Definition of RT
Reverberation time (RT) is defined as the time span needed for the sound level to decrease by 60 dB from the moment the sound source is stopped. This is also written as RT60.
Looking at a typical reverberation curve, one can note that there typically are three regions, as displayed in Figure 2.7.
It can be of interest to assess the early reverberation time (early decay time (EDT)), which is assessed over the first 10 dB decrease (and sometimes written as RT10). This will be discussed in Chapter 6.
By the way, looking at a typical reverberation curve, one can sometimes note a change of slope appearing in the decrease, as displayed in Figure 2.6. This usually indicates the presence of a flutter echo.
Simple Computation of RT
A simple predictive assessment of the reverberation time can be carried out from such basic quantities as the volume and absorptive areas of the room. The famed Sabine formula is
Figure 2.6 Example of stylized reverberation time curve.
Figure 27 Spatial sound level decay.
where V is the volume of the room, A is the equivalent absorptive area, and K is a coefficient with a value of 0.163 in metric units.
While simple, this formula is actually supposed to be valid for volumes with rather reflective surfaces and similar dimensions. The greater the absorption, the smaller the accuracy of the prediction is. Nevertheless, it does give interesting information regarding tendencies (e.g., increasing the volume, or adding extra absorption in a room).
To try to adapt to more current room volume shapes or partial absorptive treatment, similar formulations have been developed by Eyring and Millington. But on the whole, the Sabine formula is best suited to most situations .
Which RT for Which Use?
The reverberation time is akin to the time constant of a physical system: The longer the time, the slower the reaction speed and the better the stability. One may deduce that for harmonic music (i.e., music where chords are the primary expression, such as Gregorian singing), a long reverberation time is required (e.g., over 3 s), while for melodic music, where each note counts, a shorter reverberation time (e.g., 1.6 s) is needed. For speech and singing, where the text is of importance, an even shorter RT value is looked for (e.g., 1.1 s). These aspects are developed in Chapter 6.
The reverberation time may also indicate the eventual lack of acoustic absorption in the space. As less absorption means a higher sound level, one must keep in mind that for noise control purposes, long reverberation times should be avoided (e.g., in a dining room).