What is a hydrophone?

A hydrophone is an underwater device that detects and records ocean sounds from all directions.

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“You would think that the deepest part of the ocean would be one of the quietest places on Earth. Yet there is almost constant noise,” says Robert Dziak, NOAA PMEL research oceanographer and scientist. “The ambient sound field is dominated by the sounds of earthquakes, both near and far, as well as by the distinct moans of baleen whales, and perhaps the clamor of a Category 4 typhoon that just happened to pass overhead.”

Just as a microphone collects sound in the air, a hydrophone detects acoustic signals under the water. Most hydrophones are based on a special property of certain ceramics that produces a small electrical current when subjected to changes in underwater pressure. When submerged in the ocean, a ceramic hydrophone produces small-voltage signals over a wide range of frequencies as it is exposed to underwater sounds emanating from any direction.

By amplifying and recording these electrical signals, hydrophones measure ocean sounds with great precision. While a single hydrophone can record sounds from any direction, several hydrophones simultaneously positioned in an array, often thousands of miles apart, result in signals that can be manipulated to “listen” with greater sensitivity than a single device. Omni-directional and hemi-directional hydrophones pick up sound from a particular direction and can be used to track fish movements.

In addition to NOAA’s National Marine Sanctuaries, NOAA’s Pacific Marine Environmental Laboratory (PMEL) also frequently uses hydrophones. PMEL acquires long-term data sets of the global ocean acoustics environment to identify and assess acoustic impacts from both human activities and natural processes, such as underwater volcanoes, earthquakes, and icequakes on the marine environment.

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