Sound pressure level is the logarithmic measure of the root mean squared (RMS) sound pressure of a sound relative to a reference value.
A simple sound wave may be represented by a sine wave. A diagram of a sine wave which would be the typical output signal of a single tone noise level (i.e. a calibrator tone), if shown on an oscilloscope. The fundamental parameters associated with a sound wave are Peak, Peak-Peak, Periodic Time, RMS (root-mean-square). Sound level meters measure acoustic pressure and by international agreement they are calibrated in decibels (dB). The sound pressure level (Lp) in decibels is defined as: Where:
Note that the decibel is a ratio of two quantities which have dimensions of power and is not a unit.
The reference sound pressure (P0) is by agreement as 20µPa, which is the minimum audible pressure to a person with ‘normal’ hearing. The use of a logarithmic scale, such as the decibel, permits the wide range of audible sound pressures (approximately 1,000,000 to 1) to be compressed into a scale of 120 units. Hence a faint whisper may be measured as 20dBA where as a chipping hammer of a road workman may produce 105dBA.
Addition of Lp values can be accomplished by taking an anti-log of each Lp value to give a total Lp value, as shown in the equation: