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Nordic Prediction Method (1996)

Calc exactly one Reflection Order

With this option deactivated the maximum order of reflection is specified via the „Reflection“ tab (see Reflection Tab).

see also Nordic Prediction Method 1996

Calc outer Lanes separately

By default, the center lines of the outermost lanes of a road are the emission lines (at a height of 0.5 m). As in NPM just the road’s center line is the emission line, this option has to be deactivated.

Note

When active, the emission level of each outer lane is -3 dB compared with the road‘s axis being the emission line. The total emitted sound power of the entire road is, however, the same in both cases (see Calculation Protocol).

Note

According to NPM, when screening occurs, the source location for receivers behind the barrier is shifted to the barrier’s top edge.

Parameters for calculation of maximum Pass-By-Level

Select evaluation parameters LmaxD/E/N to calculate the maximum Pass-By-Level or the maximum level for the n-th vehicle passing by (see Evaluation Parameters Tab).

The A-weighted maximum sound pressure level LAFmax is an additional descriptor of noise from road traffic in Nordic Prediction Method NPM525-1996. The different corrections are calculated in complete analogy with those of the A-weighted equivalent continuous sound pressure level LA,eq with the one exception that the shortest distance source-receiver is used instead of the distance of calculation.

To calculate maximum levels, CadnaA moves a point source along each road according to its speed and specified sampling interval. The source is starting from either ends of the road due to the two emission lines representing the road where different corrections may apply (e.g. gradient correction). The maximum level for this point source for each position with the specified sound power level and length is calculated.

LAFmax X%

Enter the percentage of vehicles X% exceeding the calculated maximum level LAFmax (default value: 5%). For details of the calculation see Nordic Prediction Method 1996, LAFmax X%, 10m.

LAFmax N-th vehicle

By this option the maximum level for the N-th vehicle is calculated (e.g. for the 5th loudest vehicle during night enter N=5). For details of the calculation see Nordic Prediction Method 1996, LAFmax, N-th vehicle.

Option „Norwegian Method“

With this option enabled, the former limit of 10 or less cars/trucks during night (using X%=50% for X%>50%) is replaced by the X%=60/70/80/90/99,99% value of the normal distribution. Thus, for low number of vehicles, the level LAFmax,N-th vehicle depends on the number N entered. This approach, however, is no longer compatible with the original Swedish procedure.

Sampling Interval (s)

In order to determine the highest level, CadnaA shifts the source along the road in steps based on the sampling interval (in seconds). The default value for the sampling interval is 0.1 seconds. Consequently, the resulting distance difference between two source positions is:

\(s=v*t_{\text{sampling}}*\frac{1000}{3600}\)

with

  • s: distance steps (m)
  • v: speed (km/h), as entered on dialog Road for „Autos“
  • t: sampling interval (s)

Example: The maximum level for the road is calculated using the default sampling interval of 0.1 seconds. The maximum speed for autos is 100 km/h resulting in a sampling distance of:

\(s=100*0.1*\frac{1000}{3600}=2.78\:m\)

The stated maximum level is the maximum level from all sub-sources.

Calculation of Numbers above Threshold (NATs)

This option enables to calculate the number of events NATd/NATn above the entered threshold level from the hourly traffic counts (for details of the prescription see Nordic Prediction Method 1996).

With the option „Calc maximum Pass-By-Level“ not active a threshold level of 70 dB(A) is applied always. By activating this option any other threshold level can be entered.

Further, the following additional settings are required:

  • The evaluation parameters NATd (for Day) and NATn (for Night) are selected, e.g. as 3rd and 4th evaluation parameters (see Evaluation Parameters Tab).
  • The length of the evaluation periods Day (e.g. 16 h) and Night (e.g. 8 h) periods has to be set on the „Reference Time“ tab by entering the letters D/N for the respective hours (see Reference Time Tab). This holds especially when using diurnal patterns as road traffic figures.

Note

When entering the emission level LAeq*,10m directly neither the maximum Pass-By-Level, nor NATs can be calculated.

Calc planes of reflection

When this option is activated, the shape of terrain is approximated by at least a single or several planes of reflection when calculating the ground effect.

Note

The use of reflection planes is required with strict application of Nordic Prediction Method (NPM).

The intention with this approach is to model the direct path and the reflected path (as reflected by the ground) separately. Furthermore, NPM states that:

„If more than one valid plane of reflection can be found, the one giving the highest noise level shall be selected.“

The principle of the procedure to construct the plane/s of reflection as simple terrain slope is studied (see figure below).

Constructing the reflection plane in Nordic Prediction Method (NPM)

The black line represents the terrain slope between source and receiver. In this simple scenario, two reflection planes can be constructed (each with different source and receiver heights referring to the respective reflection plane). According to the above rule the reflection plane giving the highest level for each receiver point is relevant.

