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Average Absorption Coefficient from RT    1/1, 1/3

Strutt|Building Acoustics|Alpha from RT back-calculates the average room absorption coefficient `bar alpha` from the reverberation time in the room, using either the Sabine or Eyring formulae.

Using the Sabine formula, the average absorption is calculated as follows:

`bar alpha = (55.3 V)/(c T S) - (4mV)/S`

where: `V` is the room volume (m)
`c` is the speed of sound (m/s). When using 343 m/s the constant term `55/c` reduces to the more-familiar 0.161
`T` is the reverberation time (s)
`m` is the air absorption coefficient

Using the Eyring formula, the average absorption is calculated as follows:

`bar alpha = 1-e^((55.3 V)/(c T S)) - (4mV)/S`


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