ISO 9613 (2024)

Lateral Diffraction

see ISO 9613 (1996), Lateral Diffraction.

if Distance smaller (m)

see ISO 9613 (1996), if Distance smaller (m).

Screening & Ground Attenuation

The following options are available to account for the ground attenuation in conjunction with the calculation of the screening effect.

Exclude Ground Attenuation over Barrier

With this setting the ground attenuation is excluded from the total attenuation of screened rays in case of non-reflecting ground (Agr > 0). This is the default setting in CadnaA.

Explanatory Remarks:
ISO 9613-2 (2024), equation (16) states (for top edge and Agr > 0 ): $A_{bar}=D_z-A_{gr}>0dB$
In conjunction with equation (5): $A=A_{div}+A_{atm}+A_{gr}+A_{bar}+A_{misc}$
it results that for screened rays no ground attenuation A_gr is considered:
$A=A_{div}+A_{atm}+A_{gr}+(D_z-A_{gr})+A_{misc}=A_{div}+A_{atm}+D_z+A_{misc}$
This setting is, therefore, in line with the specifications in ISO 9613-2 (2024)

In case of reflecting ground (Agr < 0) equation (17) is used also if „Exclude Ground Attenuation over Barrier“ is selected.

Include Ground Attenuation over Barrier

The ground attenuation is always included in the total attenuation of screened rays even for non-reflecting ground (Agr > 0).

Explanatory Remarks:
With this option the equation (17) of ISO 9613-2 (2024) is applied for the top edge independent of the value of Agr: $A_{bar}=D_z>0dB$
This equation holds as well for the two lateral paths. For the total attenuation it results according to equation (5):
$A=A_{div}+A_{atm}+A_{gr}+D_z+A_{misc}$
In this case, the ground attenuation A_gr is considered also for the diffracted rays.

10 m-Criterion

When the source is higher than 10 m above ground and at least one screening edge is higher than 10 m above ground, accord. to ISO 9613-2 (2024), note 1, equation (17) is used to calculate the attenuation by a barrier A_bar (with A_bar = D_z > 0 dB) - equivalent to „Include Ground Attenuation over Barrier“. For sources below 10 heigth or if all screens are under 10m high Abar is calculated as described in „Exclude Ground Attenuation over Barrier“.

Barrier Attenuation

see ISO 9613 (1996), Barrier Attenuation.

Dz without limit

With this option activated the barrier attenuation D_z increases with increasing path difference z without any limitation (accord. to equation (18) in ISO 9613-2 (2024)).

Foliage (Afol) acc. to A.2.3 (detailed method)

If this option is activated, the attenuation due to foliage (Afol) is calculated according to ISO 9613-2 (2024) Annex A.2.3 (detailed method) based on forestal parameters ( see Built-Up Areas and Foliage, Calculation according to ISO 9613-2 (2024) detailed method). If the option is deactivated, A.2.2 (simplified method) is used.

Point sources are wind turbines

For point sources with relative height of ≥ 50m, the calculation according to ISO 9613-2 (2024), appendix D.3 and D.5 or IoA Good Practice Guide IOA-2013 can be applied. The following options are available:

Checkbox D.3

If this option is activated, barrier attenuation Abar is limited to 3 dB. Unlike described in ISO 9613-2 (2024), CadnaA does not distinguish between the obstacle types terrain and building, so the limitation is always applied. The user must ensure, on a case-by-case basis, that screening is only provided by the terrain.

Checkbox IoA (2 dB)

According to IOA-2013, "screening effects of the terrain should be limited to a reduction of no more than 2 dB, and then only if there is no direct line of sight between the highest point on the turbine rotor and the receiver location." CadnaA implements this method with the option "IoA" activated. To define the highest point of the turbine rotor, it is possible to define the rotor diameter via a memo variable ROTOR_DIA in the point source. If the memo variable is missing, the default value is the height of the hub.

Example: If the point source is defined at 100 m height with a memo variable ROTOR_DIA=50, the highest point on the turbine is considered at 125 m. If the memo variable is missing, the highest point is considered at 150 m.

Checkbox D.5

If this option is activated, CadnaA will check for concave ground profile using formula D.2 of ISO 9613-2 (2024), appendix D.5. This is done for each propagation path and - if applicable - consider dAgr with -3 dB according to formula D.1.

Formula D.2: $h_m \ge 1.5 \times \frac{h_s+h_r}{2}$

Checkbox IoA (hs - hr)

If this option is activated, CadnaA will check for concave ground profile using formula (2) of IOA-2013. This is done for each propagation path and - if applicable - consider dAgr with -3 dB (or -1.5 dB if G=0). The correction is only applied if the ray is not screened (Abar = 0).

Formula (2): $h_m \ge 1.5 \times [\frac{abs(h_s-h_r)}{2}]$

Ground Attenuation

In ISO 9613-2, two methods are mentioned to calculate the ground attenuation A_gr:

„general method“: method in frequency bands according to section 7.3.1 with the ground factor G as parameter,
„alternative method“: method for A-weighted levels according to section 7.3.2 not applying the ground factor G.

As in projects the sound emission of sources can either be given in spectral form or as A-weighted levels, several possibilities exist to calculate the ground attenuation. In CadnaA four calculation options are available:

Designation	Procedure
none	no ground attenuation at all However, the ground reflection is still accounted for by using equation (15) of ISO 9613-2 (2024).
not spectral	method 7.3.2 is used in all cases For spectral sources the same value of A_gr results for all octaves resp. 1/3-octaves.
spectral, spectral sources only	The method 7.3.1 is used for all spectral sources, in all other cases the method 7.3.2 is used.
spectral, all sources	Method 7.3.1 is used for all types of sources, for non-spectral sources the value of A_gr of the octave band where the entered frequency is located. When calculating in third-octaves all third-octaves within an octave have the same value A_gr. This is the default setting in CadnaA since this is the „general method of calculation“ according to ISO 9613-2.
WEA interim (-3 dB)	A fixed value for Agr of -3 dB is used with A-weighted or for all octaves resp. third octaves with spectral calculations (according to the German LAI interim method for wind turbine noise assessments LAI-2015). The directivity index D_omega is 0 dB in both cases.