By Noral D. Stewart (copyright 2019)
Reverberation Time is the most common factor used to characterize acoustics of a room. Though not actually a valid concept under some conditions, it is still used even in some of those cases to evaluate and specify the needed acoustical characteristics. Reverberation time does not answer all questions about sound behavior in a room. Many other problems can exist, and recent work in concert hall design has developed many new measures of room acoustics. However, most rooms are still designed based on reverberation time and geometric analysis of specific reflections.
The term was first defined by W. C. Sabine, the father of modern architectural acoustics who is recognized as the first acoustical consultant. He observed it took a long time for a clap or shout to decay in a large empty room with all hard surfaces. The sound would bounce around the room or reverberate. He conducted experiments in several rooms at Harvard using seat cushions from one room. He varied the number of cushions in each room, and measured the time for a suddenly stopped sound to decay to the point he no longer heard it. The difference between the level of the initial sound and that he could no longer hear was about 60 decibels. Thus, later, the reverberation time was defined as the time required for a sound to decay by 60 decibels. He found that the decay time was longest in the largest rooms. The more seat cushions he added to a room, the shorter this time became. He recognized that too much reverberation made it difficult to understand speech. However, composers, musicians, and listeners had come to expect a certain amount of liveliness in music halls, and music was composed with this expectation. He was trying to develop a way to predict room behavior to match the expectations. He realized that the seat cushions were absorbing most sound rather than reflecting it. He came up with a simple equation. It says that the reverberation time increases with room size and decreases as absorption is added. A perfectly absorbing material was defined as one that would reflect none of the sound incident on it. Today, we call a unit of one square foot of perfect absorption a "sabin." Very few materials are perfectly absorptive. A square foot of a material that is 60% absorptive contributes 0.6 sabins of absorption to the room. All materials, even people, absorb some sound, though often only a very small amount.
Different rooms require different reverberation times, depending on the use of the room. The ideal value for music rooms depends on the style of music. Ideal rooms for public speaking or for social uses should have more absorption for a shorter reverberation time. The reverberation time should usually be around a half second in smaller rooms and up to a second in larger rooms for clear speech. It if is too long, syllables of speech from a single speaker will blur together. Much classical music, on the other hand, usually sounds best with reverberation times on the order of 2 seconds. Some cathedrals reverberate for several seconds. The ideal value for a room also depends partially on its size, with longer times acceptable in larger rooms.
Expertly designed sound systems can be used to provide speech clarity in rooms with high reverberation times. However, this dependence on the sound system can limit the flexibility in the use of the room. Compromises on reverberation time are often necessary in rooms with many uses. These compromises will leave the room less than ideal for any one purpose. However, acceptable compromises have been developed for churches and general purpose auditoriums. These compromises are often slanted in one direction or the other depending on the emphasis of the room users.
Materials behave differently in their reaction to sounds of different pitches. The soft materials often thought of as "acoustical" absorb high-pitched sounds better than low bass sounds, unless the materials are very thick. On the other hand, solid panels with air spaces behind them can absorb a large amount of low-pitched or bass sound without absorbing much high-pitched sound. This often occurs with gypsum board or wood paneling. Thus, the reverberation time in a room will be different for different sound pitches. In most rooms it is highest for low-pitched sounds and decreases for high-pitched sounds. We have come to expect and accept this within limits. On the other hand, for music in particular, we must avoid a situation where the reverberation time increases with frequency or pitch. For rooms where music is of any concern, reverberation times should be estimated and controlled over a wide frequency range. Usually, the range used is from an octave below Middle C to the top end of the Piano scale. For rooms designed solely for music, this range is sometimes extended for an octave lower and higher. The absorption of high-pitched sound by air in the room becomes important. The number of people present changes the character of the room. This can be minimized by using upholstered seats or pew cushions that are covered when occupied. Another option is to make the room oversized, and add absorption to surfaces. Then the absorption added by the people is a smaller proportion of the total.
(An additional controlled by the amount of absorption in the room is the loudness of sound in the room. If used for social functions with many people talking at once, more absorption is needed to control the loudness. While reverberation time depends on absorption and room volume, the loudness depends on absorption and the strength and number of sound sources.)
The equations for predicting reverberation time include certain assumptions. Unless these are met, a true reverberant field may not exist. The room should have a "regular" shape and the absorption should be evenly distributed. The traditional Sabine equations are invalid if the length or width of the room is very much larger than the ceiling height, the absorption is not well distributed, or the reverberation time is very short. The equations will predict a result for the room, and give some indication of the room character. However, the result will not accurately describe the behavior of sound in the room. As indicated earlier, many other factors also affect whether a room will be acoustically acceptable. These need evaluation. Recently, computer programs for room analysis have become widely available. These use alternative techniques that do not have the shortcomings of the Sabine method. However, they do have shortcomings and can produce misleading results if the limitations of the methods and data used are not understood. These programs have unfortunately fallen into the hands of some who do not fully understand their limitations. Those needing assistance on problems in room acoustics should assure that the person providing advice fully understands and considers all aspects of the problem.
The information in this document is not provided as a consulting service or as a solution to any specific problem.