Science of Sound in the Sea
Advanced Topic
Introduction to Decibels
What is a dB?
The intensity of a sound wave is the average amount of energy transmitted per unit time through a unit area in a specified direction. The amount of energy per unit time is power, and intensity is therefore the amount of power transmitted through a unit area in a specified direction. Power is measured in watts, and intensity is therefore measured in watts per square meter. Scientists often specify sound intensity as a ratio, however. They have given the name "Bel" to the logarithm of the ratio of a sound's intensity to a reference intensity. (The name Bel is in honor of Alexander Graham Bell, the inventor of the telephone.)
The human ear is very sensitive. Humans can detect changes in relative intensity of as little as 1/10 of a Bel. A decibel is 1/10 of a Bel. For that reason, relative sound intensities are often reported in decibels (written as dB). The decibel is a relative unit, not an absolute one.
The relative intensity, I, in decibels is calculated as the ratio of the intensity of a sound wave to a reference intensity:
Acoustic intensity is rarely measured directly, however. Underwater microphones, called hydrophones, measure the pressure (amplitude) of a sound wave rather than its intensity. Because the intensity of a sound wave is proportional to the square of its pressure p:
("ρ" is the density of medium carrying the sound and c is the speed of sound), the intensity in dB can be computed directly from the measured pressure:
To be able to compare relative intensities given in dB to one another, a standard reference intensity or reference pressure must always be used. It is therefore essential that sound levels expressed in decibels include the reference pressure. Scientists have agreed to use 1 microPascal (µPa) as the reference pressure for underwater sound. In air, however, scientists have agreed to use a higher reference pressure of 20 microPascals. Sound intensity given in dB in water is therefore not directly comparable to sound intensity given in dB in air. To make it clear for the reader, this website will use "underwater dB" for underwater sounds.
The logarithmic nature of the dB scale means that each 10 dB increase is a ten-fold increase in acoustic power. A 20-dB increase is then a 100-fold increase in power, and a 30-dB increase is a 1000-fold increase in power. A ten-fold increase in acoustic power does not mean that the sound is perceived as being ten times louder, however. Humans perceive a 10 dB increase in sound intensity as only a doubling of sound loudness, and a 10 dB decrease in sound intensity as a halving of sound loudness.
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