Amplitude measurement
Peak, peak-to-peak, RMS (root-mean-squared)
Sound pressure is measured in decibels (dB) on a log10 scale relative to a reference level
dB = 20 log10 P1/Pr where Pr is a reference pressure level, usually the threshold of human hearing at 4 kHz. This is referred to as sound pressure level (SPL)
A sound with twice the SPL is 6 dB louder