Professional Definition

Reference sound pressure equal to 20 µPa ($2 \cdot 10^{-5}$ Pa) is a normalized threshold value, accepted as the smallest sound pressure that produces a hearing sensation in an average, healthy person for a frequency of 1000 Hz.

This value is denoted by the symbol $p_0$ and serves as the denominator in the logarithmic formula for sound pressure level ($L_p$). It is precisely at a pressure of 20 µPa that the loudness level is exactly 0 dB. This is the international calibration standard for sound level meters and audio equipment in air.

Acoustics in Simple Words

This can be compared to sea level in geography. When we say a mountain is 2000 meters high, we measure it from sea level, not from the center of the Earth. In acoustics, our 'sea level' is indeed 20 µPa. This is the faintest sound that a young, healthy person can hear. Everything louder (having a higher pressure than 20 µPa) has a positive decibel value, while quieter sounds (theoretically possible but inaudible to the ear) would have a negative value.

Summary

A pressure of 20 µPa is the acoustic zero point for human hearing in air. It serves as a base for calculating sound intensity levels in decibels, defining the threshold of hearing as 0 dB SPL.