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:

• P is the measured root mean squared (rms) sound pressure
• P0 is the reference rms sound pressure (20µPa)

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: