Sounding out the secrets of the sea
Box 2 | Measurement of sound levels
The level of noise energy in the atmosphere is called the sound pressure level, measured in decibels (dB) above a reference level or 20 micropascals, which is about the threshold of human hearing. Every rise of 10 dB increases the sound power by a factor 10. More subtle measurements, given as dB(A), allow for the variation of human hearing ability with frequency. Noise in a quiet suburb at night is about 30 dB(A), noise beside a busy street in the city about 80 dB(A), and a rock concert about 110 dB(A). Above about 120 to 130 dB(A) the listener experiences severe discomfort. Levels above about 85 dB(A) can damage hearing if maintained continuously, and higher levels can do the same in a much shorter time.
Sound is measured in decibels under the sea too, but the reference level is one micropascal, rather than 20. When the sound pressure is converted to sound power there are other ‘adjustments’ to be made, because water is 1000 times as dense as air and the speed of sound in water is about five times greater than in air. Sound levels quoted for the ocean are generally about 60 dB higher than those in air for the same actual sound power.
Boxes
Box 1. Comparison of the properties of sound in air and water
Box 3. Use of sonar in the sea
Box 4. Disturbing beaching events
Related sites
Sounds in the sea: How does sound in air differ from sound in water? (University of Rhode Island, USA)
How loud is loud? (Cornell Laboratory of Ornithology, USA)
Posted August 2007.