Lower a microphone over the side of a boat and you will hear almost nothing of what is happening below. The surface of the water acts like a mirror for sound, bouncing nearly all of it back down into the depths or up into the air.
A hydrophone gets around that problem by sitting inside the water itself. It is the tool behind the most famous whale recordings ever made, behind live orca streams on the internet, and behind a growing body of sound art made in fjords, rivers and glacier crevasses.
This guide explains what a hydrophone is, how it came to reveal whale song, and what artists should know before they drop one into a harbour, a lake or the open sea.
What a hydrophone actually is
A hydrophone is an underwater microphone. Most use a piezoelectric element, usually a small ceramic disc or cylinder, which produces a tiny voltage when the pressure of a passing sound wave squeezes it.
The element is sealed in rubber or polyurethane so it can be submerged, with a waterproof cable running back to a recorder. Unlike most studio microphones, there is no delicate diaphragm to protect, which is why hydrophones can survive salt water, cold and depth.
Water also changes how sound behaves. Sound travels at roughly 1,500 metres per second in seawater, more than four times faster than in air, and low frequencies can carry for enormous distances. A hydrophone is built to work with that medium rather than against it.
From submarine hunting to whale song
Hydrophones were developed in earnest during the First World War, when navies needed a way to hear submarines they could not see. Through the Cold War, listening stations kept the ears of the military pointed at the sea.
One of those listeners changed how the world thinks about whales. In the 1950s Frank Watlington, an engineer working on a hydrophone station off Bermuda, kept picking up strange moans and wails while monitoring for Soviet submarines. He saved the recordings.
In 1967 he played them to the biologists Roger and Katy Payne. Listening closely, they found that the sounds repeated in long, structured sequences, some lasting more than half an hour. Humpback whales, it turned out, were singing.
The album Songs of the Humpback Whale followed in 1970 and became one of the best-known nature recordings ever released. It was later added to the US National Recording Registry, and it helped turn public opinion toward protecting whales.
Using a hydrophone for whales today
Listening for whales no longer requires a naval base. Orcasound, a cooperative hydrophone network in the Salish Sea between Washington State and British Columbia, streams live underwater audio online so anyone can listen for orcas, humpbacks and passing ships.
Its hosts note that different orca populations use distinct sets of calls, so trained listeners can tell which group is nearby from sound alone. That is a reminder of how much information a single underwater microphone can carry.
If you want to record whales yourself, think about frequency first. Blue and fin whales call so low, often below 40 Hz, that cheap hydrophones and small speakers barely register them. Humpback song covers a wider range and is far easier to capture on modest equipment.
Location and timing matter even more than gear. Whales follow seasonal routes, and local research groups and whale-watching operators usually know when animals pass. Always follow local rules on approach distances, and never chase animals or play recordings back to them.
Choosing an underwater hydrophone
For artists, hydrophones fall into three broad groups. At the low end are DIY builds made from a piezo disc sealed in rubber coating, which are cheap, noisy and surprisingly good for experiments in ponds and buckets.
In the middle are purpose-built hydrophones made for recordists, sold by small specialist makers. These are the usual choice for field work in lakes, rivers and coastal water, and they pair well with ordinary handheld recorders.
At the top are calibrated scientific instruments, such as the DPA and Brüel & Kjær models used in research. They offer flat frequency response and known sensitivity, which matters if you need to measure sound levels rather than just capture them.
Before buying, check how the hydrophone is powered. Some run from a recorder's plug-in power, while others need phantom power or a dedicated preamp to cope with the high output impedance of piezo elements. A mismatch here is the most common cause of thin, hissy recordings.
Hydrophone microphone technique
Recording underwater is less about pointing and more about placing. Hang the hydrophone from a weighted line so it sits still, and keep the cable from rubbing against rocks, hulls or jetties.
Handling noise travels straight down the cable, so let go once it is in position. On a boat, switch off the engine and any pumps if you can, since mechanical noise carries far underwater.
Monitor on closed headphones from the start. You will hear things you did not expect: snapping shrimp, gravel rolling in a current, the creak of ice, distant propellers. Note the time, depth and conditions for each take, because the sounds are hard to identify later.
Many recordists pair a hydrophone with an ordinary stereo pair above the surface. Our guide to field recording and how artists capture the sound of a place covers that above-water side, including wind protection and choosing a recorder.
Artists who work below the surface
The Norwegian artist Jana Winderen is one of the best-known hydrophone users in sound art. Trained in fish ecology as well as fine art, she records fish, crustaceans and moving water in fjords, the Barents Sea and Greenland's glacier crevasses.
Her album Energy Field, released on the Touch label in 2010, was made with four DPA hydrophones alongside conventional microphones. It won the Golden Nica for Digital Musics and Sound Art at Prix Ars Electronica in 2011.
The composer Annea Lockwood has spent decades recording rivers. Her installation A Sound Map of the Danube, completed in 2005, follows the river from the Black Forest to the Black Sea, mixing sound from the banks with underwater recordings.
Both sit within a wider tradition of listening to environments as living systems. If that interests you, our piece on acoustic ecology from the World Soundscape Project to the Great Animal Orchestra traces where those ideas came from.
Listening stations that never switch off
Some hydrophones listen for decades. The Comprehensive Nuclear-Test-Ban Treaty Organization runs eleven hydroacoustic stations around the world, six of them using hydrophones suspended deep in the ocean and cabled to island shore stations.
They sit near the SOFAR channel, a layer of the ocean where low-frequency sound travels unusually far. Their job is to detect underwater explosions, but researchers have also used the data to study whale calls, iceberg calving and undersea volcanoes.
For artists, these networks are a reminder that the ocean is never quiet. Ships, weather, ice and animals fill it with sound, and human noise is a growing part of that mix.
Taking a hydrophone into the field
Cold water is hard on batteries, cables and fingers. Pack spares, keep the recorder in an inner pocket, and rinse salt off everything once you are back on land. Rubber cable jackets stiffen in the cold, so coil them loosely.
Safety comes before the recording. Work in pairs near deep or fast water, wear a flotation aid on boats and jetties, and tell someone where you are going. Remote coasts and frozen lakes add their own risks.
If your plans take you north, our notes on the overlooked logistics of Arctic fieldwork cover transport, permits and timing that affect any recording trip.
A hydrophone will not tell you what to listen for. What it offers is access to a part of the world that is normally silent to us, and the chance to bring some of that sound back to the surface.
About Dr. Mira Solheim
Dr. Mira Solheim is an art historian and writer focused on artistic research, Nordic visual culture and the intersection of art with technology and film. She writes for Artistic-Research.no on methodology, institutions and practice.