devices that detect and record ocean sounds coming from every direction.

A hydrophone in Gray's Reef National Marine Sanctuary in Georgia, one of four places in the National Marine Sanctuary system where hydrophones record soundscape data. Credit: NOAA.

How a hydrophone works

A hydrophone does underwater what a microphone does in the air. Most rely on a special property of certain ceramics: when underwater pressure changes, the ceramic produces a small electrical current. Submerged, a ceramic hydrophone turns sounds arriving from any direction into small-voltage signals across a wide range of frequencies. Amplified and recorded, those signals measure ocean sound with great precision.

Listening with many ears

One hydrophone records sound from every direction. Several placed in an array — often thousands of miles apart and recording at the same time — produce signals that can be combined to listen with more sensitivity than any single device. Omni-directional and hemi-directional hydrophones pick up sound from a particular direction and can be used to track the movements of fish.

Who is listening

Besides the national marine sanctuaries, NOAA's Pacific Marine Environmental Laboratory (PMEL) uses hydrophones often. PMEL collects long-term recordings of the global ocean's acoustic environment to identify and assess how human activities and natural events — underwater volcanoes, earthquakes and icequakes — affect marine life. It was a PMEL hydrophone array that solved the mystery of "the Bloop."

Sources

Based on "What is a hydrophone?" from NOAA's National Ocean Service, a work of the United States government in the public domain.