Reverberation Time

The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of the room. In a more reflective room, it will take longer for the sound to die away and the room is said to be 'live'. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'. But the time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer. In order to provide a reproducible parameter, a standard reverberation time has been defined as the time for the sound to die away to a level 60 decibels below its original level. The reverberation time can be modeled to permit an approximate calculation. .

Rationale for 60dB Reverberation Time

The reverberation time is perceived as the time for the sound to die away after the sound source ceases, but that of course depends upon the intensity of the sound. To have a reproducible parameter to characterize an auditorium which is independent of the intensity of the test sound, it is necessary to define a standard reverberation time in terms of the drop in intensity from the original level, i.e., to define it in terms of relative intensity.

The choice of the relative intensity to use is of course arbitrary, but there is a good rationale for using 60 dB since the loudest crescendo for most orchestral music is about 100 dB and a typical room background level for a good music-making area is about 40 dB. Thus the standard reverberation time is seen to be about the time for the loudest crescendo of the orchestra to die away to the level of the room background. The 60 dB range is about the range of dynamic levels for orchestral music.

What is a desirable reverberation time?

The optimum reverberation time for an auditorium or room of course depends upon its intended use. Around 2 seconds is desirable for a medium-sized, general purpose auditorium that is to be used for both speech and music. A classroom should be much shorter, less than a second. And a recording studio should minimize reverberation time in most cases for clarity of recording.

The reverberation time is strongly influenced by the absorption coefficients of the surfaces as suggested in the illustration, but it also depends upon the volume of the room as shown in the Sabine formula. You won't get a long reverberation time with a small room.

Reverberation Time Calculation

Approximate reverberation times can be calculated from the Sabine formula:

Modeling a room of Height H =m = ft Length L =m = ft Width W =m = ft with absorption coefficients:

awalls=,

afloor

=,

aceiling

= for average of

aavg

=

gives an effective absorbing area of Se

=m2 = ft2

for a room of volume V =m3 = ft3.

The corresponding reverberation time is

RT60

=seconds.

Examples of Reverberation Times

One way to respond to the question "What is a good range of reverberation times for concert halls?" is to give examples of some of the most famous halls in the world. For the overall average reverberation times:

But the overall average reverberation time does not tell the whole story. The variation of reverberation time with frequency is also important.: