Scientists measure ocean noise by recording underwater sound—usually with hydrophones—and analyzing how it varies by frequency, time, and location. To determine what those sounds may mean for marine life, they then consider which animals were exposed, how much sound reached them, and what they were doing. A recording can describe acoustic conditions; by itself, it cannot prove an animal was harmed.
What counts as ocean noise?
The ocean soundscape combines natural, biological, and human-made sounds. Storms, earthquakes, and animal vocalizations are among its natural sources; vessels and other human activities add sound too. A measured level alone does not reveal which source produced it or whether it affected an animal. Scientists first characterize the sound and its setting, then assess exposure and biological evidence. NOAA Fisheries explains the potential effects of ocean noise, while NOAA’s Soundcheck overview describes the broader soundscape.
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How do scientists measure underwater sound?
Hydrophones detect pressure changes
A hydrophone is an underwater sensor broadly comparable to a microphone. Many use ceramic materials that produce a small electrical signal as water pressure changes; electronics amplify and record that signal. A single hydrophone detects sound arriving from different directions but generally cannot identify a source’s direction on its own. NOAA’s hydrophone explainer, updated September 23, 2026, describes how these sensors work. A research measurement also depends on the recorder and its calibration: an ordinary consumer hydrophone should not be assumed to have the frequency response or sensitivity of a calibrated scientific instrument.
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Much ocean-noise monitoring is passive acoustic monitoring: instruments record sounds already present rather than transmitting sound into the water. Recorders may be fixed to moorings, installed in cabled arrays, lowered or towed from vessels, or carried by mobile platforms. Autonomous instruments can run from days to years, depending on their design. Cabled systems can provide real-time access, but require substantial infrastructure. NOAA Ocean Exploration’s acoustic-monitoring overview describes these approaches.
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Instrument choice follows the question being asked. Frequency range and response, sample rate, sensitivity, self-noise, dynamic range, depth limits, recording duration, deployment ease, and cost all affect what a system can detect. A recorder that is suitable for one frequency band or deployment may miss sounds important to another study. NOAA’s 2016 Ocean Noise Strategy Roadmap, Chapter 3 outlines these considerations and platform trade-offs; it is a planning document, not a current inventory of equipment.
Pressure and particle motion are not the same measurement
Underwater sound includes changing pressure and movement of water particles. Hydrophones commonly measure pressure, a scalar quantity. Particle motion has direction and can be expressed as displacement, velocity, or acceleration; researchers may use accelerometers or geophones to measure it. In simplified deep-water, far-field conditions, particle motion can sometimes be estimated from pressure, but that inference has limits. The measurement needs to fit the question and environment.
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Decibels also require a stated reference quantity. Underwater pressure levels conventionally use a reference of 1 micropascal, while particle-motion metrics use different references. An underwater decibel figure should not be compared directly with an airborne decibel figure without accounting for the different references and measurement context.
How do arrays and deployment choices change what can be learned?
Synchronized hydrophones can improve sensitivity and help researchers distinguish the direction of sounds. Other deployment choices trade spatial coverage against continuity, access, and operating demands:
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- Fixed moorings can record a location over an extended period, helping describe baselines and changes through time.
- Mobile or vessel-based systems can survey a wider area, but sample locations and times rather than providing uninterrupted records everywhere.
- Cabled arrays can deliver sound data in real time, with infrastructure requirements that can make deployment more demanding.
- Autonomous recorders can collect data without a continuous connection, for periods ranging from days to years depending on the instrument and deployment.
These are broad capabilities, not a ranking of products. There is no single best setup for every study: researchers match coverage, directionality, duration, frequency response, sensitivity, depth constraints, real-time access, and cost to the research objective. NOAA’s Ocean Noise Reference Station Network page describes a U.S. network for tracking noise over time. Its page presents 12 underwater reference stations in U.S. waters; because the count can change, treat it as the count stated on that page, not a permanent or global total.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can recordings reveal about marine life?
Many marine animals use sound to communicate, find food or mates, detect predators, navigate, and maintain social behavior. Researchers can analyze recordings to study animal presence, distribution, density, or behavior. Acoustic detection, however, is not automatically an abundance estimate: translating detections into density requires appropriate methods and ecological context.
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Human-made sound may cause temporary or permanent hearing loss, stress responses, displacement from preferred habitat or migration routes, or disruption to feeding, breeding or spawning, nursing, and communication. These are potential effects, not inevitable outcomes of every exposure. Their likelihood and severity depend on the sound source, its duration and location, and the animal’s exposure. Effects may be acute or accumulate over time. Evidence and thresholds differ among species and sound types; findings for one group, including particular invertebrates studied in the 2023 review of marine invertebrates and noise, should not be generalized to all marine life.
Long-term passive recordings can help establish acoustic baselines, track changes in the soundscape, and study animal distribution and habitat use. To link a change in sound to a biological effect, researchers need more than a high reading: they must characterize the signal, estimate what animals experienced, and interpret the evidence in its ecological setting.
What do the historical and network figures mean?
NOAA’s hydrophone explainer reports a 10-decibel increase in low-frequency sound levels off the U.S. West Coast from about the mid-1960s through the early 2000s. NOAA describes this as an indication from limited long-term data. It is a historical finding for that region and period, not evidence of a uniform global trend. The same explainer quotes NOAA PMEL research oceanographer Robert Dziak: “You would think that the deepest part of the ocean would be one of the quietest places on Earth. Yet there is almost constant noise,”
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