SPL = Sound Pressure Level (SPL)
P = sound pressure RMS
Pref = reference sound pressure
The reference sound pressure p(ref) in the air is generally taken as 2*10E-5 Pa. This value is the sound pressure value of the normal human ear to the presence of the 1 kHz sound, that is, the audible threshold of the 1 kHz sound. a.
Generally speaking, below this sound pressure value, the human ear can no longer detect the existence of this sound. It is obvious that the sound pressure level of the audible threshold sound pressure is zero decibels.