ing the effects from single sources are frequency, source level, pattern of amplitude versus time (time series), directionality of radiation or beam pattern, and distance from the source. Effects from multiple unidentified sources are primarily characterized by frequency, directionality, and level at the receiver. To underwater acousticians, the term “ambient noise” refers to the second type of noise from multiple and unidentifiable sources as stated in Chapter 1.
Models are used to assess the interactions of sound fields created by multiple sources, propagation through space and time, and interactions with marine mammals. The term “models” refers to a variety of tools, including empirical fits to measured data, such as the Wenz curves, computer simulation models, and numerical models, which can be either physics or empirical based. Physics models rely on known relations such as those expressed in Equations 1-1 to 1-5. Empirical models are based on observed data rather than underlying physics. In many cases the dominant mechanisms of natural sources of ocean ambient noise, for example, those associated with wind-generated noise, have not yet been conclusively identified. Therefore, physics-based approaches that incorporate actual source mechanisms are still in their infancy in underwater acoustics. In contrast, empirical models such as the Knudsen curves (Knudsen et al., 1948) and the Wenz curves (Wenz, 1962) have been extremely successful; they remain the basis of standardized noise spectra used by the U.S. and British navies.
The first part of this chapter describes current acoustic models and efforts to model underwater noise effects on marine mammals. Gaps that must be filled to model the effects of noise on marine mammals are identified in modeling efforts and current databases.
Some ocean noise can be traced to a single identifiable source. High-quality models exist to predict the time series of the received signal from a source of specified directivity and given transmitted signal time series. Propagation models utilize bathymetric databases, geoacoustic information, oceanographic parameters, and boundary roughness models to produce estimates of the acoustic field at any point far from the source (see Glossary for definitions). The quality of the estimate is directly related to the quality of the environmental information used in the model. For example, in continental shelf waters, geoacoustic parameters such as compressional sound speed, attenuation, and sediment density can significantly affect the acoustic propagation. Variability introduced in these parameters can substantially affect model predictions; propagation loss can be incorrect by as much as 20 dB as a result of inaccurate geoacoustic parameters.
There are four main categories of acoustic propagation models prima-