Sources of Human-Generated Ocean Noise
Commercial, scientific, military and recreational marine activities can all generate noise sufficient to impact marine life.
Explosives are detonated in the ocean by the military, scientific researchers, and the oil and gas industry for demolition purposes, seismic exploration, or testing of equipment - such as shipshock trials, whereby ships are deliberately struck with explosives to test their durability. Explosions are created by chemical devices; they cause extremely high noise levels in the wideband frequency range and are characterized by rapid rise times - a sudden burst which can permanently damage hearing structures.
Seismic airgun arrays are used primarily for oil and gas exploration and research purposes. The airguns produce sound by introducing air into the water at high pressure, usually directed toward the sea floor, with up to 20 guns being fired in synchrony, while "streamers" of hydrophones listen for echoes. Seismic surveys with airguns can last for many weeks at a time. During the surveys, every airgun in the array produces a pulse of noise lasting 20 to 30 milliseconds which is repeated every 10 seconds, often for 24 hours a day.
Active sonar is used by military vessels during exercises and routine activities to hunt for objects in the path of the vessel. These Mid-Frequency Active (MFA) and Low-Frequency Active (LFA) sonar systems usually emit 100-second-long "pulses" of sound that can be deployed for hours and are designed to focus as much energy as possible in narrow ranges in a horizontal direction. LFA sonar is a type of long-range surveillance sonar that saturates thousands of cubic miles of ocean with sound. Frequencies commonly used by sonar systems range from around 0.1 to 10 kHz, with source levels in excess of 230 decibels.
Ships produce noise that generally falls in the low frequency band, between 10 Hz and 1 kHz - capable of propagation over immense distances in all directions. These low frequencies coincide with the frequencies used, in particular, by baleen whales, fish, seals, sea lions and dolphins for communication and other biologically important activities. Ships generate sound primarily by propeller action, hull-mounted machinery, and hydrodynamic flow over the hull and the flexing of the hull. Over 90 percent of world trade is transported by ship, effectively producing an ever-present and rising aural "fog" that masks crucial natural sounds and is the most pervasive source of ocean noise today. In general, noise increases with vessel speed.