Nordic Prediction Method 1996
Evaluation Parameters
Two types of evaluation parameters are calculated according to Nordic Prediction Method (NPM) for railway noise (TemaNord 1996:524 NPM524-1996):
- the A-weighted equivalent continuous sound pressure level LA,eq and
- the A-weighted maximum sound pressure levels LmaxM (energy average over train length) or LmaxF (highest level with „Fast“ meter setting).
As the equivalent continuous level LA,eq is calculated per day (24 h) no specific day- or nighttime values exist. In contrary, a separate procedure is applied to calculate the maximum level LAFmax.
Emission Level L’wo
The emission parameter for evaluations based on Leq is the linear (unweighted) sound power level per unit length L’wo NPM524-1996:
\(L_{wo}'=a*\lg (\frac{v}{100})+10 \lg(l_{24h})+b+\Delta L_c\:in\:dB\:re\:10^{-12}\:W\)
with
- a,b: constants depending on train type and frequency
- v: train speed, in km/h
- l24h: total passing train length within 24 hours, in m
- ΔLc: track correction, in dB
Emission Level L’wt
The emission parameter for maximum level evaluations is the linear (unweighted) sound power level per unit length L’wt NPM524-1996:
\(L_{wt}'=a*\lg (\frac{v}{100})+10 \lg v+43.8+b+\Delta L_c\:in\:dB\:re\:10^{-12}\:W\)
with
- a,b: constants depending on train type and frequency
- v: train speed, in km/h
- ΔLc: track correction, in dB
Two types of levels are used to describe the maximum level of a train passage: the level LmaxM based on the energy average over train length, and the level LmaxF the highest level occurring at a passage with „Fast“ meter setting. For all frequency bands it holds:
\(L_{max\:F}=L_{max\:M}+3-(\frac{3d_c}{100})\:dB\), for electric trains
\(L_{max\:F}=L_{max\:M}+6-(\frac{3d_c}{100})\:dB\), for diesel trains
with dc distance receiver - train centre (m) and the further conditions:
dc > 100 m (electric trains): \(L_{max\:F}=L_{max\:M}\)
dc > 200 m (diesel trains): \(L_{max\:F}=L_{max\:M}\)
Note
In CadnaA, both types of evaluation levels are available, see Nordic Prediction Method (1996)
Track Condition
In case the option „Train Classes and Penalties“ is selected the track condition can be entered manually. The track correction ΔLc is also used to correct the emission level for joints, crossings, and bridges.
| Type of Track | ΔLc (dB) |
| ballasted tracks with continuously welded rails on concrete or wooden sleepers | 0 |
| rails with joints | 3 |
| 10 m track length for each unit of switches and crossings | 6 |
| partial track length on a bridge without ballast | 6 |
| partial track length on a bridge with ballast | 3 |
Note
CadnaA provides no list box for track conditions as NPM states ranges of the track correction, rather than distinct values.
Application of the track correction
In particular, the track correction ΔLc at switches and crossings requires some additional modeling effort as the correction does not apply for the entire (and arbitrary) length of a railway section, but just to a finite length of 10 m. For both, LA,eq and LmaxM or LmaxF calculations, an additional railway section of 10 m length has to be entered at the location of each switch or crossing having the same emission as the neighboring railway sections, but with a track correction of ΔLc=6 dB. Further, the following rules regarding it‘s geometry apply:
- with LA,eq calculations: The additional railway section representing a switch or a crossing replaces by its length the continuous railway track. In other words, the 10 m section cannot be drawn on top of the existing railway track as this would cause additional ray paths. Thus, a single railway segment needs to be broken into three peaces resulting in a single section of 10 m length in between two other sections (see Break into pieces in the manual „Introduction to CadnaA“).
Example
Path: Examples \Sources\Railway\ NPM 96\10 m switch correction.cna
- with LmaxM or LmaxF calculations: In this case, the additional railway section representing a switch or crossing can be entered on top of the continuous railway track. Deactivate the option "Extrapolate trains to 1/2 length on both sides of rail" (see Nordic Prediction Method (1996)).
Source Position
The source heights with respect to the octave band centre frequency are listed on the following table (approximative):
| Partial Source | Octave band centre frequency(Hz) | ||||||
| 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | |
| Rails | x | x | x | ||||
| Wheels | x | x | |||||
| Engine etc. (Diesel power trains) | x | x | x | ||||
| Curve screech | x | x | x | ||||
| Carriages, wagon (primarily goods trains) | x | x | x | x | |||
| Braking | x | x | x | ||||
| Source Position on the track centre line (meters above rail head) | 2 | 1.5 | 0.8 | 0.3 | 0.4 | 0.5 | 0.6 |
vmax (km/h)
When specifying a track-relevant maximum speed vmax the train speed will be adjusted if necessary. In the calculation the smaller of both values is used.
Train Types
In CadnaA, the following train types are available from the global library (Tables menu, Libraries (global)|Railway Groups).
| Abbreviation | Train Class Name |
|---|---|
| Npas | N-Pass El |
| Ngoo | N-Goods El |
| Nb65 | N-B65 Pass El |
| Nb70 | N-B70 Pass El |
| Nb69 | N-B69 Pass El |
| Sx2 | S-X2 Pass El Concr |
| Spas | S-Pass El Concr |
| Spaw | S-Pass/W El Wood |
| Sx10 | S-X10 Pass El Concr |
| Sgod | S-GoodsDi Diesel Concr |
| Sgoo | S-Goods El Concr |
| Fsm | F-Sm Pass El Concr |
| Fsr1 | F-Sr1 Pass El Concr |
| Fgoo | F-Goods mEl Wood |
| Rgoo | R-Goods mEl Concr |
| B71t | BM 71 El today maint |
| B72t | BM 72 El today maint |
| B73t | BM 73 El today maint |
| B71i | BM 71 El improved maint |
| B72i | BM 72 El improved maint |
| B73i | BM 73 El improved maint |
| B93D | BM 93 Di |
| SL79 | SL 79 Leddtrikk |
| Tban | T-bane |
| T2k | T-2000 |
| X31 | X31/32 Contessa |
| X52 | X52-53 Regina |
| Y31 | Y31 Itino |
| B71f | BM 71 flytoget |
| B74 | BM 74/75 |
Note
In case those train classes are not listed on the Tables menu, import the library „Train Classes“ from Type „Default“ into the global library.
Train Classes and Penalties
With this option activated, the types of trains and train classes, numbers of trains and corrections, can be edited locally, or an existing list with number of trains can be selected. CadnaA will automatically calculate the emission level from the specified values.
Note
In the following, the procedure to edit a local train list (with numbers of trains) is described. To enter a train list in the local library, see Common Input Data, Number of Trains.
Local Train List
Train Type
The train type is selected from the list Train Types on the Library (global)|Railway Group (also called „Train Classes“). By this selection all relevant parameters are specified.
Number of Trains Day/Evening/Night
Numbers of trains within 24 hours. With the same numbers of trains for time periods D/E/N the same power level L’wo,i D/E/N results (see also Nordic Prediction Method (1996)).
Speed v (km/h)
relevant speed of trains (km/h)
Length of one train (m)
length l of the respective train class (m)
Emission Lm,E,i (dB)
The emission level (sound power per unit length) L’wo,i for a train class resulting from the specified parameter is displayed here.