Concawe
Obstacles within Area Sources do not shield
When this option is active, any obstacle (e.g. buildings, foliage, barriers etc.) located within area sources (including parking lots and optimizable sources) provide no screening for any sub-source of that area source.
Example
Area source with 2 buildings inside, option activated: sources with free propagation (with all rays displayed)
With this option activated the result levels are calculated as if the obstacles were not present (i.e. no screening effect).
Area source with 2 buildings inside, option deactivated: sub-sources behind the buildings are screened, sub-sources in front of buildings with free propagation (screened rays in RED)
With this option deactivated lower levels result at the receiver points due to the screening effect of the obstacles inside. Nevertheless, for all other sources (i.e. sources outside of the are source’s border) the obstacles inside cause always screening.
Sources in Building/Cylinder do not shield
By default, a screening effect results for sources inside buildings and cylinders as the outer walls act as barriers.
Note
In this case, prior to the calculation, a message will be displayed dialog Consistency Check) which enables to proceed with the operation (button „Continue“).
This procedure is due to the fact that buildings and cylinders are treated as open boxes within the calculation of propagation (despite they are displayed as „closed“ objects, i.e. with roof, on the 3D-Special View).
With this option activated, sources located inside of the geometrical borders of buildings or cylinders are not screened as if the obstacle were not present. For sources outside, those obstacles are always screening and reflecting. This option enables to model e.g. sound radiating chimneys by using point or line sources located inside the cylinder.
In this case, the directivity pattern of the chimney - whether entered or selected from the default directivities - will not distorted by the cylinder’s screening edge.
Example
see manual „Introduction to CadnaA“, chapter Directivity
Temperature / rel. Humidity
The air attenuation is calculated based on the temperature and relative humidity. Intermediate frequency values result from linear interpolation. The temperature specified serves also as the ambient temperature when calculating the exhaust stack’s directivity.
Calculation in 1/3-Octave Bands
see ISO 9613 (1996)
Meteorology
In the CONCAWE propagation model the meteorological correction is named „attenuation factor for meteorological effects K4“ CON-1981. The attenuation is a function of frequency, distance, and meteorological category.
With non-spectral sources CadnaA applies the correction K4 independent of frequency as stated in CONCAWE, appendix II, section 2. Thus, the spectral K4 at the entered frequency (500 Hz per default) is not used.
Note
Despite the specification in CONCAWE, appendix II, not to use the attenuation curves for receiver distances below 100 m, in CadnaA the value of K4 for d<100 m is linearly interpolated to 0 dB at the source location.
Stability Class D/E/N
The assessment of atmospheric temperature gradient is replaced by use of Pasquill Stability Categories A to G (for periods D/E/N separately). The definition of the categories depends on wind speed, time, and cloud cover:
| Wind Speed * m/s | Day Time, Incoming Solar Radiation(mW/cm²) | 1 hour before Sunset or after Sunrise | Night-Time, Cloud Cover (octas) | |||||
| > 60 | 30-60 | < 30 | Overcast | 0-3 | 4-7 | 8 | ||
| <=1.5 | A | A-B | B | C | D | F or G** | F | D |
| 2.0 - 2.5 | A-B | B | C | C | D | F | E | D |
| 3.0 - 4.5 | B | B-C | C | C | D | E | D | D |
| 5.0 - 6.0 | C | C-D | D | D | D | D | D | D |
| > 6.0 | D | D | D | D | D | D | D | D |
* wind speed is measured to the nearest 0.5 m/s
** category G is restricted to night-time with less than 1 octa of cloud and a wind speed of less than 0.5 m/s
Based on literature, category A represents a strong lapse condition (large temperature decrease vs. height), category G a temperature inversion observed early on a clear morning. Categories E and F apply to slight inversions. However, inversion conditions, particularly those over water, are highly variable and not easy to predict. The acoustical effect depends on additional factors (e.g. geometry).
From this classification six new meteorological categories have been defined, based an a combination of Pasquill Stability Categories (representing the temperature gradient) and vector wind speeds (v (z=10 m) in m/s). The effect on the attenuation is indicated by the arrow.
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Meteorological Category | Pasquill Stability Category | ||
| A, B | C, D, E | F, G | ||
| 1 | v < -3.0 | - | - | |
| 2 | -3.0 <= v <-0.5 | v <= -3.0 | - | |
| 3 | -0,5 <= v < + 0.5 | -3.0 <= v < -0.5 | v < -3.0 | |
| 4 (*1) | +0.5 <= v <+3.0 | -0.5 <= v < +0.5 | -3.0 <= v < -0.5 | |
| 5 | v >= +3.0 | +0.5 <= v < +3.0 | -0.5 <= v < +0.5 | |
| 6 | - | v >= +3.0 | +0.5 <= v < +3.0 (*2) | |
*1 category with assumed zero meteorological influence
*2: For speeds v >= +3.0 also category 6 is assumed.
Note
In CadnaA, the Pasquill Stability Category D at wind speed 0 m/s is selected by default, resulting in the meteorological category 4 with a correction K4=0 dB.
