Underwater Acoustics

Underwater Acoustics

A brief introduction to the science and practical application of underwater acoustics.
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Overview

Other than for military applications, the study of underwater noise and its effects on the marine environment is an ongoing area of research and guidance is constantly evolving. This is in contrast to the understanding and assessment of airborne environmental noise which has been established for many decades and the processes and methodologies are extremely mature.(More information about airborne noise)

It is increasingly evident that noise from human activities in and around the marine environment has an impact on the marine species in the immediate area and beyond. Unlike light and other stimuli, sound is transmitted very efficiently through water with sound often being detectable underwater at distances many times that which would be expected with noise in air. Piling noise can be detected at a range in excess of 100km in water of 30m depth and this noise extends considerably further in deep waters.

The efficiency of sound propagation under water allows marine animals to use sound as a method of communication, sometimes over long distances, and to sense the presence and location of objects including prey. Marine animals may also obtain a great deal of information about their environment by listening to the sounds from natural sources. Examples may include the sound of surf indicating the presence and direction of the shoreline and/or ice noise.

The impact on marine life from manmade noise varies considerably according to the source of the noise and the species considered. Continuous noise (such as from shipping or dredging) can have the effect of raising the background noise level and preventing detection of other sounds important to the animal (known as masking). High-level, impulsive noise (such as from piling or seismic surveys) can have a significant behavioural effect and possibly cause life-limiting injury.
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Underwater Acoustics

For an in-depth explanation of the physics of sound and how it propagates, we recommend the Discovery of Sound in the Sea (DOSITS) website.

Sound levels can be characterised in a number of ways, the most common being sound pressure level (SPL). Depending on the nature of the sound, different forms of the SPL may be used: 

 Peak SPL (and peak-to-peak): The maximum acoustic pressure of the wave, used for impulsive sounds (such as from impact piling or blasting).
 RMS SPL: The ‘average noise level’ used to describe continuous sounds (such as from vessels, vibro-piling, drilling etc.); 
 Sound Exposure Level (SEL): The noise ‘dose’. Used when considering the overall acoustic energy that a receptor may experience over a given period.

Other acoustic measures such as particle motion* are believed to have greater significance than pressure for some animals i.e. some fish may be more sensitive to being shaken (particle motion) than squeezed (pressure). However, research is ongoing in this field and at present there remains several knowledge gaps that prevent the use of particle motion in routine assessments. 

It should be noted that noise levels in air cannot be directly compared to noise levels in water. This can be confusing and has led to misleading comparisons in media reports. To learn more about the distinctions between noise levels in air and in water, see the DOSITS website: https://dosits.org/science/sounds-in-the-sea/how-does-sound-in-air-differ-from-sound-in-water/.

*The term particle motion, as used by biologists, often refers to the whole-body motion of an animal. This differs from particle motion as understood by physicists.

Sensitivity to Underwater Noise

Many marine mammals and fish have highly evolved hearing and use sound extensively to navigate, communicate and find food but the extent to which this is the case varies considerably between species. 

While human hearing is sensitive to sound pressure within a frequency range of 20 Hz to 20 kHz, large whales can detect sounds from 7 Hz to 22 kHz, some species of porpoise have a range of 200 Hz to 180 kHz and other animals can be anywhere in between. This means that assessments are always tailored to the specific location receptor species of concern. 

Current regulatory guidance groups fish and marine mammals into the following groups according to their sensitivity to sound pressure:

Mammals:
  Low-frequency cetaceans (Baleen whales) 
  Mid-frequency cetaceans (most dolphins, beaked whales, killer whales)
  High-frequency cetaceans (porpoises, river dolphins)
  Phocid pinnipeds (true seals)
  Otariids pinnipeds (fur seals, sea lions)

Fish:
  Swim bladder involved with hearing (most sensitive)
  Swim bladder not involved with hearing
  No swim bladder (least sensitive)
  Fish eggs and larvae

Turtle

Some species of fish and other marine animals (such as cephalopods and crustaceans) are understood to be insensitive to sound pressure may be sensitive to other acoustic parameters such as particle motion and research is ongoing in these areas.
Airborne noise has well established international standards for the measurement and assessment of the effects of noise in the environment.

Underwater Noise Assessments

Airborne noise has well established international standards for the measurement and assessment of the effects of noise in the environment. The existence of such standards means that, provided the assessor is competent and the standards are followed, there is little room for disputing how a study was undertaken.

With underwater noise there is a greater reliance on the experience and expertise of the consultant to determine the best approach. Regulatory bodies will frequently draw on the latest scientific research to inform their advice and expect those undertaking assessments to incorporate relevant studies into new projects (and often revise previous studies to bring them up to date.). This extends to measurement and modelling methodologies where best practice guidelines do exist to inform assessments but are much less prescriptive than the ISO or BS standards utilised in airborne noise assessments.

The following publications represent the criteria and guidelines currently accepted in the UK (and other jurisdictions), however, each country has its own approach to marine licencing and may draw on other criteria.


Due to the differences in receptor species and the high dependence on the local environmental conditions, assessments are always site and project specific. Whilst some generalisations may be made, it is not usually possible to apply the findings from one project to a different site.
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