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Indoor Barrier Insertion Loss    1/1,1/3

Strutt|Building Acoustics|Indoor Barrier calculates the transmission loss through an indoor barrier, taking into account the reverberant sound field and reflections from other surfaces, and inserts the result into the active work sheet.

Strutt implements the formula as:

`IL = 10log_10(Q/(4 pi r^2)+4/(S_0 bar(alpha_0))) - 10log_10((QF)/(4 pi r^2)+(4 K_1 K_2)/(S(1-K_1 K_2)))`
`IL` is the barrier insertion loss
`Q` is the source directivity factor in the direction of the receiver (back-calculated from directivity index DI via `DI=10log(Q)`
`r` is the distance between source and receiver in the absence of the barrier
`S_0` is the total room surface area
`bar(alpha_0)` is the mean root Sabine absorption coefficient
`S` is the open area between the barrier perimeter and the room walls and ceiling
`F` is the diffraction coefficient and is given by the following equation:

`F = sum_i 1/(3+10N_i)`

`N_i` is the Fresnel number for diffraction around the `i`th edge of the barrier and is given by the following equation:

`N = +-(2/lamda)(A+B-d)`

`K_1` and `K_2` are dimensionless numbers given by the following equation:

`K_1 = S/(S+S_1 bar(alpha_1)), K_2 = S/(S+S_2 bar(alpha_2))`

`S_1` and `S_2` are the total room surface areas on sides 1 and 2 respectively of the barrier
`bar(alpha_1)` and `bar(alpha_2)` are the mean Sabine Coefficients associated with areas `S_1` and `S_2` respectively

Bies and Hansen, Engineering Noise Control, Second Edition, E & FN Spon, London 1996

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