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Ship noise is not universally the loudest sound in the ocean. Its importance comes from being widespread, persistent and concentrated at low frequencies, where it can form a near-continuous background. Seismic airguns, sonar, construction, storms, earthquakes and animals can produce higher levels in particular places, frequency bands or events.
A fair comparison must specify frequency, acoustic measurement, distance, duration, location and background conditions. A ship’s source level is not directly comparable with an ambient sound level recorded at a distant monitoring site.
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The short answer: persistence makes shipping different
The ocean has a soundscape rather than one noise level. Wind and waves, storms, earthquakes, ice, landslides and animal calls mix with ship traffic, sonar, seismic exploration, drilling and construction. Each source has its own frequency range, intensity, timing and propagation pattern.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Shipping stands out because it is often present for long periods over large areas. NOAA Fisheries describes many human-produced sounds as intermittent, while shipping creates “an almost constant rumble in the ocean.” The International Maritime Organization (IMO) likewise characterizes commercial shipping as one of the main contributors to underwater radiated noise in its revised guidelines. Those descriptions refer to shipping as a broad, persistent contributor—not to every vessel being louder than every other source.
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In practice, a seismic survey may dominate low-frequency sound during an operation, a sonar transmission may briefly dominate a particular band, and a storm may overwhelm all human sources locally. Between such events, thousands of vessels can maintain an elevated low-frequency background that affects how far other sounds travel and how clearly animals detect calls.
Why “loudest” is not a single, valid ranking
Underwater acousticians compare sound using several different quantities. Treating their decibel values as interchangeable can produce a misleading ranking.
Frequency band
Sound level is measured over a frequency band or across a broadband range. A source can be prominent below 500 hertz (Hz) yet relatively weak at several kilohertz. Low-frequency ship machinery and propeller noise overlap the hearing and communication ranges of some large whales, while higher-frequency sonar or construction components may matter more to other species.
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A source level estimates the sound a source would radiate at a reference distance, commonly expressed in decibels relative to 1 micropascal at 1 metre (dB re 1 μPa m). A received level is what a hydrophone measures at its actual location after spreading, absorption, reflection and environmental effects. An ambient level describes the combined sound field at a site, often in a one-third-octave band. These are different measurements.
For example, a 2025 Gulf of Mexico study reported an average broadband monopole source level of about 183 dB re 1 μPa m for cargo ships and tankers over 20–1,000 Hz. That source-level figure cannot be compared directly with a monitoring station’s daily ambient level in a 63 Hz band.
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Duration and repetition
A short, high-level pulse can have a different biological and ecological significance from a lower-level sound that persists for hours. Repeated transits, continuous machinery noise and a steady flow of vessels can raise background conditions even when no individual passage is exceptional.
Distance and propagation
Sound levels generally decline with distance, but the rate is not fixed. Water depth, temperature, salinity, seafloor properties, surface conditions and frequency affect propagation. Low-frequency sound can travel long distances under suitable conditions. A distant source can therefore contribute to a regional soundscape, while a nearby source can dominate a local recording.
Location, season and background
Busy shipping lanes, ports and narrow straits have different baselines from remote polar waters or offshore areas with little traffic. Seasonal ice, storms, migration, vessel schedules and changing ocean conditions can alter both natural and human-made sound.
What ship noise contains
Underwater radiated noise from a vessel is a mixture rather than one tone. The IMO-hosted 2023 Vard Marine technical report identifies three broad contributors:
- Flow noise: turbulence and water movement around the hull and appendages.
- Machinery noise: engines, generators, pumps, gearing and other onboard equipment transmitted through the hull.
- Propeller noise: pressure fluctuations and cavitation as propeller blades move through water.
Ship speed is the strongest predictor of vessel source level among the factors examined in the 2025 Gulf of Mexico study; vessel size and environmental parameters also contributed. That result describes the study’s statistical analysis, not a universal rule that applies identically to every ship or operating condition.
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Because commercial vessels move continuously and follow predictable routes, their sounds can create a broad, recurring low-frequency footprint. A single transit may be temporary at one hydrophone, but traffic density determines whether another vessel arrives before the soundscape returns to its previous level.
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How shipping compares with natural ocean sound
Wind, waves and storms
Wind and breaking waves generate sound over broad frequency ranges. Calm conditions can reveal vessel passages that would otherwise be masked; heavy weather raises the natural background and can mask ships, animals or sonar. A storm may therefore be the dominant source for its duration even in a busy region.
Earthquakes, landslides and ice
Earthquakes, submarine landslides and ice interactions can produce intense, impulsive or prolonged signals. Their occurrence is episodic and geographically limited, so they cannot be assigned one global decibel value or placed permanently above shipping in a ranking.
Marine animals
Whales, dolphins, fish and invertebrates add biological sound. Calls may be highly directional, seasonal or concentrated in a narrow band. A vocal chorus can dominate a local recording even while shipping sets the wider low-frequency background. The relevant comparison depends on the species’ hearing, call frequencies and whether masking occurs at the receiver.
How shipping compares with other human-made sound
| Source | Typical timing | Frequency or acoustic pattern | How it can compare with ships |
|---|---|---|---|
| Commercial shipping | Frequent to nearly continuous in busy routes | Broad low-frequency rumble from flow, machinery and propellers; cavitation can add higher components | Often important as persistent background; regional effect depends on traffic density and propagation |
| Seismic airguns | Intermittent survey operations, with repeated pulses | Strong low-frequency impulses; in one Gulf of Mexico study, dominated below 500 Hz | Can exceed or mask vessel noise locally during a survey, while vessels still contributed substantial high-amplitude sound in that band |
| Sonar | Intermittent transmissions, varying by system and mission | Frequency-specific pulses or sweeps | May dominate its transmission band briefly; not equivalent to a continuous shipping background |
| Underwater construction and drilling | Project-based, often intermittent or scheduled | Equipment-dependent; pile driving can be impulsive, while drilling can be more continuous | Can dominate near a work site even if its geographic footprint is smaller than shipping |
NOAA’s Gulf of Mexico analysis found seismic airguns dominated the low-frequency soundscape below 500 Hz in that study area. It also found vessel traffic contributed substantial high-amplitude noise in the same range. The result illustrates why “ship versus seismic” has to be framed for a place, time and frequency band rather than answered with one universal winner.
