Underwater noise

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Definition of Underwater-Radiated Noise (URN):
Underwater-Radiated Noise (URN) is the sound energy that a marine vessel or other humanmade source emits into the ocean
This is the common definition for Underwater-Radiated Noise (URN), other definitions can be discussed in the article


The radiated noise level (RNL), expressed in decibels (dB), is defined as [math]RNL = 20 \log_{10} (r \, p_{rms}(r))[/math]. It is a property of the source, commonly measured at a distance [math]r[/math] from the source; [math]p_{rms}[/math] is the root-mean-square pressure and [math]r\, p_{rms}[/math] is expressed in units [10-6 N/m].


Pile driving for the installation of an offshore wind turbine. Source https://www.deingenieur.nl/


Sources of underwater sound

Sound propagates through water as a longitudinal pressure wave: water particles oscillate back and forth in the direction of wave propagation, producing alternating compression and rarefaction. Sound in seawater has various origins: environmental, biotic and anthropogenic. Environmental sound is caused by e.g. breaking waves, turbulent water motions, earthquakes, thunder and raindrops. Biotic sound is produced by marine animals. Anthropogenic sound is what we call 'underwater-radiated noise' (URN) here. It is produced by human activities such as trawling, dredging, military exercises, oil and gas exploration, seismic surveys, commercial shipping, recreational boats, windfarm construction (e.g. pile driving), sounds emitted by turbines, etc. These noise sources have grown tremendously over the past century. High-intensity impulsive noise is produced by pile driving, underwater blasting, seismic exploration and active sonar application.[1]

Sound perception by marine animals

Underwater noise can travel large distances before being dissipated. For example, pile-driving noise can elicit strong avoidance behavior in harbor porpoises at distances of many kilometers, with effects reported up to about 20 km in some studies. A highly sensitive animal, the bottlenose dolphin, suffers auditory injury within 100 m of the pile-driving location, and exhibits behavioral disturbance up to 50 km. Similarly, behavioral disturbance of minke whales may occur as far as 40 km from the pile driving site. [2]

Low-frequency noise (below 500 Hz), which is typically produced by activities such as commercial shipping, offshore oil, gas and wind infrastructure, as well as trawling and dredging, is generally attenuated less strongly than high-frequency sound and can therefore propagate over very large distances (up to hundreds of kilometers where water depth and seabed conditions favor propagation). High-frequency noise (frequencies of several tens of kHz and higher) is produced for example by echo sounders and fish-finding sonar; it reaches less far but can be critical for species relying on echolocation.

Marine mammals have highly sensitive hearing mechanisms with ears adapted to hearing in underwater environments. Fish hear with otolith organs that are sensitive to sound through relative motion between the sensory epithelium and the denser otolith, which bends the sensory hair cells[3]. In many fish, gas-filled structures such as the swim bladder enhance this stimulation and thereby increase hearing sensitivity and extend the detectable frequency range. Cephalopods can sense water-borne vibrations through statocysts. Crustaceans such as crabs can detect mechanical disturbances through statocysts, chordotonal organs and other mechanosensory structures. Bivalves such as clams and mussels also possess mechanoreceptors capable of detecting water- or substrate-borne vibrations[4][5]

Importance of sound for marine animals

Biotic sound is produced by fish, invertebrates, marine mammals and other marine organisms, and plays an important role in communication, orientation, mate and prey detection, and echolocation. Swimming of pelagic animals and burrowing and feeding of benthic animals produce sound. Some fishes can produce special sounds by vibrating their swim bladder, other species produce sound by rubbing hard parts of their body.[1][4]

Many marine organisms depend on the interpretation of acoustic information of their environment for their survival. Animals use sound vibrations to learn more about their environment, predators, prey, potential mates, and competitors. Therefore, anything that interferes with the animal's ability to detect sounds, can affect their survival as individuals and populations. Noise pollution can affect marine organisms’ acoustic communication through physiological damage of the hearing system and through auditory masking where the perception of one sound is affected by the presence of another sound. Anthropogenic noise can drown out biologically important cues or signals, with possibly detrimental consequences such as the inability to find shelter or the right migratory route, finding food (prey), or even detecting a predator.[6]

