Reverberation Time

Sound is reflected from surface to surface with energy being absorbed on each reflection.

A room with hard surfaces is said to be 'Live' whilst one with lots of soft surfaces is 'Dead'.

A reduction to one millionth =
a 60db reduction (6 zeros).

Hence the use of the term RT60 when describing reverberation time. The time for the sound pressure to reduce to one millionth of the original sound pressure.