Underwater Acoustics

[Noise-Contaminated Sea and Ocean]

ㅇ 원인: anthropogenic noise (인간이 만들어 내는 잡음)

- Military sonars

- Ship propellers

- Oil & gas exploration devices

ㅇ 해양동물(marine animals)의 음파사용: 항행, 먹이탐지, 짝 찾기, 포식자 회피, 상호통신

1 μPa = -26 dB SPL

[Underwater Exploration]

Seismic airguns are primarily used for oil and gas exploration on the seabed. Air is driven into the water and towards the seabed at high pressure. The sound can penetrate thousands of metres of ocean before heading up to hundreds of kilometres into the earth crust. Up to 20 guns are fired at the same time, with each of them emitting sound every ten seconds, often for 24 hours per day and for several weeks on end in the same spot. Hydrophones are used to listen and chart the echoes.

[Sonar]

Active sonar is used by military vessels during exercises and routine deployments to search for objects such as hostile submarines. These mid- and low-frequency sonar systems emit pulses of sound for over 100 seconds at a time for hours on end. These pulses are emitted with as much energy and in as narrow a range as possible. Low-frequency sonar serves as a way of putting large areas under surveillance and saturates thousands of cubic kilometres of water with sound. Military sonar uses frequencies between 0.1 and 10 kHz and can reach up to 230 decibels. That is equivalent to the sound generated by a space rocket launch.

[Shipping Traffic]

Ships tend to produce low-frequency sound between 10 Hz and 1 kHz that can spread over huge distances. Noise of this frequency interferes with the sounds of whales, dolphins, seals, fishes and other marine animals. More than 90 per cent of the global transportation of goods is made with ships. These vessels are generating an ever-present and constantly rising acoustic “fog” that masks natural sounds and is the most common source of ocean noise along with seismic airguns.