About Reverberation Time
Sound Level Versus Reverberation Time
Sound level is the most important parameter to measure when solving environmental or occupational noise problems. In the open air (or the anechoic room) it is often all you need to measure. But indoors, the Reverberation Time (RT) affects not only whether you can understand what others say or enjoy music, but also the level and distribution of sound. The problem is usually that the RT is too long, but it may also be too short or not properly balanced over the frequency spectrum.
Getting the RT Right
Reverberation time that is too long will muffle speech sounds so that one sound (or word) cannot be distinguished from another, making it impossible to understand what is being said. This is critical in classrooms, auditoriums, churches, theatres and airport buildings. On the other hand, reverberation time that is too short will make levels too low at a distance, and make the sound too "dry". The optimal RT for music depends on the type of music and the volume of the room. Church music requires 2-4 seconds while concert studios need 1-2 seconds. The RT frequency spectrum should be reasonably flat and even. Excessive RT will cause the sound level to rise, causing annoyance or risk of impaired hearing. Typical examples are concrete stairwells in apartment blocks or workplaces with hard reverberant walls.
RT in Practice
Reverberation time is measured using either interrupted noise (pink or white) from a loudspeaker source, or impulsive noise from a starting pistol. It is measured in 1/1- or 1/3-octaves, serially or simultaneously. It is usually averaged over several positions in the room and over several decays in each position. Quite often a wide-band average is calculated by mathematically averaging the RT for a range of frequency bands. For critical applications, the shape of the decay curve is also important. Deviations from the straight line can reveal acoustical defects.