HJ 2.4 (2009 / 2021)
If not stated otherwise, the information applies to both HJ 2.4 (2009) HJ24-2009 and HJ 2.4 (2021) HJ24-2021.
Version of HJ 2.4
The calculations can be performed according to version:
Lateral Diffraction
Three options for lateral diffraction are available for selection:
- none: No lateral diffraction considered, just the direct path source-receiver is evaluated.
- only one object: Lateral diffraction is not calculated if more than one object intersects the line connecting the source to the receiver point.
- some objects: The two shortest possible convex rays around the arrangement of objects are determined and used for the calculation of the lateral diffraction.
if Distance smaller (m)
This setting defines the distance between source and receiver for which lateral diffraction is taken into account. The default value is 1000 m.
2D distance for hm
If the checkbox is activated, the calculation of hm for formula A.20 of HJ24-2021 uses the 2D distance instead of the 3D distance. With this setting activated, a level drop above the source can be prevented. For a detailed description of this effect, please see ISO 9613 (1996), Non-spectral Ground Attenuation.
Note
Strictly according to HJ 2.4, this checkbox has to be inactive. As shown in figure A.4, hm is determined by the 3D distance considering the heights of source and receiver.
Barrier Attenuation
According to chapter „A.3. 4.2 Double diffraction“ of HJ24-2021, „the attenuation due to ground is no longer considered after Abar has been calculated“. CadnaA offers multiple implementations for the handling of Aground in conjunction with Abar:
- Aground not considered in case of single and multiple diffraction:When choosing this option, the above-mentioned condition is also applied for single diffracted rays to calculate a continuous transition between single and multiple diffraction. This interpretation was implemented in CadnaA for all versions previous to CadnaA 2023. When loading an older file with CadnaA version 2023 or newer, this option is selected automatically.
- Aground not considered in case of multiple diffraction:This option strictly follows HJ24-2021 and disregards Aground exclusively in case of multiple diffraction. Since the transition between single and double diffraction is not continuous when Aground is not zero, it seems not advisable to use it in these cases. As an example: adding an additional obstacle can make it louder behind the obstacle.
- Use maximum of Aground and Abar:For positive values of Aground this setting considers the maximum value of Aground and Abar for single and multiple diffracted rays.
- For negative values of Aground the sum (Aground + Abar) is considered. The method does not refer to any interpretation of HJ24-2021, but is used with other calculation methods and generates a continuous transition. This option is the new default setting for projects created with CadnaA version 2023 or newer.
Ground Attenuation
The setting refers to chapter A.3.3 of HJ24-2021. The functionality is similar as described for ISO 9613-2, see ISO 9613 (1996), Ground Attenuation with following terms:
- „not spectral“ refers to calculation according to formula A.20 of HJ24-2021
- „spectral“ refers to the calculation according to GB/T 17247.2. For the calculation of ground attenuation, the frequency of 500 Hz is used.
The selection of „none“ or „- 3dB“ is not relevant when calculating according to HJ 2.4.
Calculation of Foliage and Housing
For HJ 2.4 (2009/2021), the calculation of attenuation due to foliage and housing is handled as described in ISO 9613-2 ISO-9613-2-1996 (for foliage, 500 Hz is used). To consider foliage / housing, the respective objects need to be entered, see Built-Up Areas and Foliage.