Strutt|Environmental Noise|Barrier Attenuation provides a barrier attenuation calculation, using either the Maekawa equation, ISO9613-2 equation, Degout equation or the CORTN method, which is entered in the active row of the worksheet.
The CNOSSOS method of Barrier + Ground attenuation is also available and is discussed in detail here.
The calculation is valid for the shadow or illuminated zone of the barrier and is limited to a maximum attenuation of (-) 25 dB at any frequency.
The Maekawa equation is implemented within the shadow zone as:
`IL = -[5 + 20log_10(sqrt(2 pi N_F)/tanh(sqrt(2 pi N_F)))]`
where,
`N_F = 2(delta/lambda)` is the Fresnel number
The CORTN method gives a single-number dB(A) value, which is placed in Column E of the active row. In the Barrier Attenuation user form, the attenuation is displayed in the octave band cells, but the final result is a broadband value only, and selecting the CORTN method will NOT insert spectral values into the active row. The CORTN method is implemented using the following equation:
`IL = A_0 + A_1 x + A_2 x^2 + A_3 x^3 + ... + A_n x^n`
where,
`x = log(delta)` and `delta` is the path difference, m
The coefficients `A_n` are:
A0 |
A1 |
A2 |
A3 |
A4 |
A5 |
A6 |
A7 |
| -15.4 | -8.26 | -2.787 | -0.831 | -0.198 | 0.1539 | 0.12248 | 0.02175 |
The ISO 9613-2 method includes an option to calculate the effect of meteorological conditions on the barrier attenuation, and is calculated using:
`A_(b a r) = -10log_10(3 + 20/lambda C_3 delta K_(met))`
Where,
`lambda` is the wavelength of sound at the frequency in question
`delta` is the path difference between the diffracted sound and direct sound, calculated using:
The Degout formula is calculated as follows:
`IL = {(0 " for " \ N_F < -0.25 ),(-6 + 12 sqrt(-N_F) \ " for " \ -0.25 <= N_F < 0 \ ),(-6 - 12 sqrt(N_F) \ " for " \ 0 <= N_F < 0.25 \ ),(-8 - 8 sqrt(N_F) \ " for " \ 0.25 <= N_F < 1.0 \ ),(-16 - 10log_10(N_F) \ " for " N_F > 1.0 \ ):}`