The Source Level of a noise source is defined as the "effective" level of sound at a nominal distance of one metre, expressed in dB re 1 µPa. However, the assumptions behind this simple definition warrant careful explanation.
As illustrated in the diagram above, in the near field (immediate vicinity) of a source of noise the sound level may be very complex, vary rapidly, and not be representative of the level of sound further from the source (the far field). Usually, a means of estimating the level at greater distances is needed. Hence it is normal to measure the sound pressure in the far field at sufficient distance from the transducer that the field has "settled down", and to use this pressure to estimate the apparent (or effective) level at a nominal one metre from the source. The apparent level may bear no relation to the actual level.
A measurement of the apparent level can be accomplished by assuming inverse dependence of pressure on the range from the noise source, or by extrapolating the far field pressure. For instance, if measurements were made in the range 100 metres to 10000 metres, the apparent level would, as illustrated by the extrapolation, be much higher than the actual level.
There is in general no reliable way of predicting the noise level from sources of man-made noise, and hence it is normal to directly measure the Source Level where a requirement exists to estimate far-field levels.
The figure to the right shows measurements of peak pressure levels from an airgun array. The lines in different colours represent measurements made on different days. The best fit of the data is given by:
The effective Source Level is therefore 262.9 dB re 1 µPa at 1 metre.
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