My hypothesis,

Our ear take in a lot of different sound everyday.
It is logical to assume there exists some form of pre-processing to filter out unwanted i.e. ambient/background sound in order to have greater clarity on the information that we actually need.

But when you entered a noise proof room, your normal noise filtering mechanism over compensated and resulted in the perception of noise instead. If this is true, you would need some time of staying and living in the same noiseless room until your system readjust itself.

I think TS meant the Sound Pressure. See here ->Examples of Sound Pressure Levels

dB always has a reference value, just often this reference level is implied and thus not stated.
In this case 0dB is the human auditory threshold in air.