Impacts of underwater noise on marine animals

In field studies it is often not possible to isolate the impacts of anthropogenic noise from other environmental stressors, such as pollution, climate change, and ocean acidification. Hence, studies of physiological and auditory impacts of URN have mainly been conducted under laboratory or controlled conditions. However, the results obtained are not fully representative of real natural conditions. Field evidence for persistent effects on fitness, recruitment or population size is still scarce.[1]

In the field, commonly observed responses include avoidance and changes in swimming, surfacing, respiration and diving behavior. Severe responses to intense sonar exposure have also been implicated in some beaked-whale stranding and mortality events[7]. Declines in fish catch rates for rockfish, cod, haddock and saithe have been attributed to seismic shooting and vessel-generated noise[8]. Cumulative noise effects are still difficult to measure and poorly understood. A major challenge exists between integrating results from captive and wild studies, largely because the contexts are so fundamentally different. Wild animals exposed to noise are the animals of primary concern, and exposures in the wild encompass the natural and anthropogenic contexts. [9].

Laboratory studies have revealed numerous impacts of underwater noise on animals.

  • Auditory injury. For example, permanent and substantial alterations of the sensory hair cells of the statocysts in several cephalopod species (squid, octopus, cuttlefish) caused by low-frequency noise exposure; ablated hair cells of the sensory epithelial hair cells in the pink snapper following exposure to air-gun blasting[10]
  • Physiological stress. For example: Increased levels of stress-related neurotransmitters in beluga whales after exposure to high-level noise from a seismic water gun; changes in the expression profile in genes related to oxidative stress, energy homeostasis, metabolism, respiration and immune response observed in biomarker analyses of sessile invertebrates such as bivalves and sea squirts[5]
  • Impaired feeding/growth. For example: Reduction in growth and significantly smaller body weights of lined seahorses exposed to loud noise; significant increase in metabolism and a reduction in growth rate in brown shrimp from noise exposure in aquarium systems[1]

Underwater-radiated noise (URN) may have both short and long-term negative consequences on marine life, especially marine mammals. Thresholds have been established for acute temporary auditory injury and for injuries that do not recover after exposure. TTS (Temporary Threshold Shift) is a temporary, reversible increase in the threshold of audibility at a specified frequency or portion of an individual’s hearing range above a previously established reference level. PTS (Permanent Threshold Shift) is a permanent, irreversible increase in the threshold of audibility at a specified frequency or portion of an individual’s hearing range above a previously established reference level[11].

Impact assessment

Underwater noise levels can be predicted by combining estimates of the sound emitted by a source with a model of sound propagation. However, the determination of source level in shallow water is complicated by repeated interactions of the propagating sound with the sea surface and sea-floor, and multi-path arrivals at the target. Source characteristics are generally well known for some controlled sources, such as seismic airgun arrays, but are much more variable for shipping, dredging and other marine activities. The underwater radiated noise of individual ships is generally not known and is commonly estimated from empirical relationships with vessel type, dimensions and speed, using AIS information. Propeller cavitation noise is usually dominant at normal transit speeds, but its strength depends strongly on propeller design, loading, fouling, hull condition, draught and actual operating state. Predictions of regional underwater noise therefore have substantial uncertainty and are generally complemented by acoustic measurements.

For impact assessment, the estimated sound level or sound exposure at different distances from the source and its duration has to be compared with the sensitivity of the species living in the affected habitat. Impact assessment may use acoustic effect criteria where these are available. Such criteria are best developed for marine mammals, for which threshold exposure levels have been established for hearing impairment and, more provisionally, behavioral disturbance. For fish, proposed criteria exist for intense impulsive sources such as pile driving, whereas generally accepted exposure thresholds are still lacking for many fish and invertebrate species.