Wind Direction (°) D/E/N
The wind direction is the direction from which a wind originates (with 0° being North wind, i.e. wind blowing from North, with 180° being South wind etc.).
Wind Speed (m/s) D/E/N
CONCAWE-method does not specify at what height the wind direction and speed shall be assessed.
Note
The commonly used height for evaluation of wind speeds in meteorology is 10 m above ground (plus further criteria regarding nearby obstacles etc.).
Interpolate Meteorological Category
As the velocity vector of the wind in source-receiver direction is relevant for the calculation of the meteorological correction level steps may result between categories. This option causes to smoothen the noise contours at the limits of areas with different meteorological categories by interpolation.
Example
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horizontal grid from a point source with meteo settings: - stability class: A - wind direction: 180° - wind speed: 5.0 m/s option „Interpolate meteorological Category“ deactivated |
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as above option „Interpolate meteorological Category“ activated |
Worst case wind direction
With this option active uses the wind direction causing the highest level will be used for all directions.
Note
In version 2025 MR1 a bugfix was implemented. Before, K5 was not calculated when varying K4 to dedermine the maximum level. The previous state can be used with local text block OPT_OLD_CALC with text conc_k5_wc_x209=1.
Example
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| option „Worst case wind direction“ deactivated | option „Worst case wind direction“ activated |
Batch Processing
Different meteorological configurations for a distinct situation can be processed automatically. The procedure to be followed consists of the following steps:
- the situation file with several receiver points (
*.cna), - the macro-file specifying the configuration of calculation and the results to be exported (
*.cnm). - The macro-file is loaded into CadnaA via menu File|Open. Select the option „All Files“ from the list box „File Type“ to select the cnm-file.
- Upon opening of the file the batch processing start automatically and saves the results to a text file with file extension
*.log.
Example
Path: Examples\Immissions\Harmonoise_Concawe
An example file is provided. The settings in the example refer to the method „Harmonoise“. Please see Harmonoise for more details.
Additional Information
Ground Attenuation K3
The ground attenuation K3 is controlled in CadnaA via the „Default Ground Factor“ G specified on tab „Ground Absorption“ (see Ground Absorption Tab). With the default ground factor G=0 reflective ground is assumed (leading to K3=-3 dB), while with a default ground factor G>0 absorbing ground is considered. The object „Ground Absorption“ can be used to model mixed ground (with G=0 or G>0). In this case, CadnaA just evaluates the ray’s path length across the absorptive ground (see CONCAWE-report, section 5.1.3).
Note
Despite the specification in CONCAWE, appendix II, not to use the attenuation curves for receiver distances below 100 m, in CadnaA the value of K3 for d<100 m is linearly interpolated to 0 dB at the source location.
Meteorological Correction K4
In CONCAWE, the so-called „vector wind velocity“ for each ray path based on the entered overall wind velocity and direction is evaluated (see chapter 2.5 of CON-1981) considering the angle between the entered wind direction and the ray‘s path. Thus, the relevant speed for K4 can be smaller than the specified wind speed resulting in a different meteo category. For downwind the cosine is positive, for upwind negative.
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| Example 1: Relevant wind speed in a downwind situation |
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| Example 2: Relevant wind speed in an upwind situation |
Source/Receiver Height Correction K5
The function for gamma using the grazing angle ψ is limited to γ>=0 (i.e. no negative values for γ), Thus, the function plotted in figure 9 of CONCAWE report is in CadnaA assumed to end at γ=0 asymptotically (as a specification is missing).
Barrier Attenuation K6
In the CONCAWE-report, section 2.7, it says:
„The presence of a discrete barrier may reduce ground effects and it is proposed that this be covered by recalculating K5 based an the barrier height and barrier-receiver distance.“
Furthermore, it says in section 5.1.6:
„This is not, however, necessary if the barrier is a topographical feature.“
This proposal is not respected in CadnaA as it causes level jumps due to different path length differences.
Distance Source-Receiver
In CONCAWE, it is unclear what the mentioned „distance d“ exactly means, the 2D or the 3D distance. On page 8 of the CONCAWE-report it says “d source – receiver distance” (i.e. not specifying whether 2D or 3D distance is meant). This has, with respect to the different attenuation terms, the following consequences with regard to the implementation in CadnaA:
- In CONCAWE, chapter 5.1.1, the attenuation term K1 is based clearly on 3D distance (CadnaA uses 3D distance for K1).
- In CONCAWE, chapter 5.1.4, regarding the attenuation term K4 nothing is said whether to use 2D or 3D distance (CadnaA uses 2D distance).
- In CONCAWE, chapter 5.1.5, the attenuation term K5 in the equation for psi the distance d must be the 2D distance (as 3D distance does not make sense).
Type of Wind & Wind Data
CONCAWE does not specify what kind of wind regarding stability class, wind direction and wind speed is to be used. For example, it does not say whether the wind data should be based on average values, on peak values, or on the „representative/dominating wind“ at a location or in an area. Further, it is not said in CONCAWE at what height the wind direction and speed shall be assessed.
Furthermore, CONCAWE does not address the objective on how to proceed in order to assess a long-term average level based on several distinct wind situations.