In the absence of shipping, natural forces are the dominant sources of the long-term time-averaged ocean noise at all frequencies. 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. 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. 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. Natural biological sound sources make a noticeable contribution at certain times of year. For example, a peak around 20 Hz created by calls of large baleen whales is often present in deep-ocean noise spectra. Groups of whistling and echolocating dolphins can raise the local noise level at the frequencies of their signals. 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. Fish can add to ocean noise in some locales.
Whether intentional or unintentional, anthropogenic noise in the marine environment is an important component of ocean noise. Sound is a widely used tool for a broad range of marine activities. In the search for new hydrocarbon reserves, the rock underlying the seafloor is characterized using air-guns. Marine researchers use sound waves to investigate the properties of seawater both for local and global studies. Sonars used for civilian navigation and defense purposes use sound waves to locate objects under the sea surface. Unintentional contributions to marine noise arise from transiting ships, coastal and marine construction activity, mineral extraction, and aircraft overflights. These anthropogenic sound sources contribute to ocean noise over the complete 1-Hz to 200-kHz band of interest in this report. In the lowest bands, 1-10 Hz, the contributors are ship propellers, explosives, seismic sources, and aircraft sonic booms. In the 10-100 Hz band, shipping, explosives, seismic surveying sources, aircraft sonic booms, construction and industrial activities, and naval surveillance sonars are the major contributors. For the 100-1,000 Hz band, all the sources noted for the 10-100 Hz band still contribute. Also, the noise from nearby ships and seismic air-guns can extend up into the 1,000-10,000 Hz band. This band also includes underwater communication, naval tactical sonars, seafloor profilers, and depth sounders. The 10,000-100,000 Hz band includes the systems listed, in addition to mine-hunting sonars, fish finders, and some oceanographic systems (e.g., acoustic Doppler current profilers). Anthropogenic contributors at and above 100,000 Hz are limited to mine hunting, fish finders, high-resolution seafloor mapping devices