Subjective quality:
The most commonly perceived quality of room acoustics is how sound energy decays in a space, referred to as the room's reverberation. In a large gothic cathedral, the sound takes a long time to die away, sometimes up to 9 seconds! In a small conference room, however, the sound does not linger but decays very rapidly. Longer reverberation generally accentuates music, but can cause speech to be muddled. For concert halls, musicians will often refer to a reverberant space as being very "live". If there is not very much sound reverberation, a hall may be referred to as being "dead".
Click button to listen to different reverberation times.
Objective measure:
where V is volume in cubic meters, S is total surface area of the room
in square meters, and 
is
the average absorption coefficient in the room.
Click button to calculate a room's reverberation time.
Optimum values:
Optimum reverberation times for concert halls depend on the type of music for which the hall is being designed. Generally, good concert halls have a reverberation time between 1.8 and 2.2 seconds at mid-frequencies.
Click button for RT's of famous concert halls.
How to design:
To increase reverberation time, one could increase the volume of the room, or reduce the amount of absorption in the room.
Other notable info:
More recently, acousticians have studied early and late portions of the sound decay separately. The early part of the sound decay seems to determine better how an audience perceives music. An objective measure for this is called the Early Decay Time (EDT). EDT is calculated by measuring the amount of time it takes sound energy to decay the first 10 dB, and multiplying that by six. In concert halls, it is desirable to have a shorter EDT to improve clarity, but a longer RT to provide liveness to the music.
discussion|reverberation|clarity|intimacy|warmth&brilliance