Optimisable Source

Note

This type of source requires option BPL. BPL stands for „Back-Tracing of Power Levels“.

This object enables to enter an area source the emission of which is maximized by an optimization procedure. The characteristics of this type of source are:

In the calculation, the area of the optimisable source is segmented into sub-areas in the same way as with the general area source (see Industrial Sources) taking into account the receiver distance and screening objects. Each sub-area is replaced by a point source.
The configuration option on the „Industry“ tab „Obstacles within Area Sources do not shield“ (see Industry Tab) shall be activated when determining noise quotas using the optimisable source. This will cause screening objects inside the optimisable source’ area not to affect its emission.
On the dialog Optimisable Source, switch from Day to Night depending on what period shall be used in the optimization. The specified sound power refers to the selected period (sound power level PWL or sound power level per unit area PWL“).
The further input parameters at the bottom of the dialog differ from the "normal" area source. These parameters are required for the optimization with respect to the allocation of noise quotas (or noise allotment).

Dialog options

Period Day/Night

Select the daily period which shall be used in optimization (Day or Night). The specified sound power refers to the selected period.

Note

The optimization procedure in conjunction with noise allotment occurs in CadnaA for the levels Ld (Day) and/or Ln (Night) only. This means that mixed level (such as Lden or Lde) cannot be used for an optimization calculation, since the emissions of the partial periods can not be optimized separately. Therefore, make sure that the levels Ld and Ln are selected for the first two evaluation parameters on tab „Evaluation Parameters“ (see Evaluation Parameters Tab).In case a mixed level is selected as the first evaluation parameter (e.g. Lde), the optimization occurs, however, for the level Ld. When the optimisable source is converted afterwards into a normal area source, the receiver level increases by 1.93 dB assuming that the same emission level Day=Evening/Recreat.Time was specified and, for example, with 13 hours for the Day period, 3 hours for the Night period and a penalty of 6 dB for the Evening/Recreat.Time (here with Ld=Le=60 dB (A)):

$L_{de}=10\lg\frac{1}{16}(1310^{Ld/10}+310^{Le+6/10}) = 61.93 dB(A) $

Therefore, when optimizing the daytime level Ld and after having converted the optimisable source into an area source whose emission shall be - in this example - decreased by 1.93 dB in order to respect the limiting value given as Lde, again assuming the same emission during Day and Evening/Recreation Time.

Power Level PWL or PWL"

Enter alternatively the sound power level PWL or the the sound power level per unit area PWL".

Option „Apply immediately"

This option is activated by default. In this case, a change in emission cause an immediate effect.

Salient Point of Usability Function

The usability function assigns a usability value w% to any sound power level per unit area. The continuous shape of the usability function is approximated in CadnaA by two straight lines and a salient point.

Range PWL",min to PWL",max

Via these input boxes the emission of the optimisable source can be restricted:

PWL",min: This value is the minimum required emission. It is determined based on the absolutely necessary noise-relevant processes (e.g. a 10-minute truck ride across an area of 1000 m² results in a PWL"= 55 dB(A)).
PWL",max: This value is the maximum exploitable emission (depends on the branch of industry and may be up to 80 dB(A) for industrial installations with permanent truck traffic).

After optimization the slider control bar is at the location of the resulting emission value the value of which is displayed on the top of the dialog.

Button „Directivity“

Clicking this button opens a dialog to select a 2D directivity (see User-defined Directivities).

The coordinates xy of the reference point define the location within the BPL source where the selected 2D directivity is positioned.

Note

By defining a 2D directivity in angular sectors, the procedure according to DIN 45691, Annex A.2 „Increase of emission quotas for individual directional sectors“ (translation from German, see 45691-2006), can be approximated.

Button "Optimize"

When clicking this button the sound power of the actual optimisable source is optimized with respect to all active receiver points. In this case, the emission of all further optimisable sources is no changed. To optimize all optimisable sources in a project at once select the command Optimize Area Sources on the Calculation menu.

Noise Allotment

CadnaA supports this optimization in a flexible manner without applying a specific fixed optimization strategy, but by use of user-definable usability function. The optimization procedure aims at determining the permissible emissions, ensuring that

the relevant limiting values at receiver points in the vicinity are respected, and
the intended use of the individual areas is not restricted as far as possible.

Usability Function

In order to achieve this, a usability function is assigned to each area.

The decreasing slope of the function with increasing sound power level means that a further increase in emission cannot be exploited due to the intended use. On the other hand, the usability decreases excessively when a particular emission required for the intended use is further reduced.

Power Level PWL" (dBA) at the salient point: The emission value where the gradient of the usability function changes. A reduction of the power level by 1 dB below this point results in an even stronger reduction in usability than the same reduction above the salient point. Enter here the value that should be "retained" in the optimization (default value: 60 dBA).
Usability K (%) at the salient point (default value: 80%)

Due to this concept the individual requirements can be accounted for in the maximization Pro-Lärmkont. The usability function of CadnaA is approximated by two lines. This requires three emission values for each partial area as a basis for the maximization. The usability specified at the turning point determines the curve and thus the „maximizing strategy“. Common data for industrial areas is:

PWL’’ min	57 dB(A)
Turning Point	80 % / 60 dB(A)
PWL’’ max	65 dB(A)

The bottom part of the edit dialog for the optimisable source contains a horizontal scroll bar enabling to select an emission value within the range from min to max like. The current value which is displayed in the dialog above. For a first test, push the slider to the extreme right, thus selecting the maximum emission value.

Calculation

The most critical receiver points are entered with their relevant limiting levels. Clicking the calculator icon on the toolbar starts the calculation. If no receiver turns red either for day, nor for night, this means that the limiting levels are respected using the maximum value assumed. In this case, no further reduction of the emission is necessary.

In the calculation, all sources existing in the project - roads, railways, industry or aircraft noise - are taken into account. By the deactivating objects using groups (see Groups) different scenarios can be analyzed. The procedure described so far is the same as for the calculation of ordinary area sources.

So far, the described procedure does not differ from the calculation of normal area sources.

Manual Optimization

The red color shows that the limiting level is exceeded at a receiver point. In this case, reduce the emission of the optimisable source with the strongest noise impact on the relevant receivers in steps.

CadnaA does not require a new calculation for each step. Double-clicking the respective optimisable source and shifting the position of the slider will show the resulting change at the receivers immediately. Depending on the desired strategy, the emission values can be reduced until compliance with the standard levels is ensured at all receiver points.

Apply this procedure separately for day-time and night-time as required.

Automatic Optimization

Selecting the command Optimize Area Sources on the Calculation menu causes to start an automatic, iterative reduction of the emission values. It is always the reduction of emission resulting in the smallest decrease of the area-related usability according to the concept explained above which is carried out in this step-by-step procedure.

With

Si	area in m² of the ith partial area
wi	area-related usability in % of the ith partial area

the total usability is

$w=\frac{\sum w_i * S_i}{\sum S_i}$

This value is displayed at the beginning and the end of the maximization. After optimization, the position of the horizontal scroll bar represents the emission optimum value and is displayed in the top of the dialog.

Note

The iteration step width is 0.1 dB by default. Alternatively, the iteration step width can be controlled using a local text block (Name: BPL), for example, in order to optimize the calculation time. The text DELTA_L=1.0 sets - for example - the iteration step width to 1 dB.

The result calculated by CadnaA needs not be the best if further aspects, not included in the usability function, are to be considered. In this case, the automatic optimization is a suitable starting point, whereas manual modifications of the emission values using the slider enable to further adapt the scenario in the required way.