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”.

Listen to different reverberation times.

Objective measure:

Wallace Clement Sabine was the first to quantify this subjective quality in the early 1900s. He developed the quantity Reverberation Time (RT), which is defined as the time it takes for sound energy to decay 60 decibels (dB). One can measure this in an existing room, by measuring the length of time for sound to decay 20 dB (from -5 dB down from the peak to -25 dB down) and multiplying that period by three. Or for prediction, one can use the following equation:

where V is volume in cubic meters, S is total surface area of the room in square meters, and α is the average absorption coefficient in the room.

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 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 live-ness to the music.

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