Sound rays obey Snell’s law of reflection. When they strike a surface the fraction of sound energy absorbed is known as the absorption coefficient, which varies with frequency. The time for the reverberant sound in a room to drop by 60dB is known as the reverberation time R. This can be calculated from the formula R = 0.16 V/A where V is the volume in cubic metres and a is the total absorption in metric Sabin i.e. the sum of the surface areas times their absorption coefficients. If R is large the sound in the room will lack clarity but the sound level will be large. If R is small the sound will be clear but the sound level will be low. Typically R will be 1s for a theatre and 1.5-2s for a concert hall.