Monitoring of underwater sound exposure is well developed. However, monitoring of biological impacts is still comparatively underdeveloped. Field studies can detect changes in behavior, distribution or abundance, but establishing persistent effects on fitness, recruitment or population size is much more difficult because responses are variable and many other environmental factors act simultaneously.

Mitigation of underwater noise

Guidelines prepared by IMO for the operation, design, construction and modification of ships, provide a tool for reducing underwater radiated noise (URN)[12].

Several methods have been developed to mitigate anthropogenic sound in the ocean, primarily targeting steady sources such as ships and turbines. Passive sound absorption using foam-type polymer, metal, or ceramic materials is possible, but these materials are typically effective over narrow frequency bands, transmit low-frequency noise, are costly, and have limited practical applicability.[13]

Bio-inspired structural adaptations can reduce noise generated by turbine-induced turbulence and cavitation, while other design approaches focus on shifting resonance frequencies and on noise damping through internal baffles, vortex generators, and water injectors. Active noise control in the marine environment remains challenging due to the difficulty of identifying multiple sound sources and accurately predicting their frequency ranges, intensities, and propagation. However, active noise cancelation shows potential for mitigating low-frequency noise. Overall, mitigation of underwater sound remains an active and developing field of research.[13]

Noise from pile-driving operations can be substantially reduced in water depths of up to about 45 m by installing surrounding bubble curtains. An alternative approach is the use of encapsulated resonator systems, which consist of curtains of resonating elements placed around the pile and tuned to attenuate specific frequency bands[14]

The noise disturbance of shipping depends on the ship speed and is considerably lower if the ship speed is reduced - especially in the case of container vessels. Reduction of greenhouse gas emissions is an important co-benefit. A modest 10% reduction in speed would cut global underwater sound energy from shipping by around 40% [15].

Legislation

Under the EU Marine Strategy Framework Directive (MSFD), Good Environmental Status for underwater noise is assessed for impulsive and continuous noise. EU Member States have agreed to implement threshold values based on the proportion of habitat of a target species exposed above the Level of Onset of Biologically Adverse Effects (LOBE). For continuous noise this proportion should not exceed 20% in any month; for impulsive noise it should not exceed 20% on any day and 10% on average over a year. Species-specific LOBE values are still not available (2026) for many species and regions. The UK Marine Strategy similarly requires impulsive and continuous underwater noise to remain below levels that adversely affect populations of marine animals. Operational disturbance thresholds are applied in some harbor porpoise protected areas, while development of broader risk-based thresholds is ongoing[16].

Population-level consequences of noise exposure can be modelled, but data to parameterize such models are currently unavailable for most species. Because of this difficulty, most regulations represent soft laws in the form of guidelines and recommendations that are rarely enacted in national legislation. To date, the application of measures to reduce noise levels (quieting/noise abatement) has been limited. [17][18]


Further reading

Erbe, C., Houser, D., Bowles, A., Porter, M.B. (Eds.) Marine Mammal Acoustics in a Noisy Ocean. Springer and ASA Press, open access https://link.springer.com/book/10.1007/978-3-031-77022-7


