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62
Spécial “ 5es Assises sonore ”
Acoustique
&
Techniques n° 51
Noise mapping for railway noise: assessment of NMPB method as implemented in different software, comparison with RMR method
Then, the difference in terms
of sound pressure level and
spectrum leads to a difference
in the isophones curves as
presented figure 5. In such a
case, NMPB overestimates the
levels compared to RMR.
Another fundamental difference
between both methods is the
number and the location (height)
of the sources used to define
the trains. For the high speed
trains, four sources located at
four heights (0,5 m, 2 m, 3 m
and 4 m) are proposed in the
Dutch method whereas the NMPB
uses two sources allocated at 0
m and 0,8 m. This difference in
the position can be significant
when calculating the effect of a
noise barrier as presented in the
following example.
Table 1 shows the attenuations
calculated for high speed trains
passing-by behind a 2.5 meters high
noise barrier located at a distance of 4
meters from the end rail. Attenuations
calculated with the RMR method are
considerably lower due to the sources
located on the roof of the train.
The figure 6 shows the corresponding
isophones. The influence of the global
directivity of the train source model is
clearly depicted.
Fig. 4 : Noise levels at the reference point (25 m from track centre and 3, 5
meters above ground) measured for fret train (+), and RMR category 4
calculated with CadnaA (
Δ
) and RMR category 4 calculated with Immi (o)
Niveaux de bruit au point de référence (25m du centre des voies et 3.5m
au dessus du niveaus du terrain) mesurés pour le train Fret (+), et sous
RMR catégorie 4 pour CadnaA (
Δ
) et RMR catégorie 4 pour Immi (o)
Fig. 5 : Noise levels for freight train at 100Km/h with absorptive ground and meteorological conditions 100% favourable.
Above NMPB Fret train, below RMR category 4 both calculated by means of CadnaA
Niveau de bruit pour un train calculé à 100 km/h avec un terrain absorbant et des conditions météorologiques
100% favorables. Train fret avec la NMPB, et RMR catégorie 4 calculés avec CadnaA
Attenuation
TGV-D (NMPB)
Attenuation
Category 9 (RMR)
d = 7,5 m
h=2m
15,1
8,6
h=3,5m
7,4
2
h=10m
-0.4
0
d = 25 m
h=2m
13,5
4,8
h=3,5m
13,6
4,7
h=10m
6.2
0
Table 1. : Attenuations calculated for high speed trains passing-by behind a 2,5 meters high
noise barrier located at a distance of 4 meters from the end rail (NMPB and RMR methods)
Atténuations calculées pour un passage de train à grande vitesse,
avec un écran de 2,5 m de hauteur et à 4 m du bord du rail (méthodes NMPB et RMR)
Fig. 6 : Difference in noise propagation of the high speed train noise behind a sound barrier (in black) with
the RMR method (left) and the NMPB method (right) presented with the same colour scale
Différences de propagation du bruit pour un train à grande vitesse avec un écran (en noir) : Les
méthodes RMR (à gauche) et NMPB (à droite) sont présentés sous la même échelle de couleurs