# Equivalent sound absorption

The equivalent sound absorption area A in m2 is the area representing a room’s entire sound absorption (walls, ceiling, decoration etc. – sound absorption coefficient 100%). It is the sum of the products of all limiting partial surfaces Si (including the surfaces of objects within the room) and their sound absorption coefficients αi. The larger a room’s equivalent absorption area, the more muted and the quieter it is.

The equation for the equivalent sound absorption area A is illustrated in figure. It allows for the determination of A in relation to room volume V and reverberation time T.

The difference

between sound power level LWA and sound pressure level LPA is depicted in relation to the equivalent sound absorption area A in figure 5.62b. It facilitates the determination of the sound pressure level present in a given room based on measured sound power levels as contained e.g. in the design documents for ventilated luminaires.

With multiple sound sources (number N) of identical sound power level LWA, the overall sound power level LWA, total is increased by ΔL2 according to the formula

This formula is analysed in figure c.

Example
For an office room with a room volume V = 200 m3 and a reverberation time T = 0.5 s – see table – the resulting equivalent sound absorption area A = 65 m2, figure a.
Hence, the difference between sound power level LWA and sound pressure level LPA is 12 dB(A); figure b. For a sound source (e.g. a ventilated luminaire) with a sound power level LWA = 30 dB(A) in this room, the expected sound pressure level LPA = 30 dB(A) - 12 dB(A) = 18 dB(A). If the room contains e.g. 12 sound sources of identical loudness, the overall sound power level rises from 30 dB(A) to ca. 41 dB(A) according to figure c.