In conjunction with grid calculations the effect of this rule becomes obvious: At some grid point the relevant reflection plane switches from one to another causing a level jump on the grid (see figures below).

Vertical grid: road on plane ground, reflecting ground (G=0), highest level occurs from reflection plane 2 (see before) for the entire grid, no ground effect to the right of the road as highest level occurs from plane 2 assuming even ground

Vertical grid: road on slope, reflecting ground (G=0), considering reflection plane 1 (see above) causing ground effect to the right and level jumps to the left of the road

Maximum Deviation (m)

According to Nordic Prediction Model, section 2.4.1.4, the vertical deviation from the real terrain shall not exceed +/- 1 m. In addition, it is stated: „+/- 0.5 m desirable for „hard“ ground“.

In CadnaA, the default value is 1 meter. The smaller this value the more reflection planes are to be generated, increasing also the calculation time.

Use Correction for Thick Screen DeltaLts

By activating this option the screening model is extended by the „Correction for Thick Screens“ DeltaLts according to part 2, section 2.6.4.

hb interpreted as source height

Nordic Prediction Method specifies (section 2.4.2, equation 2.37 NPM525-1996) the height hb - relevant in the calculation of DeltaLm and DeltaLms - as „height (m) of the elevated road relative to the plane of reflection“. In a Swedish reference software („Buller Väg“), however, the height hb was interpreted as source height.

With new projects this option is OFF by default. In former CadnaA files (prior to version 2019 MR1), however, this option is ON since this was the previous common interpretation used. Thus, in order to receive the results as with former versions of CadnaA activate this option.

Other Properties: See Industry

This hint refers to the following settings on the „Industry“ tab:

  • option „lateral diffraction“
  • option „if distance smaller“
  • temperature/relative humidity

Further Information

Ground Absorption

In NPM, the ground can either be acoustically „soft“ or „hard“. There are no specifications for intermediate ground properties.

In CadnaA, the ground factor G is used to distinguish both cases (see Ground Absorption Tab):

  • soft ground: G > 0.5 (enter G=1)
  • hard ground: G <= 0.5 (enter G=0)

According to NPM, the valid ground type for the situation is the ground type (absorption factor G) in the area of the reflection point +/- 10 m. In CadnaA, it depends on the ground absorption factor between the source line and the reflection point (resp. line) whether soft or hard ground is considered.

Option „Roads/Parking Lots are reflecting (G==0)“

Since NPM does account for mixed ground, the option „Roads/Parking Lots are reflecting (G==0)“ on the „Ground Absorption“ tab (see Ground Absorption Tab) should be deactivated. Otherwise, in case of default G==1 and this option activated propagation effects will result for receivers or grid points close to or in direction of the road‘s axis (or of road segments). In this case, these rays cause a higher level due to reflection by the road surface along their path. This will lead to unsteady noise contours and level jumps.

Default G==1 and option „Roads/Parking Lots are reflecting (G==0)“ activated: levels increase or level jumps for receivers/grid points in direction of the road‘s axis

Distance Correction

The distance correction in NPM consists of two parts:

  • the „average distance correction“ DeltaLav according to equation 2.34 and
  • the correction for „short distance to the road“ DeltaLka according to equation 2.54 (both in NPM525-1996).

Note

DeltaLka is a function of the (perpendicular) distance a and the road width 2b (see chapter 2.5.7 of NPM525-1996). This means that the result depends on the road width entered in CadnaA, even when there is just a single emission line.

In CadnaA, both corrections are only applied when the option „Calc outer Lanes separately“ is deactivated (i.e. „strictly NPM“). With the option activated the distance correction is just based on DeltaLav. In both cases, the relevant distance is the perpendicular (!) distance to the road‘s axis and not the distance from the (partial) source to the receiver.

Correction for Angle of View

As NPM is basically a „long infinite road“ model it requires an angle correction to correct for the finite length of a road section. This angle correction DeltaLalpha (see equation 2.51 NPM525-1996) disappears for receiver locations in direction of the road (with α=0°).

Note

The angle correction does not apply when calculating maximum levels (see NPM525-1996, chapter 2.6.3).

CONCLUSION

Provided the option „Calc outer Lanes separately“ is deactivated, both, the two parts stated in NPM for the overall distance correction, may - in conjunction with the angle correction - lead to unexpected level jumps and weird grid contours close to the source. This is immanent to the NPM model and not a CadnaA specific effect.

Combined Assessment Road/Railway

In case of assessments with combined impact by road and railway noise the maximum level of both has to be evaluated according to local legal provisions. In CadnaA, this can be handled within the configuration, „Evaluation Parameters“ tab (see Evaluation Parameters Tab), by using the expression „max(strd, schd)“ for the evaluation type „f(x)“: