According to the scale on this page, 0 dB (aka 0 dB-SPL) is the sound level in a recording studio:
We said above that the decibel is a ratio. So, when it is used to give
the sound level for a single sound rather than a ratio, a reference
level must be chosen. For sound pressure level, the reference level
(for air) is usually chosen as pref = 20 micropascals ...