A marine sound is any acoustic signal that travels through the ocean, whether produced by animals, natural forces, or human activity. The ocean is far from silent. It is filled with a complex mix of sounds that travel faster and farther than they do in air, making sound the primary way information moves underwater. Light fades within a few hundred feet of the surface, but sound can cross entire ocean basins.
How Sound Behaves Underwater
Sound travels roughly four times faster through seawater than through air. Two main factors control its speed and range: water temperature and pressure. Near the surface, where water is warmer, sound moves quickly. As it descends through the thermocline (the layer where temperature drops sharply with depth), it slows down. Below the thermocline, temperature stays fairly constant, but increasing pressure causes the speed to climb again.
This interplay creates a natural phenomenon called the SOFAR channel, short for Sound Fixing and Ranging. At a certain depth, the combination of cold temperature above and rising pressure below traps sound waves in a band where they bounce up and down without losing much energy. Low-frequency sounds caught in this channel can travel hundreds or even thousands of miles. In early experiments, an underwater explosion was detected 900 miles away. Whales rely on the SOFAR channel to communicate across vast stretches of ocean during migration.
Biological Sounds
Marine animals are the ocean’s most diverse sound producers. Taken together, marine mammals alone generate sounds spanning from below 10 Hz to above 100,000 Hz, a range far wider than human hearing (roughly 20 Hz to 20,000 Hz). Different species occupy different parts of this spectrum depending on what they need sound to do.
Blue whales and fin whales call at extremely low frequencies, between 10 and 40 Hz, with source levels reaching up to 190 underwater decibels. These calls are infrasonic, meaning they fall below the threshold of human hearing entirely. Despite being among the loudest animals on the planet, you would hear nothing standing next to one. Low frequencies travel enormous distances, which suits animals that need to find mates or coordinate movement across open ocean.
Toothed whales and dolphins use the opposite end of the spectrum. Their echolocation clicks are often ultrasonic, well above 20,000 Hz. Beaked whales, for example, produce clicks with most of their energy between 40,000 and 90,000 Hz. These high-frequency bursts are ideal for detecting prey and navigating in dark water, functioning like a biological sonar that builds a picture of the surroundings from returning echoes.
It isn’t just mammals. Snapping shrimp are one of the most pervasive sound sources in tropical and temperate waters. Each tiny snap produces a broadband burst extending up to 200,000 Hz, with peak energy between 2,000 and 5,000 Hz and source levels of 183 to 189 underwater decibels per snap. Colonies of these shrimp create a constant crackling background noise that can interfere with sonar equipment.
Natural Non-Biological Sounds
Wind, waves, rain, and geological activity all contribute to the ocean’s background noise. In the absence of animal calls and human activity, wind-driven waves dominate the soundscape from below 1 Hz up to at least 50,000 Hz. Below about 5 to 10 Hz, the main source is the interaction of ocean surface waves moving in opposite directions, which generates deep, rumbling vibrations called microseisms.
Breaking surf is surprisingly loud. Plunging waves can raise underwater noise levels by more than 20 decibels a few hundred meters outside the surf zone, across a wide band from 10 Hz to 10,000 Hz. Rain is even more dramatic: a heavy downpour can boost ambient noise by up to 35 decibels across frequencies from several hundred hertz to above 20,000 Hz. Even a light drizzle creates a distinct spectral peak near 15,000 Hz that rises 10 to 20 decibels above normal background levels.
Tectonic processes add another layer. Earthquakes, volcanic eruptions, and hydrothermal vents along mid-ocean ridges all generate sound that can travel long distances. These geological sources make significant contributions to the overall marine noise field, particularly at low frequencies.
Human-Made Sounds
Commercial shipping, oil and gas exploration, and military sonar have added a growing layer of noise to the ocean over the past century. Seismic airguns, used to map the seafloor for oil deposits, are among the loudest human sources. A large airgun array produces pulses that dominate the sound spectrum from 200 Hz to 22,000 Hz at distances up to 2 kilometers. Even 8 kilometers away, the pulses remain clearly above background noise levels up to 8,000 Hz, well within the hearing range of dolphins and other marine mammals.
Shipping noise is lower in intensity per vessel but constant and widespread. The collective hum of global shipping has raised baseline ocean noise levels in some regions, particularly at low frequencies where large whales communicate. This matters because sound is not optional for marine life. It is the primary sense many species use to find food, avoid predators, locate mates, and navigate. When background noise rises, animals must call louder, shift frequencies, or simply lose contact with each other.
Why Measuring Marine Sound Is Tricky
One detail that causes confusion: decibel levels in water and in air use different reference pressures, so the numbers are not directly comparable. Underwater decibels are measured against a reference of 1 micropascal, while airborne decibels use 20 micropascals. This means 150 decibels underwater is not the same intensity as 150 decibels in air. The underwater number will always look higher for the same actual sound energy. NOAA explicitly warns against comparing the two scales without accounting for this difference.
This distinction matters when interpreting claims about how loud marine animals or human activities are. A blue whale call at 190 underwater decibels is extraordinarily powerful, but it does not translate to 190 decibels in air (which would be louder than a rocket launch at close range). The physics of sound transmission through water versus air are fundamentally different.
How Noise Affects Marine Life
Prolonged or intense exposure to human-made noise can cause measurable hearing damage in marine mammals. NOAA’s 2024 technical guidance sets thresholds for when sound is loud enough to cause the onset of auditory injury. These thresholds vary by species group: animals that rely on very high-frequency hearing, like porpoises, are more vulnerable to noise damage than low-frequency specialists like baleen whales. For very high-frequency cetaceans, non-impulsive noise (steady sounds like ship engines) can begin causing auditory injury at cumulative exposure levels significantly lower than for other groups.
Beyond direct hearing damage, noise pollution disrupts behavior. Whales have been observed changing their migration routes, altering call patterns, or abandoning feeding areas in response to seismic surveys and heavy shipping traffic. For species that depend on the SOFAR channel to stay in contact across hundreds of miles, rising background noise can effectively shrink their communication range, isolating populations that were once acoustically connected.