Underwater, sound is the only sense that travels. How sound speed works in seawater, what the SOFAR channel is, and why the ocean is far noisier than it seems.
Divers know the sensation: a boat appears overhead, audible long before it comes into view, the engine's growl arriving from everywhere at once, with no sense of direction at all. Underwater, sound behaves so differently from the air-world that evolution redesigned an entire sensory economy around it. Whales sing across ocean basins. Fish larvae navigate home by reef noise. Snapping shrimp fry the shallows with static. Sound is the ocean's long-distance sense — and, increasingly, its most polluted one. Here's how the physics works, and why it matters to every kelp forest, whale and survey vessel on the coast.
Sound is a pressure wave, and water — dense and nearly incompressible — conducts it superbly. The numbers tell the story:
That single trade — high frequency for precision at short range, low frequency for range — runs through every natural and human use of underwater sound.
Sound speed in the ocean is not constant; it follows a simple sensitivity to three variables, the same three that oceanographers measure with a CTD cast:
The layered result is the ocean's famous sound-speed profile. Descending from the warm surface, sound speed falls as temperature drops through the thermocline, reaches a minimum at a few hundred to ~1,000 metres in mid-latitudes (as the last cooling peters out and pressure begins to win), then rises steadily toward the abyss. In polar seas the temperature term weakens, and the profile simplifies: sound speed just climbs with depth.
Waves bend toward the slow lane. Wherever sound speed changes with depth, sound rays curve toward the slower region — the same physics that bends light through a lens, applied to pressure waves in a layered fluid. In the deep sound-speed profile, rays emitted near the minimum are bent back toward it from above and below: sound gets trapped in a waveguide. This is the SOFAR channel (Sound Fixing And Ranging), also called the deep sound channel, centred on the sound-speed minimum.
The consequences are startling. A low-frequency sound injected into the SOFAR channel loses energy only at its shallowest grazing angles, and can propagate across entire ocean basins with barely any loss. During the Cold War, the SOSUS network of seafloor hydrophones used it to track submarines across the Atlantic. When cold-war hydrophones were declassified to science, researchers found the whine of distant whales threaded through the same channel — fin whales have been tracked from one side of an ocean basin to the other. Even in the atmosphere's equivalent handful of channels, nothing in air rivals it; with sound, the ocean thinks in continental distances.
Near coasts the picture is different: down-slope propagation guides sound from shelf waters into the channel, and sounds that don't couple into it fade with range. Whales crossing the deep Pacific can talk basin-wide; a kelp forest dweller in the shallows mostly deals in metres and hundreds of metres.
A healthy coastal habitat is anything but silent. On a kelp reef the ambient mix includes:
This soundscape is information. Fish and invertebrate larvae, drifting for weeks after hatching, use habitat sound to choose where to settle — playback experiments have shown settlement rates rising near speakers playing healthy-reef noise, which has made acoustic enrichment a genuine tool being tested for restoration. We explore how scientists eavesdrop on all this in our explainer on reef soundscapes and acoustic monitoring.
Sound is also how humans see underwater. Light dies within metres beyond the euphotic zone's limits; sonar carries on. Three instruments matter most on a working coast:
Tidewright's survey approach leans on exactly this toolkit: our autonomous vessels combine sonar seabed maps with optical imagery to choose reseeding sites — the Drifter platform describes how that mapping pipeline works in practice. Passive listening matters as much as pinging: hydrophone recordings let us detect vessel traffic, local snapping-shrimp bands and biological choruses without ever entering the water.
For most of ocean history, control of the soundscape belonged to biology and weather. Now it doesn't. Commercial shipping roughly doubles background low-frequency noise in many heavily trafficked regions by the sheer thrum of propellers and engines; seismic airgun surveys for oil and gas fire some of the loudest artificial pulses in the sea; pile driving, naval sonar and small-boat traffic add piercing local layers.
The costs are increasingly well documented: whales and dolphins — whose lives run on sound — raise their voices, shift frequencies, abandon habitats, or strand in acute exposures; fish startle and altered-breed near loud operations; even invertebrate larvae show changed settlement near noise. Studies of natural morphological stress in scallops have found developmental abnormalities after seismic exposure nearby. The problem is compounded by acoustics: because sound carries so far, a single ship pollutes an enormous acoustic footprint — a coastline's quiet never stands a chance in the shipping lanes.
The policy response is emerging: quiet-ship designs, speed restrictions, seasonal exclusion windows during whale migrations, and a growing literature that treats the underwater soundscape as a metric of habitat health rather than a backdrop — a philosophy that guides our own monitoring and restoration projects.
About 1,500 metres per second on average — roughly 4.3 times the speed in air. The exact value depends on temperature, salinity and depth (pressure), typically ranging from about 1,450 to 1,570 m/s across the ocean's layers.
It's a natural sound waveguide centred on the depth where sound travels slowest — usually a few hundred to 1,000 metres down in mid-latitudes. Sound rays bend back toward this slow zone from above and below, so low-frequency sounds emitted there can travel across whole ocean basins losing little energy. Whales and Cold War submarines both exploited it.
Your brain locates sound using the tiny arrival-time and loudness differences between your two ears, calibrated for the speed of sound in air. Underwater, sound is much faster, and it also conducts through your skull, reaching both ears almost simultaneously — the cues collapse, and every direction sounds overhead or everywhere.
Yes — superbly. Fish hear through inner ears, often aided by gas-filled swim bladders as resonating chambers. Baleen whales produce and hear low-frequency songs; toothed whales and dolphins hear extreme high frequencies and use clicks for echolocation. Many species spend their lives inside acoustic habitats that human noise now intrudes on.
Evidence says yes, at many levels: chronic ship noise raises stress and forces marine mammals to alter calls and abandon habitat; very loud events — seismic surveys, naval sonar — have been linked to strandings and injury. Because sound is the ocean's main long-distance sense, raising the noise floor shrinks the usable world of every animal that depends on it.
Multibeam and side-scan sonar build detailed maps of seabed terrain and texture; echosounders can detect the canopy's acoustic backscatter; and autonomous platforms knit sonar with camera imagery. Sonar does what light can't — it reaches through turbid, dark water to show the reef's shape before a single diver gets wet.
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