It is an acoustical property of most internal spaces which can be both predicted and measured. It represents the time in seconds for reflected sound to decay and become inaudible after a loud sound, such as a hand clap, has ceased.

Reverberation time is an indicator of the sound quality within a space for both speech and music. Shorter RTs (<1.0 sec) are preferred for good intelligibility in teaching, conference and meeting spaces to ensure optimum speech communication. However for music making and listening in halls and auditoria, longer RTs (>1.5 secs) are desirable.

In several European countries, legislation acknowledges the need for optimum acoustic conditions in public spaces (particularly in education and health) and provides guidance and criteria for acceptable RT values.

The RT of a space is primarily determined by its volume and how much sound absorbing material is present. Larger volumes result in longer RTs whereas increased amounts of sound absorption result in shorter RTs as shown by the relationship: RT = Volume/Absorption x Constant.

For example, a small classroom with a suspended acoustic ceiling and carpet will have a short RT, whereas a large multi-purpose room with a drywall ceiling and hardwood floor will have a long RT.