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Underwater sound reference detachment12/27/2022 ![]() ![]() Sonars used for civilian navigation and defense purposes use sound waves to locate objects under the sea surface. Marine researchers use sound waves to investigate the properties of seawater both for local and global studies. In the search for new hydrocarbon reserves, the rock underlying the seafloor is characterized using air-guns. Sound is a widely used tool for a broad range of marine activities. Whether intentional or unintentional, anthropogenic noise in the marine environment is an important component of ocean noise. Fish can add to ocean noise in some locales. Snapping shrimp are an important component of natural noise from a few kilohertz to above 100 kHz close to reefs and in rocky bottom regions in warm shallow waters. Groups of whistling and echolocating dolphins can raise the local noise level at the frequencies of their signals. For example, a peak around 20 Hz created by calls of large baleen whales is often present in deep-ocean noise spectra. Natural biological sound sources make a noticeable contribution at certain times of year. Nonlinear interactions between ocean surface waves called microseisms (see the Glossary referred to as “Surface Waves-Second-Order Pressure Effects” in Plates 1 and 2) are the dominant contributors below 5 Hz, while thermal noise (i.e., the pressure fluctuations associated with the thermal agitation of the ocean medium itself) is the dominant contributor above 100 kHz. The dominant source of naturally occurring noise across the frequencies from 1 Hz to 100 kHz is associated with ocean surface waves generated by the wind acting on the sea surface. In the presence of distant shipping, contributions from natural sources continue to dominate time-averaged ocean noise spectra below 5 Hz and from a few hundred hertz to 200 kHz. In the absence of shipping, natural forces are the dominant sources of the long-term time-averaged ocean noise at all frequencies. The distinction between source level and received level also is discussed both in Chapter 1 and in the Glossary. ![]() The text clearly differentiates between the properties of the sources and those of the received field. Therefore, in such situations, the measured properties of the received acoustic field (which are obtained directly and require no additional information, computation, or assumptions, but which contain the effects of propagation) will be presented. However, accurate estimation of the source properties for many types of naturally occurring sounds is impossible, given the lack of knowledge of the individual source locations, of the spatial distribution of multiple contributing sources, and of the complex propagation conditions. Parameters such as source level (in units of dB re 1 µPa at 1 m), source spectral density level (units of dB re 1 µPa 2 per Hz at 1 m), and time-integrated source pressure amplitude squared for use with transient signals (units of dB re 1 µPa 2 at 1 m) are presented for many of these sources, particularly man-made sources. This chapter focuses on the properties of the sources and does not describe in detail the effects on the environment as the acoustic energy travels away from the vicinity of the sources. A more thorough description of modeling efforts is contained in Chapter 4. Gaps in our knowledge or available data are identified that will need to be addressed in future research in order to develop predictive models of the effects of noise on marine mammals. In this chapter the major natural (physical and biological) and anthropogenic contributors to ocean noise are discussed. ![]()
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