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

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Report No. 6945 BBN Systems and Technologies Corporation 6.2 Recommendations The modeling procedure developed in the study provides a means of ranking source - species encounter situations using acoustic principles. While the principles employed have been used in similar ways to predict human annoyance by industrial noise, their application here to marine mammals has involved the use of several untested assumptions. Moreover, it has been necessary to use estimated and inferred values for many of the required model inputs where 'measured data are not presently available. When appropriate data become available the procedures used in this study should be augmented and modified where required. While humans have been found to respond as energy detectors with a fundamental stimulus integration time roughly equivalent to 8 hours, the hypothesis built into the SER model of a 2 hour integration time for marine mammals should be tested. This concept is useful for comparing different types of sources on an energy equivalent basis, but other procedures can be devised if it is found not to be appropriate for marine mammal psychoacoustics. Possible testing procedures could be devised which will allow the stimulus integration concept to be tested concurrently with testing for adaptation using repeated controlled noise exposures. Appropriate weighting factors should be investigated for use in the SER modeling procedure which provide for the apparent increase in sensitivity of certain marine mammals during special situations. This increase in sensitivity occurs for the sudden onset of a new sound (startle effect), for a sound that is increasing in level (indicating approach), and for sounds indicating a known threat. The use of weighting factors in human response modeling has been found to provide the flexibility needed to accommodate the effects of special stimuli. The accuracy and utility of the modeling procedures developed in this study need testing with field data. Ideally this testing would initially employ benchmark acoustic and biological data obtained from an area prior to the onset of development. This would be followed up using data obtained during the course of increasing industrial activity. The models would be run and the results compared with observations of mammal reactions in the area as the acoustic environment changed. The goal of this procedure is the refinement of the present preliminary and largely untested models into a marine mammal acoustic response model which would predict potentially significant acoustic impact situations during the course of environmental impact statement research and thereby allow time for assesment of the problem and determination of mitigation procedures.
~epbrt No. 6945 BBN Systems and Technologies Corporation GLOSSARY GENERAL ACOUSTIC TERMINOLOCY 1/3 Octave Band Filter A bandpass filter having a bandwidth equal to 23% of the center frequency. Absorption Loss, A, The reduction in sound level caused by volumetric absorption of sound energy by the transmission medium. Acoustic Normal Mode Theory A solution to the acoustic wave equation which considers sound propagation as a series of acoustic standing waves (normal modes) which match the boundary and source conditions specified. The pressure contributions from'a series of modes are added to give the total acoustic pressure at a selected observation point (similar to room acoustic theory); useful for shallow water and low frequencies. Acoustic Ray Theory A solution to the acoustic wave equation which considers sound propagating as uniform phase wavefronts along a path (ray) determined by the initial radiation direction from the source and the refractive properties of the medium; (similar to optical theory for light) useful for deep water and high frequencies. Critical Angle The reflection loss is 0 for grazing angles less than the critical angle. Equivalent Sound Level, Leq The constant sound level which produces the same acoustic exposure dose as the actual time-varying sound field. Exposure Period A reference period of time for calculating a behavioral response measure such as the equivalent sound level. This period should be related to the activity cycle of a specific species (i..e., 8 hours for humans). Grazing Angle The angle between the sound propagation direction and a reflecting surface.
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FIG. 3.2 NOISE SPECTRA IN SHALLOW W
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FIG. D. 1 HUMAN VOCALIZATION AND AU
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