From Dick Hazelwood and Andrew Ward
I note with interest the compromise plan for the ATOC acoustic ocean temperature measurements (This Week, 17 June). These are vital for the assessment of global warming, given the huge heat capacity of the sea.
It would help to dispel some common misunderstandings if the strength of the signal were better described. Many readers will be familiar with the sound pressure levels used for airborne noise, measured in dB(A). A noise limit of 90 dB(A) Leq (a time-averaged intensity) is used in factories. They may thus conclude that the “195-decibel booms” to which the article refers exceed this limit by 105 dB, a power ratio of over 30 billion. The poor whales!
The reality is very different. The 195 decibels quoted in the story is actually a source level. It describes the power of the source, not the intensity at the receiver.
Such an omnidirectional source in the sea is more simply described as a 263-watt projector. Whilst the sound intensity close to it will be large, it falls quickly as the sound spreads into larger volumes. This source power is minute compared with many other regular sources of noise, both artificial and natural.
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It has proved very difficult for acoustic scientists to convey to the public and even other scientists the relative sizes of underwater noise sources. I hope you will provide reference levels for any future data quoted in decibels.
Compared with the ATOC bangs, for example, seismic air guns really boom, with a peak power of 330 kilowatts, repeated every few minutes all day long. Nevertheless, whales have been observed following seismic survey ships voluntarily.
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A loud bang is particularly disturbing to any animal if it is unexpected. The effect on marine life of the proposed 195-decibel booms could be both reduced and monitored by preceding them with lower-intensity warning signals.
If the main bangs are indeed harmful, then the animals may be able to limit their exposure (can whales shut their sonar receivers?), and are likely to give an observable conditioned response to the warning signal.
