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Ship noise is not always the loudest sound in the ocean, but it is one of the most widespread and persistent. Commercial vessels create a long-lasting, low-frequency background rumble. Seismic airguns, sonar, underwater construction, storms, earthquakes and animal calls can be more intense in a particular place or frequency band, yet many of those sounds are intermittent or localized.
There is no scientifically meaningful global ranking of the “loudest” underwater source. A valid comparison must specify frequency, distance, duration, location, propagation conditions and the acoustic quantity being reported—such as a vessel’s source level, a received level at an animal, or an ambient sound level measured at a monitoring site.
The ocean has a soundscape, not one noise level
Underwater sound is a mixture of natural and human-made sources. Wind and waves, storms, ice movement, landslides, earthquakes and animal calls all contribute. Human sources include commercial shipping, sonar, seismic exploration, drilling and underwater construction. Each source has its own loudness, frequency range, timing and geographic footprint.
That mixture changes continually. A storm can dominate a coastal recording for hours; an earthquake can produce a powerful but short-lived signal; whales and other animals may create repeated calls; and a busy shipping lane can maintain a low-frequency background even when no single vessel is unusually loud. NOAA’s ocean-noise explainer summarizes the central point as: “Not all sound is created equal.”
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Sound also propagates differently at different frequencies. Low-frequency energy can travel long distances under suitable water-temperature and depth conditions, while higher-frequency sound generally attenuates more quickly. Consequently, a source that is modest at a nearby recorder may still contribute to a broad regional background.
Why ship noise stands out
A persistent low-frequency rumble
Ship-radiated noise comes from several mechanisms: water flow around the hull, onboard machinery and propellers. Propeller cavitation—the formation and collapse of vapor bubbles around blades—can add strong broadband and tonal components. A 2023 Vard Marine report hosted by the International Maritime Organization (IMO) describes these flow, machinery and propeller contributions.
NOAA Fisheries characterizes many human-produced ocean sounds as intermittent, while shipping creates “an almost constant rumble in the ocean.” That distinction is as important as peak level. A brief sound may be intense but have little effect on the long-term acoustic background; repeated vessel passages can raise background conditions over a much larger period.
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Speed and operating conditions matter
In a 2025 Gulf of Mexico study, vessel speed was the strongest predictor of ship source level among the factors examined. Ship size and environmental parameters also contributed. This is a result for that study population and operating range, not a universal rule that predicts every vessel or a guaranteed operational prescription.
Shipping is recognized as a major contributor
The IMO’s revised underwater-radiated-noise guidelines describe commercial shipping as one of the main contributors to underwater radiated noise. That statement concerns the sector’s overall contribution; it does not mean ships dominate every location, time period or frequency band.
How to compare underwater sources fairly
Before comparing two decibel values, identify what each value represents. The following checks prevent the most common errors.
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| Comparison question | Why it changes the answer |
|---|---|
| Which frequency or band? | Sources have different spectra. A low-frequency ship rumble cannot be ranked directly against a high-frequency sonar pulse without specifying the band. |
| What acoustic quantity? | A source level describes the sound referenced to a notional distance from the source; a received level is what reaches a recorder or animal; an ambient level summarizes background sound at a site. They are not interchangeable. |
| How long and how often? | A continuous or frequently repeated source can shape exposure even if its instantaneous level is lower than a rare event. |
| How far from the source? | Levels generally decrease with distance, but the rate depends on depth, seabed, water temperature, salinity and frequency. |
| Where and when? | Shipping density, season, storms, ice and local geography alter both the source and the background against which it is measured. |
| Which species’ hearing range? | A sound can overlap one animal’s communication band while falling outside another’s most sensitive range. |
Decibels are logarithmic, and the reference matters. “dB re 1 μPa” is commonly used for underwater received or ambient pressure levels. “dB re 1 μPa m” is used for a monopole source level. A number without its reference, band and measurement position is incomplete.
