This report proposes and offers validation of a frequency weighted scale, the dBht(Species), as a metric for the assessment of the behavioural and audiological effects on underwater animals of man-made underwater noise.
There is concern over the environmental effects that may result from man-made underwater noise caused by increasing exploitation and exploration of the coastal and deep sea. While the hazards associated with, for instance, the dispersal of toxic materials are now well understood, the understanding of the effects which may be associated with noise is in its infancy, despite its status as a significant underwater environmental issue. This results primarily from the lack of an objective scale (usually termed “metric”) which may be used to relate the observation of environmental effects to recorded levels of noise, and hence to allow both prediction of the effects of noise and the development of a body of case history relating noise, expressed in an appropriate metric, to effect to be developed.
It is common that much higher levels of noise are measured underwater than is the case in air, as a result of the relative incompressibility and density of water. These high levels often cause concern, and there have been many explanations offered as to why they should not have an undue effect on marine animals. For instance, it is often pointed out that the sound energy carried by noise in water is much lower than would be the case in air; however animals do not possess receptors that are sensitive to energy, so this explanation is fallacious. The authors suggest that the true explanation lies in the relative insensitivity of marine animals to sound.
Noise causes both auditory and non-auditory effects. The non-auditory effects of noise may be obvious, for instance when underwater blast results in floating dead fish. Other injuries, such as swim bladder rupture in fish, may be observed by observation and dissection of exposed individuals. These effects only occur at high levels of sound, for instance typically within tens of, or at most a few hundred metres from underwater blast, and hence affect relatively small areas and numbers of individuals (Nedwell and Edwards 2004).
The auditory effects of sound comprise temporary or permanent noise induced deafness, and the behavioural effects of underwater noise, typified by a species avoiding an area of high noise. Both of these are poorly understood, yet behavioural effects may have an influence over great ranges, often kilometres or tens of kilometres, reaching very much larger numbers of individuals. High noise levels have been cited as having the potential to impede communication amongst groups of animals, to drive them away from feeding or breeding grounds, to cause strandings, or to deflect them from migration routes.
The authors suggest that behavioural effects are primarily caused by noise that is uncomfortable or painfully loud to an animal. In order to quantify the “loudness” of a sound to a marine animal, the sensitivity of that animal to the noise must be assessed. Therefore, the frequencies contained in a sound must be considered when judging its likely effect.
Consider the significance of noise which is ultrasonic or infrasonic to a species, i.e. respectively above and below the range of hearing for that species, and hence inaudible. For man, sound is ultrasonic above about 20 kHz, and infrasonic below about 10 Hz. Bats (the loudest terrestrial animal) and dog whistles have no effect on humans, who do not perceive the sound because it is outside the auditory range for a human. Sounds above 1 kHz, such as generated by many sonar systems, are ultrasonic for most fish, which mainly have a limited hearing range of up to a few hundred Hz. Many marine mammals cannot perceive sounds below 1 kHz or so, and hence much (although not all) of the energy of a seismic airgun may be infrasonic to them.
Current regulatory limits for underwater noise are often expressed as a simple unweighted peak pressure, RMS pressure or unweighted Sound Exposure Level. However, the preceding considerations indicate that in order to objectively investigate the auditory and behavioural effects of underwater noise any valid scale must judge the loudness of the noise by reference to the frequency range and sensitivity of the animal's hearing. These considerations indicate the importance of considering hearing ability when evaluating the effect of underwater noise on marine animals. A number of recent reviews have highlighted this current shortfall, and have indicated that as the auditory system is frequency dependent, noise levels should be weighted for the manner in which the sound will be perceived for the marine receptor, in a similar approach to the way that noise assessments are conducted for humans (see for example Madsen et al, 2006).
For a metric to be widely acceptable to industry, pressure groups, regulators and government, it must be simple to understand and use, and must also be pan-specific, i.e. able to deal with a wide range of marine animals with greatly different hearing abilities, and a wide range of source types. However, it does not matter if it is complex to implement; for instance, one does not have to know how a sound level meter works to use it to measure noise in air.
A validated metric has many benefits which include:
- objective, rather than emotional, debates on environmental issues connected with noise;
- real problems to be sorted out from “red herrings” and hence the best use made of available resources;
- the drafting of clear and simple legislative or regulatory guidance (“the sound shall not exceed 90 dBht at the stated range for any of the critical species...”);
- a framework within which models may be used to estimate any effect of noise prior to an offshore activity being undertaken, and measurements of noise made and interpreted;
- allowing technically competent but non acoustically expert personnel to understand and evaluate the effects of noise;
- the provision of simple instrumentation, such as a Species Sound Level Meter (SSLM), enabling non-expert personnel to make evaluations of noise and its effects.
A distinction should be drawn between loudness and perception. For the purposes of this report, perception is defined as an objective measurement of the amount a noise is above the threshold of hearing. An objective measurement can therefore be made of the perception of a noise, and the dBht metric is a measure of perception. Loudness, by contrast, is a subjective assessment of the apparent level of a noise to an animal. It is likely, however, that in most cases loudness and perception will be closely related.
For instance, the dB(A) scale is a measure of the perception of noise by humans. It is well known, however, that certain types of noise that are “unpleasant”, like tonal noise from ripsaws in woodyards, has the same effect as other noise 5 – 10 dB higher in level. That is, the apparent loudness, and hence the effects of the noise, is greater than the perception would imply. This difference is applied as a “correction factor” in human noise exposure, allowing the loudness, and hence the effects of the noise, to be judged by reference to the human dB(A) perception scale.