Analysis and Ranking of the Acoustic Disturbance Potential of ...

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Report No. 6945 BBN Systems and Technologies Corporation same band (Greene 1985, 1987b). However, playback experiments using recorded sound from that same drillship showed that some bowheads initiate weak avoidance reactions when exposed to drillship sounds no stronger than those tolerated by the bowheads observed several kilometers from actual drillships. Taken together, results of drillship and dredge playback tests indicated that most bowheads do not react overtly unless the received noise levels are about 110-120 dB, or 20-30 dB above ambient levels in the corresponding band and 20- 30 dB above the assumed threshold of hearing sensitivity (Miles et al. 1987; Richardson et al. MS). Thus, the radius of responsiveness around a drillsite is apparently considerably smaller than the radius of potential audibility. Recently, migrating bowheads were monitored as they passed an operating drillship in the Alaskan Beaufort Sea. There was clear evidence that the whales avoided the area within 10 km of the ships, and some reactions were evident at greater ranges (Koski and Johnson 1987). The average level of broadband industrial noise 10 km from the drillship was 114 dB (Greene 1987a). Reactions of migrating gray whales have been studied when the whales were exposed to underwater playbacks of drillship, semisubmersible, drilling platform, and production platform sounds (Malme et al. 1983, 1984). Avoidance reactions to all of these sounds were noticed. Received sound levels at which 50% of the whales exhibited avoidance ranged from 117 to 123 dB, depending on the type of noise. These sound levels corresponded to the received levels that one would expect to find 1100 m from the actual drillship if it were operating off the California coast, and 4-20 rn from the other three sources (Malme et al, 1984). Larger radii of influence would be predicted if the same noise sources were operating in the Bering or Beaufort Sea, where sound attenuation rates are lower (Miles et al. 1987). In summary, cetaceans exhibit avoidance reactions and other behavioral effects when exposed to moderately intense levels of drilling or production sounds. Whales seem most sensitive when the sound level is increasing or when a noise source first starts up. The limited available data suggest that stationary industrial activities producing continuous noise result in less dramatic reactions by cetaceans than do moving sound sources, particularly ships. There are indications that cetaceans may partially habituate to continuous noise. At least in the case of white whales, habituation may result in greatly reduced sensitivity. Cetaceans are often observed close enough to drillsites to be within the zone where they are expected to be able to hear industrial sounds emanatfng from those sites. Thus, the radius of avoidance by cetaceans appears to be considerably smaller than the radius-of audibility. Virtually no information is available about the reactions of pinnipeds to drilling or production operations.
Report No. 6945 BBN Systems and Technologies Corporation 3. NOISE SOURCE CHARACTERISTICS AND TYPES This section contains a discussion of the various sources of sound in the Alaskan coastal environment. Procedures for describing the properties of sound are presented. Representative examples are given which show the characteristics of ambient noise and man-made sound sources. 3.1 Noise Source Descriptors Noise has been described as unwanted sound. This subjective definition is appropriate since sound that may be disturbing to some listeners may contain useful information for others - a rock music concert being one example. The procedures outlined here for describing sound energy are therefore intended to provide physical measures which can be used to classify sound sources without requiring consideration of their potential effects on listeners. The issues of annoyance and disturbance are addressed separately in Section 5. Two major categories of descriptive parameters are considered - sound level spectra and temporal statistics. 3.1.1 Sound level spectra The mammalian heacing process is capable of working over a very wide range of sound intensities and frequencies. Studies of the hearing processes of humans and of a limited number of other species, including some marine mammals, have showrthat this wide range capability is obtained by having a logarithmic hearing sensitivity characteristic; i.e., the sensation of loudness has been found to increase as the logarithm of the sound pressure. Also, humans and several other species have a proportional bandwidth hearing selectivity; i.e., the selectivity of the hearing process becomes broader in the high frequency portion of the hearing range. The logarithmic hearing sensitivity characteristic has resulted in the decibel scale of measuring sound intensity with a reference level (for airborne sound) set at the average threshold of (young) human hearing. Since sound intensity is proportional to the sound pressure squared, this results in the following definition of sound pressure level: SPL = 10 dB or SPL = 20 L0g(P/Pref) dB where, for airborne sounds, 'ref = 20 pPascal (20 pNewton/meter 2 ) and Log = LoglO For underwater sound 1 pPa is used as the reference pressure to obtain a more convenient physical scale. Underwater sound levels using this reference will be specified using Lr or Ls rather than SPL to avoid confusion.
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