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Regional ambient indicators
A NOAA-supported study by Haver and colleagues (2021) examined one-third-octave bands centered at 63 Hz and 125 Hz at five U.S. sites. At three sites with high shipping traffic, daily average levels in both bands were near or above 100 dB re 1 μPa. Hawaii was about 20 dB lower year-round, and the Alaskan Arctic was about 10–20 dB lower depending on season, where cargo vessels were less common.
Those are regional ambient indicators, not the source levels of individual ships. The authors note that the 63 Hz and 125 Hz measurements only generally help distinguish sources. At the time of the study, U.S. waters did not have standardized metrics for monitoring and quantifying shipping noise, so the results should not be treated as a universal global shipping index.
Received levels during vessel transits
In Southern Resident killer whale habitat, Veirs, Veirs and Wood (PeerJ, 2016) measured isolated ship transits within 3 km. Received ship noise was 20–30 dB above median background from 100 to 1,000 Hz. At 10,000–96,000 Hz, it was 5–13 dB above background. These figures describe the measured transits and conditions; they are not a universal biological-effect threshold or a level that applies to every vessel encounter.
A vessel source-level estimate
Johnson and colleagues (Journal of the Acoustical Society of America, 2025) reported an average broadband monopole source level of about 183 dB re 1 μPa m from 20 to 1,000 Hz for cargo ships and tankers in their Gulf of Mexico study. This source-level figure must not be compared numerically with the 100 dB re 1 μPa ambient indicators or with the received levels measured near killer-whale habitat. The reference quantity and measurement design are different.
Long-term regional change
NOAA describes a roughly 10 dB increase in low-frequency sound off the U.S. West Coast from about the mid-1960s through the early 2000s. NOAA qualifies this as an indication based on limited data, not a complete continuous record of every source.
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Natural sources can be dominant
Wind, waves and storms
Surface weather generates sound through wave action, bubbles and turbulence. In exposed waters, a storm can raise the ambient level across a broad frequency range and temporarily mask ship noise. The effect depends on wind speed, sea state, depth and the location of the recorder.
Earthquakes, landslides and ice
Earthquakes and underwater landslides produce short or episodic signals that may be much stronger than the ordinary background at the time they occur. In polar regions, cracking and collisions of sea ice add another important natural component. These events cannot be assigned one representative decibel value for all locations.
Marine animals
Whales, dolphins, fish and invertebrates produce calls, clicks, snaps and other sounds. Animal signals can be locally dominant, especially near a calling group or a breeding or feeding area. Their frequency ranges vary widely, so the relevant comparison is with the hearing and communication bands of the species being studied.
Other human-made sources
Seismic airguns
Seismic surveys use repeated high-energy airgun pulses to image geological structures beneath the seafloor. In the Gulf of Mexico study area examined by Johnson and colleagues in 2025, seismic airguns dominated the low-frequency soundscape below 500 Hz. Vessel traffic still contributed substantial high-amplitude noise in that same range. The result illustrates why “ship versus seismic” has no single answer: dominance changed with frequency and with the activity taking place.
Sonar
Sonar is intentionally produced sound. Systems differ greatly in frequency, pulse length, repetition rate and operating power. A sonar transmission may be prominent during a brief ping or operating sequence, while shipping supplies a more continuous background. Comparing a sonar peak with a daily shipping average would conflate different time scales and acoustic quantities.
Drilling and underwater construction
Drilling, pile driving, dredging and other construction activities create machinery noise, vibration and, in some cases, repeated impulsive signals. They are usually concentrated around a project site and schedule rather than spread along an entire shipping route. Their importance therefore depends strongly on distance, project duration and the local species present.
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Frequency determines which source matters
Ship noise is especially important at low frequencies, where machinery, propeller activity and hull-flow processes contribute and where sound can propagate efficiently. That range overlaps parts of the hearing and communication bands of many large whales and some fish.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchIn the Southern Resident killer whale measurements, the largest elevation above median background occurred at 100–1,000 Hz, with a smaller but measurable elevation at 10,000–96,000 Hz. The result does not establish that every ship causes a specific response. It shows that a source’s spectrum must be compared with the frequencies an animal uses and hears.
High-frequency sonar, toothed-whale echolocation and some biological clicks may be far more prominent above several kilohertz than ordinary ship rumble. Conversely, a source can be barely audible in a high-frequency recording while dominating a low-frequency band. A full-spectrum statement such as “source A is louder” hides this distinction.
Why distance and propagation change the result
A recorder close to a vessel captures a received level that is much higher than a recorder farther away. In shallow water, the seabed and surface can reflect or absorb sound; in deep water, temperature and pressure gradients can bend sound paths and create channels that carry low frequencies over long distances. Bathymetry, weather and vessel position all affect the received signal.
For that reason, two monitoring stations can report different levels on the same day even when they are influenced by the same shipping lane. A ship’s source level is intended to characterize the emission independently of a particular receiver distance, whereas an ambient level describes the acoustic conditions at one place and time.
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How underwater noise is measured
Hydrophones and calibrated records
Scientists use hydrophones—underwater microphones—to record pressure fluctuations. Calibrated systems allow measurements to be expressed in standard units and analyzed by frequency band. The Southern Resident killer whale study used hydrophone recordings to characterize received ship noise. A consumer hydrophone may capture audible signals, but without calibration, known sensitivity and controlled deployment it should not be treated as a scientific decibel measurement.
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One-third-octave bands and spectra
One-third-octave bands group frequencies into standardized intervals, making it possible to compare broad trends such as the 63 Hz and 125 Hz indicators used by Haver and colleagues. Spectral analyses provide finer detail and can reveal tones, harmonics, broadband energy and changes caused by propeller cavitation.
Separating overlapping sources
Analysts combine timing, vessel-tracking information, frequency patterns and propagation models to identify likely contributors. Separation is imperfect when several ships, weather systems or industrial activities occur together. This is why the NOAA study described its band indicators as only generally useful for distinguishing sources.
A practical way to interpret a claim about “loudest” noise
- Read the unit and reference. Determine whether the value is dB re 1 μPa, dB re 1 μPa m or another explicitly stated quantity.
- Find the frequency band. A value at 63 Hz cannot be ranked against one at 20 kHz without a spectral comparison.
- Check the distance and location. Ask whether it was measured beside a vessel, at a fixed monitoring site or as a modeled source level.
- Check duration and repetition. Distinguish a single pulse, a vessel transit, a daily average and a long-term trend.
- Identify the background. A “20 dB above background” result is relative to the local median or ambient condition, not an absolute worldwide threshold.
- Match the frequency to the species or application. The ecological relevance depends on hearing sensitivity, communication frequency and exposure time.
Following these steps usually turns an apparent contradiction into a difference in measurement design. A seismic survey may dominate below 500 Hz during operations, while shipping remains the more persistent regional contributor over a longer period. A storm may overwhelm both for several hours, and a nearby animal call may dominate a narrow high-frequency band.
What can and cannot be concluded globally
The available measurements support a consistent broad conclusion: shipping is a widespread, persistent low-frequency contributor, while natural events and other human activities can dominate locally or in particular bands. They do not support a single all-ocean ranking of ships, sonar, seismic surveys, construction, storms and animals by decibel level.
Results are tied to specific regions, years, frequencies, seasons, vessel types and methods. The U.S. ambient indicators, the Southern Resident killer whale transits, the Gulf of Mexico source-level study and the West Coast historical estimate answer different questions. Treating their numbers as though they were measurements from one common scale would produce a misleading comparison.
The most accurate answer to “How does ship noise compare with other underwater noise?” is therefore conditional: ships are unusually important because they are numerous and persistent, especially at low frequencies; seismic, sonar, construction, storms, earthquakes, ice and animals can be more prominent during particular events, at particular frequencies or near particular sources.
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