Nordic Prediction Method (1996)

Lateral Diffraction

Three options for lateral diffraction are available for selection:

none: no lateral diffraction considered, just the direct path is evaluated.
only one object: This option has in conjunction with NPM the same effect as „none“ and should, therefore, NOT be used.
some objects: In conjunction with NPM this option causes the two shortest possible convex rays around the first diffracting edge to be determined and used in the calculation.

Note

Due to restrictions in Nordic Prediction Method, lateral diffraction occurs just at the first diffracting edge. Consequently, a further barrier behind a first barrier in ray direction will not cause a lateral diffraction. Also at buildings (being multi-point objects) no lateral diffraction occurs.

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.

Obstacles withinArea Source 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.

Temperature / rel. Humidity

The air attenuation is calculated based on the temperature and relative humidity as given in annex B of the paper DAN32-1982. Intermediate frequency values result from linear interpolation. The temperature specified serves also as the ambient temperature when calculating the exhaust stack’s directivity.

Additional Information

Diffraction Model

The screening model for industrial noise of Nordic Prediction Method DAN32-1982 covers the diffraction effect with just a single screening edge. With two or more edges, there is no specification on how to apply the concept using the additional height Δh to multiple screening edges.

In section 4.5.1 of DAN32-1982 it says:„The general principle in calculating the screening correction, ΔL_s, is to identify all screening obstacles between source and immission point. ... The calculation procedure depends on the number of screens present. Usually only one screen is taken into account. ΔL_s is then calculated according to section 4.5.4. If more than one screen are present, the procedure in Appendix C [„Screening by more than one screen“, see section „Multiple Screens“ below] should be applied.“

Section 4.5.4 describes the procedure for a single screening edge. The additional height Δh above the direct path is calculated from the horizontal distance (2D-distance) from source and receiver, respectively. The remaining height to the barrier’s top edge is the so-called „effective“ height h_e which is used to evaluate the path difference.

Lateral Diffraction

For the lateral path it is stated on page 37: „When a building is represented by a single screen, δ_r and δ_l [path differences] are calculated for the edges yielding the highest values.“

This expresses that the diffraction model cannot handle several edges neither for the direct (vertical), nor for the lateral paths (horizontal). In figure 4.5.7 (sectional view) just the first edge of the building is addressed (as if it would be a flat barrier).

Multiple Screens

Annex C describes for an arrangement of multiple screens with respect to the direct path on how to evaluate „the two most effective single screens“ („effective“ with respect to the ray’s elevation angle is meant). However, nothing is said on how to account for lateral diffraction in this case.

In note 1 it says: „It must be emphasized that the recommended procedure is of rather speculative nature ...“

As annex C does not address the application of this scheme to the lateral paths in an arrangement with multiple screening objects, CadnaA applies the lateral path calculation to the first screening edge only.

Note

Applying the „ribbon band method“ for multiple objects (as with ISO 9613-2) would cause a level difference compared to the procedure used for a single screening edge only.

Example

Comparing lateral diffraction using ISO 9613-2 and NPM.

Object	ISO 9613-2	Nordic Prediction Model (1996)
building (height 100 m), option „some objects“ for lateral diffraction selected
	lateral diffraction occurring at 1st and 2nd vertical edge	no lateral diffraction due to the restrictions of the diffraction model
two barriers behind each other (height 100 m each), option „some objects“ for lateral diffraction selected
	lateral diffraction occurring at 1st and 2nd barrier	lateral diffraction occurring at 1st barrier, but not at 2nd barrier

Attenuation due to Vegetation

The Nordic Prediction Method distinguishes between a single and multiple vegetation areas in the ray path. For a single vegetation area, the attenuation depends on the length of the ray passing through the foliage (considering the additional height Δh above the direct path). For multiple vegetation areas, the attenuation depends on the number of vegetation areas, with a maximum of four.

In CadnaA, the attenuation ΔL_v for a single and multiple vegetation areas is always calculated based transmission path length d_v (considering the additional height Δh) and not on the number of groups of vegetation:

\(\Delta L_v=-n_v*a_v\) with \(n_v = \frac{d_v}{50} \le 4\)

with

d_v: transmission path length (m)
α_v: attenuation coefficient accord. to table 4.6.1 of DAN32-1982

Attenuation due to Internal Scattering

The object „Built-Up Area“ in CadnaA applies the algorithm for so-called „internal scattering ΔL_i“ given in Nordic Prediction Method, annex D. The attenuation ΔL_i“ calculates from:

\(\Delta L_i = -\alpha_i*\Delta d_i \ge -10 \:dB\)

with

Δd_i: transmission path length (m)
α_i: attenuation coefficient accord. to table D of DAN32-1982