The projector source level specifies the amount of sound radiated by the projector. It is defined as the intensity of the radiated sound at a distance of 1 meter from the source, where intensity is the amount of sound power transmitted through a unit area in a specified direction. The unit of intensity is watts per square meter. Source level is rarely given in watts per square meter, however. Source level is commonly given as a relative intensity in units named decibels (dB).
The relative source level in water is calculated as the ratio of the source intensity to the intensity of a sound wave with a pressure of 1 microPascal. Source levels given in dB in water are not directly comparable to source levels given in dB in air because a different reference pressure is used in air (See How does sound in air differ from sound in water?).
Although the source level of a sound source is defined at a range of 1 m from the source, it is normally measured at much larger ranges. The equivalent source level at 1 m is then calculated assuming spherical spreading.
The source level for large, low frequency sources must be measured far from the source. This is because the sound pressure levels close to the source form a complex pattern that is not the same as the source level calculated using measurements far from the source.
The source levels for some sound sources are given in What are common underwater sounds?
The projector source level can be used to calculate the total acoustic power transmitted by the source. A projector that transmits equally in all directions with a source level of 170.8 dB relative to 1 microPascal at 1 meter transmits 1 watt of acoustic power.
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