Hard rooms
Traditional evaluation of the acoustics in a room means in many cases that only the reverberation time is measured. In hard rooms, it is usually sufficient to have the reverberation time as the room acoustic descriptor. Both the sound level and the reverberation time here are more or less only dependent on the total sound absorption in the room. If the reverberation time and the sound effect that a sound source sends out into the room are known, the sound level in the room can be calculated. However, the hard room is, in reality, very rare.
In a hard room, the reverberation time can be determined using Sabine’s formula: 
T=0,161 x V/A     (s)

T = reverberation time (seconds)
V = room volume (m3
A = equivalent absorption area (m


The equivalent absorption area A describes the sound absorption in the room. If the room is empty and the sound absorption level is determined mainly from the absorption of the ceiling, the floor and the walls, then A = αceiling x Sceiling + αfloor x Sfloor + αwall x Swall where α is the sound absorption factor and S is the surface for the respective area of the room. We have assumed that all the walls have the same absorption factor.

Sabine’s formula shows that the reverberation time is only dependent on the total absorption in the room and not on the placement of the absorbers nor on the sound-scattering effect of furniture and other furnishings in the room. It is assumed that the sound field is diffuse, meaning that the sound, at each location in the room, disperses with the same intensity in all directions.
The sound level reduction that an added equivalent, absorption area A, gives in a room with diffuse sound will be

ΔL=10 x log((A0+A)/A0)      (dB)

where A0 is the existing equivalent absorption area in the room. If, for instance, A0=10 m2 in an empty room without an acoustic ceiling, and the acoustic ceiling contributes A=40 m2, the sound level reduction will be 7 dB.