Related articles

Coastal pollution and impacts
Threats to the coastal zone


References

  1. ↑ 1.0 1.1 1.2 1.3 Peng, C., Zhao, X. and Liu, G. 2015. Noise in the Sea and Its Impacts on Marine Organisms. Int. J. Environ. Res. Public Health 12: 12304-12323
  2. ↑ Bailey, H., Senior, B., Simmons, D., Rusin, J., Picken, G. and Thompson, P.M. 2010. Assessing underwater noise levels during pile-driving at an offshore windfarm and its potential effects on marine mammals. Mar. Pollut. Bull. 60: 888–897
  3. ↑ Popper, A.N. and Hawkins, A.D. 2018. The importance of particle motion to fishes and invertebrates. Journal of the Acoustical Society of America 143: 470–488
  4. ↑ 4.0 4.1 Roberts, L. and Elliott, M. 2017. Good or bad vibrations? Impacts of anthropogenic vibration on the marine epibenthos. Sci. Total Environ. 595: 255-268
  5. ↑ 5.0 5.1 El-Dairi, R., Outinen, O. and Kankaanpää, H. 2024. Anthropogenic underwater noise: A review on physiological and molecular responses of marine biota. Marine Pollution Bulletin 199, 115978
  6. ↑ Houser, D. 2025. Marine Mammal Hearing. In: Erbe, C., Houser, D., Bowles, A., Porter, M.B. (Eds.) Marine Mammal Acoustics in a Noisy Ocean. Springer and ASA Press
  7. ↑ Evans, D.L. and England, G.R. 2001. Joint interim report Bahamas marine mammal stranding event of 14–16 March 2000. National Oceanic and Atmospheric Administration, Washington
  8. ↑ Engas, A. and Lokkeborg, S. 2002. Effects of seismic shooting and vessel-generated noise on fish behaviour and catch rates. Bioacoustics 12: 313–316
  9. ↑ Erbe, C., Houser, D., Bowles, A., Porter, M.B. (Eds.) Marine Mammal Acoustics in a Noisy Ocean. Springer and ASA Press
  10. ↑ Guerra, A., Gonzalez, A.F., Pascual, S. and Dawe, E.G. 2011. The giant squid Architeuthis: An emblematic invertebrate that can represent concern for the conservation of marine biodiversity. Biol. Conserv. 144: 1989–1997
  11. ↑ NOAA 2024. Update to: technical guidance for assessing the effects of anthropogenic sound on marine mammal hearing. NOAA Technical Memorandum NMFS-OPR-71
  12. ↑ IMO 2023. Revised guidelines for the reduction of underwater radiated noise from shipping to address adverse impacts on marine life. I:\CIRC\MEPC\1\MEPC.1-Circ.906
  13. ↑ 13.0 13.1 Jin, X., Huang, H-L., Tao, Y., Zhang, F-G., Luo, M., Fan, C-Y. and Tang, F. 2025. A systematic review on mechanism and regulation strategy of marine hydrodynamic noise: Advances, challenges, and perspectives. Ocean Engineering 330, 121202
  14. ↑ Merchant, N.D. and Robinson, S.P. 2020. Abatement of underwater noise pollution from pile-driving and explosions in UK waters. Report of the UKAN workshop held on Tuesday 12 November 2019 at The Royal Society, London. 31pp
  15. ↑ Leaper, R. 2019. The Role of Slower Vessel Speeds in Reducing, Greenhouse Gas Emissions, Underwater Noise and Collision Risk to Whales. Front. Mar. Sci. 6, 505
  16. ↑ DEFRA 2026. UK Marine Strategy Part One: Updated assessment and Good Environmental Status
  17. ↑ Merchant, N.D., Putland, R.L., André, M., Baudin, E., Felli, M., Slabbekoorn, H. and Dekeling, R. 2022. A decade of underwater noise research in support of the European Marine Strategy Framework Directive. Ocean and Coastal Management 228, 106299
  18. ↑ Jolliffe, C., Erbe, C., Juretzek, C., Lewandowski, J., Merchant, N.D., Miller, B., Senigaglia, V. and Thornton, J.J. 2025. In: Erbe, C., Houser, D., Bowles, A., Porter, M.B. (Eds.) Marine Mammal Acoustics in a Noisy Ocean. Springer and ASA Press


The main author of this article is Job Dronkers
Please note that others may also have edited the contents of this article.
Citation: Job Dronkers (2026): Underwater noise. Available from http://www.coastalwiki.org/wiki/Underwater_noise [accessed on 7-09-2026]
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