received field. Therefore, a gap in existing shallow-water noise databases is lack of knowledge of the ocean-bottom geoacoustic properties in the regions where the measurements were made. More generally, the quality of ambient noise databases is directly related to the quality and variety of ancillary information (e.g., near-surface winds, shipping traffic, visual observations of marine animals) collected simultaneously at the same location. Development of a long-term ocean noise monitoring system requires careful consideration of which types and in what ways this supporting information will be collected.
Sound sources in the ocean can be categorized and modeled as two main types: unknown distributed sources (that is, unknown location, source level, and spectral content) referred to as ambient noise and best modeled as statistical in nature, and identified single sources best modeled deterministically. Noise from the collection from all sources is referred to as “ocean noise” in this report. The dominant source of ambient noise is associated with ocean surface wave activity. In the frequency band from 5 to 200 Hz, shipping may be dominant, at least in the northern hemisphere. The time-averaged received levels of shipping noise in some locations can be fairly well modeled. Above 200 Hz, noise levels from breaking waves are roughly modeled through the use of empirical relations between noise level and wind speed. Limitations exist in ambient noise models not just from lack of knowledge of the source characteristics and distributions but also resulting from uncertainties in the environment. The sounds from single sources, such as sonar and air-guns, are usually well modeled by propagation codes. The accuracy of these models is limited by environmental uncertainty. The effects of sound from single sources on marine mammals are beginning to be modeled by integrative tools such as AIM and ESME. The effects of distributed sources, such as shipping and wind, on marine mammals are not yet well modeled.
From field observations and threshold experiments on captive animals (see Chapter 3), it is clear that sound can disturb marine mammals both behaviorally and physiologically. Noise from shipping may be affecting marine mammals adversely. Similarly, high-intensity transient sources at short ranges may have significant effects on marine mammal physiology or behavior. Modeling these effects is possible and prudent. While modeling the physiologic effects is relatively straightforward, modeling behavioral effects is difficult and needs more effort. In all cases, field data must be collected to validate the model predictions.