Defining Areas of Potential Sound Effects
For activities that produce sound above these acoustic thresholds, it will be necessary to evaluate sound propagation from the source and estimate the area(s) within which sound levels are above the acoustic threshold(s). Propagation of sound in the sea is a complex science. Transmission loss is highly variable in nearshore environments, and hydroacoustic data are needed to accurately estimate spreading and attenuation loss. Spreading loss represents a regular weakening of sound as it spreads from the source, and can be expressed as dB loss per doubling of distance. Spreading loss is a geometric effect that is either spherical or cylindrical. Attenuation loss includes the effects of absorption and scattering, among other effects.
The Northwest Region encourages collection of acoustic data to inform transmission loss estimates, and review of previous sound propagation studies in the area that may be applicable to the project site. Knowledge of the background sound in the sea is also important to evaluate whether a sound source is audible over the background level. Through consultation with NOAA Fisheries staff, the 120 dB rms threshold may be adjusted if background sound is at or above this level. In the absence of background sound data, Fisheries acoustic effect thresholds should be used to define areas of potential sound effects. The Northwest Region and Northwest Fisheries Science Center developed the following documents for use in marine mammal ESA consultations and MMPA permit applications.