9.5.2.1 Lethality 52.
Very close to the source, the high peak pressure sound levels have the potential to cause death, or severe injury leading to death. Some of these effects may be considered to be barometric pressure effects due to the shock experienced by the animal, rather than acoustic effects per se. There has been considerable research into the levels of incident peak pressure that cause lethal injury in species of fish and human divers. The work of Yelverton et al. (1973; 1975 and 1976) on fish highlighted that for a given pressure wave the severity of the injury and likelihood of a lethal effect is related to the duration of the pressure wave (i.e., a pulse of the same peak pressure but with a longer duration would be more likely to cause injury). In the Yelverton model, smaller fish are generally more vulnerable than larger ones. Richardson et al. (1995) converted Yelverton’s expressions for fish mortality into examples representative of larger marine mammals.
9.5.2.2 Injury and Hearing Impairment 53. High exposure levels from underwater sound sources can also cause hearing impairment. This can take the form of a temporary loss in hearing sensitivity, known as a TTS, or a permanent loss of hearing sensitivity known as a Permanent Threshold Shift (PTS). The potential for injury is not just related to the level of the underwater sound and its frequency relative to the hearing bandwidth of the animal, but is also influenced by the duration of exposure. For example, for two separate piling events where the total energy expended inserting the pile is the same, but one with a lower blow energy but a higher number of strikes and one with a higher blow energy and fewer hammer strikes, the overall noise exposure of the animal would be expected to be the same assuming that the animal does not move and that the sound energy in each sound pulse is linearly proportional to the hammer energy. However, if the animal were to flee the sound at its onset, then the lower blow energy example would be expected to result in an overall lower level exposure to the sound and thus reduce the likelihood of TTS or PTS, although the potential extent of hearing damage will further depend on any hearing recovery that may occur between individual exposures (inter-pulse recovery, for example, when considering piling). Importantly, when considering the impact that any auditory injury has on the survivability of the animal, one must also consider the frequency range over which the auditory injury occurs. TTS (and likely PTS) would normally only be expected in the critical hearing bands in and around the critical band of the fatiguing sound (e.g. Kastelein et al. 2012; 2013a). Auditory injury resulting from sound sources like piling, where most of the energy occurs at lower frequencies, is unlikely to negatively affect the ability of high-frequency cetaceans to communicate or echo-locate. Auditory injury is considered further in Section 9.6.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Appendix 9.1 Underwater Noise Modelling 17
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