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Barrier Attenuation    1/1

Strutt|Environmental Noise|Barrier Attenuation provides a barrier attenuation calculation, using either the Maekawa, ISO9613-2, Degout, CNOSSOS or CORTN method, which is entered in the active row of the worksheet.

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 CNOSSOS method of Barrier + Ground attenuation is also available and is discussed in detail here.

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

delta is the path difference, m
lambda is the wavelength of sound, m

Within the illuminated zone, a log-log curve fit is used to calculate the barrier loss based on the figure in the original Maekawa paper.

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 as an insertion loss using:

A_(b a r) = D_z - A_(g r)

D_(z) = -10log_10(3 + 20/lambda C_3 delta K_(met))

Where,
A_(g r) is the ground attenuation
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:

delta = d_(ss) + d_(sr) - d

C_3 is a coefficient for double diffraction; C_3 = 1 for single diffraction
K_(met) is a meteorological correction factor calculated using:

K_(met) = e^((-1/2000 sqrt(d_(ss)d_(sr)d)/(2 delta)))

d_(ss) is the distance from the source to the first diffraction edge, m
d_(sr) is the distance from the second diffraction edge to the reciever, m
d is the straight-line distance between source and receiver, m

Note:
• The barrier attenuation D_(z), in any octave band, should not be taken to be greater than 20 dB in the case of single diffraction (i.e. thin barriers) and 25 dB in the case of double diffraction (i.e. thick barriers}.
• The ISO 9613.2 barrier attenuation is the excess attenuation above the ground attenuation. In some instances, it may not be appropriate to include ground attenuation in your calculation. For example, the ISO 9613.2 ground effect calculation is applicable only to ground which is approximately flat, either horizontally or with a constant slope. In addition, the ground effect component of the barrier calculation will break down should you attempt to use negative source or receiver heights. In some instances, you can use trigonometry to adjust the relative source and receiver heights to be positive, so that you can calculate the ground effect correctly. See image below as an example:

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 \ ):}