0 dB SPL (sound pressure level)

0 dB SPL (sound pressure level) is the standard reference for the threshold of human hearing in air. It corresponds to a root-mean-square (RMS) sound pressure of 20 µPa (micropascals), or 0.00002 Pa. This is not “zero sound,” but rather the quietest level that the average human ear can detect under ideal conditions. SPL is logarithmic, so every 20 dB increase represents a tenfold increase in pressure. SPL measurements above 0 dB reflect louder sounds; negative values are possible but indicate quieter-than-threshold levels.