Sound pressure is the dynamic variation of the static pressure of air and is measured in force per unit area (Pa). The instantaneous sound pressure is typically averaged over a certain duration to give sound pressure level. Sound pressure level is usually represented on a logarithmic amplitude scale, which is similar to the human perception of hearing. Typical values on this logarithmic scale are a sound level of 0 dB, which is the average threshold of human hearing, 60 to 70 dB for normal conversation, 110 dB at an extremely loud concert, and 150 dB for the noise of a rocket takeoff or a jet engine at close range.
The Sound Pressure Level (SPL or LP) in decibels is defined by the following equation.
SPL = 20 log10 (p/pref)
where p is the instantaneous sound pressure in Pa and pref is 20 µPa, the internationally accepted reference for sound pressure measurements, which roughly corresponds to the threshold of human hearing.
You use a microphone to measure sound pressure. The microphone acts as a transducer, creating a voltage signal that is proportional to the instantaneous sound pressure.