Sound Pressure Level / SPL
Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average, or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit for sound pressure p is the pascal (symbol: Pa).
Sound pressure level (SPL) or sound level is a logarithmic measure of the effective sound pressure of a sound relative to a reference value. It is measured in decibels (dB) above a standard reference level. The standard reference sound pressure in air or other gases is 20 µPa, which is usually considered the threshold of human hearing (at 1 kHz).
Distance law
When measuring the sound created by an object, it is important to measure the distance from the object as well, since the sound pressure decreases with distance from a point source with a 1/r relationship (and not 1/r2, like sound intensity):.[9]
The distance law for the sound pressure p in 3D is inversely proportional to the distance r of a punctual sound source.[citation needed]
p \propto \dfrac{1}{r} \,
If sound pressure p_1\,, is measured at a distance r_1\,, one can calculate the sound pressure p_2\, at another position r_2\,,
\frac{p_2} {p_1} = \frac{r_1}{r_2} \,
p_2 = p_{1} \cdot \dfrac{r_1}{r_2} \,
The sound pressure may vary in direction from the source, as well, so measurements at different angles may be necessary, depending on the situation.[citation needed] An obvious example of a source that varies in level in different directions is a bullhorn.