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Measured examples that show the scale—and the limits
Regional ambient indicators in U.S. waters
A NOAA-supported study by Haver and colleagues, published in 2021, examined daily averages in the 63 Hz and 125 Hz one-third-octave bands at five U.S. sites. At three sites with high shipping traffic, daily averages were near or above 100 dB re 1 μPa in both bands. Hawaii was about 20 dB lower year-round, while the Alaskan Arctic was about 10–20 dB lower depending on season, in comparisons with locations where cargo vessels were less common.
These are regional ambient indicators, not individual ship source levels. They combine multiple contributors and reflect the conditions at the monitoring sites. The authors also cautioned that the 63 and 125 Hz bands only generally help distinguish sources. At the time of that study, U.S. waters lacked standardized metrics for monitoring and quantifying shipping noise consistently.
Received levels during ship transits
In a 2016 PeerJ study of Southern Resident killer whale habitat, isolated ship transits within 3 kilometres produced received noise 20–30 dB above median background at 100–1,000 Hz. At 10,000–96,000 Hz, the increase was 5–13 dB. Those figures describe the measured transits, locations and conditions; they are not a universal biological-effect threshold or a prediction for every whale habitat.
The stronger increase at lower frequencies is consistent with the frequency content of many vessels and shows why species-specific analysis matters. A level that is prominent in one animal’s communication band may be less important to another species with different hearing sensitivity.
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NOAA has described an indication of roughly a 10 dB increase in low-frequency sound off the U.S. West Coast from about the mid-1960s through the early 2000s. NOAA qualifies that trend because it is based on limited data. It is evidence of changing regional conditions, not a global estimate or a direct measure of every ship’s contribution.
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How to make a meaningful comparison
When reading a report, environmental assessment or news claim, check these items before comparing numbers:
- Identify the acoustic quantity. Is the number a source level, received level, sound-exposure level or ambient band average?
- Read the reference unit. Underwater levels normally use dB re 1 μPa; source-level notation may include the 1-metre reference, such as dB re 1 μPa m.
- Note the frequency. A 63 Hz one-third-octave value is not a broadband value from 20–1,000 Hz, and neither is equivalent to a 20 kHz measurement.
- Check distance. Record the range from source to hydrophone and whether the value was corrected to a reference distance.
- Check duration and repetition. Determine whether the measurement covers a pulse, a transit, a daily average or a long-term percentile.
- Read the site and season. Traffic, bathymetry, weather, ice and biological activity can change the result.
- Ask what species or use is relevant. Compare the sound with the hearing and communication ranges of the animals being considered, not with human airborne-noise intuition.
Why low-frequency shipping noise travels and matters
Low-frequency sound is often less attenuated by seawater than higher-frequency sound and can be guided by ocean conditions over long paths. That does not mean every low-frequency ship signal is detectable everywhere: bathymetry, temperature layers, salinity, sea state and other sources still control propagation. It does mean that a busy route can influence a larger area than the immediate vicinity of a hull.
For animals, the concern is often masking rather than physical damage from one measured passage. Elevated background can reduce the range at which a call, echolocation signal or environmental cue is detectable. The effect depends on received level, frequency overlap, the animal’s hearing and behavior, and how long the elevated condition persists.
Common mistakes and how to avoid them
- Putting every decibel on one leaderboard: separate source, received and ambient measurements.
- Calling a one-time peak the “usual” level: state whether it was a pulse, transit or daily statistic.
- Assuming the nearest source is always dominant: propagation can bring distant low-frequency sound into a measurement, while local conditions can mask a nearby source.
- Using a regional indicator as a vessel specification: ambient monitoring values do not describe one ship’s radiated output.
- Ignoring natural sound: storms, waves, animals and geophysical events can raise the background independently of human activity.
- Inferring a biological threshold from one study: measured elevation above background is not, by itself, proof of harm or a universal threshold.
What a hydrophone can—and cannot—tell you
Researchers use calibrated hydrophones and documented deployment methods to measure underwater sound. A recording can reveal frequency content, timing and received level at that sensor. Turning it into a source level requires reliable information about range, propagation and calibration. An uncalibrated consumer recording may show that a sound occurred, but it cannot automatically support a scientific decibel comparison.
For that reason, a useful comparison report should publish the hydrophone’s location and depth, calibration, bandwidth, sampling period, processing method, vessel distance and environmental conditions. Without those details, a precise-looking number may be less informative than a qualified description of persistence and frequency.
The practical conclusion
Ship noise is best understood as a widespread, persistent low-frequency background source. It is not always the highest-level sound: seismic airguns can dominate below 500 Hz during surveys, sonar can dominate its transmission band, construction can overwhelm a work site, and storms or earthquakes can control the natural soundscape. The distinctive shipping issue is the combination of many vessels, repeated exposure and long-range low-frequency propagation.
There is no scientifically defensible global ranking of “the loudest” underwater noise source. The answer changes with frequency, measurement type, distance, duration, location, season and the species or ecosystem being considered. Comparing those dimensions explicitly produces a useful answer; comparing decibel numbers without them does not.
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