Utilizing Sabine’s formula
Sabine’s formula measured the time it takes for sound energy to decrease by 60 dB after the sound incident has ceased. Measured in seconds, this is the reverberation time 60 dB (RT60). The rate of decay is going to depend on the amount of absorption in a room, its geometry, obstacles, and the properties of the sound incident itself. This will vary from space to space.
The RT60 is calculated by determining the volume of the space, and the amount of surface area (i.e. walls, floor, and ceiling). An absorption or attenuation coefficient is placed on each surface. As specifiers and designers know, most building materials are more reflective and less absorptive. The RT60 is 0.161 m (0.049 ft) times the volume of the space over the surface areas and individual coefficients. This number will reflect the time in seconds it will take a sound incident in this space to drop 60 dB.
The algebraic relationship of Sabine’s equation means any variable can be determined if the remaining components are known. Working the formula forward one can determine a modelled RT60 by knowing the volume, surface areas, and associated coefficient. Conversely, knowing the volume, the desired RT60, and the surface areas, one can determine how much treatment or absorptive coefficient material will be needed to achieve the desired effect or RT60.
For example, in a lecture hall with a desired two-second RT60, the calculation can be used to find a measurement of sabins (a sabin is 0.09 m2 [1 sf] of perfectly absorptive material). If 2100 sabins are needed, and a 50 per cent absorptive material is being used, the amount of sabins is divided by the absorption coefficient, in this case 0.50, to come up with 390 m2 (4200 sf) of treatment needed to achieve the desired two-second RT60.
For general purposes, an RT60 of 1.5 to 2.5 seconds is considered acceptable for most spaces. Under the 1.5 second mark, there is a clearer articulation of speech, but the space starts to become acoustically dead, making it difficult to hear at the rear of the space, resulting in a loss of deeper bass tones. Above 2.5 seconds RT60, the space gains fullness and richness of sound but speech intelligibility suffers, and discerning words become difficult. Luckily, algebraic prowess is no longer a necessity for running these calculations, as several free, online acoustic reverberation calculators are readily available. These calculators can work forward to determine a space’s RT60, or backward to determine the amount of sabins or absorption needed to reach a desired RT60.
