Measuring Sound Pressure

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 normally 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 as the following:

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 can 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.

Some analog input devices support programmable AC or DC coupling. If your device supports this feature, your sound pressure measurement task defaults to AC coupling. If you want a different coupling behavior, you can change this property programmatically.