What is Reverberation Time?
It's an acoustical property of all architectural spaces. It represents the time (seconds) that it takes for sound reflections within a space to become inaudible after a loud sound, such as a handclap, was made.

Why is this important?
It's an indicator of the sound quality within a space for both speech and music. Short RT's (< 1 sec) are preferred for high quality intelligibility in classrooms and other instructional spaces, whereas long RT's (> 1.5 sec) are preferred for music listening in theaters, auditoriums, etc. There is an ANSI standard that addresses the need to provide good listening environments within classrooms and other learning spaces, and criteria are given for the maximum acceptable RT for these spaces.

What is the relationship between architectural design and reverberation time?
RT = Constant (Volume/Absorption)
The RT is proportional to the volume of the space, and inversely proportional to the amount of sound absorbing material within the space. For instance, a small classroom with a suspended acoustic ceiling and carpet will have a short RT, whereas a large multi-purpose room with drywall ceiling and hardwood floor will have a long RT.