# NORD2000 VS. THE EXISTING NORDIC PROPAGATION MODELS

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2 GROUND EFFECT The reflection from the terrain has great influence on the frequency spectrum of the entire transfer function from source to receiver. The interference between direct transmission and ground reflections causes the frequency spectrum to change strongly with the absorbing properties of the surface as well as the distance from the source. The old methods characterized the ground properties either as soft of hard whereas NORD2000 defines 7 ground classes. The flow resistivity σ (normally in kns/m 4 ) is used as a parameter. The interference between direct and reflected sound creates a series of peaks and dips where the two waves are in-phase or out of phase. The lowest frequency at which the waves are 180 out of phase will be the most important to the ground effect. In Fig. 2, 3 and 5 some tendencies can be read. The figures show the ground effect (the sound pressure level relative to free field) as a function of frequency. source above a flat and hard ground, yet the shape of the contour lines are very complex due to the ground effect. SPL re free field, db Hr = 5 m Hr = 2 m Frequenc y, Hz Fig 3. Ground effect. Parameter: Receiver height In Fig. 2 the source height Hs and the receiver height Hr is both 2 m and σ = kns/m 4 (hard ground). The distance between source and receiver is the parameter. The interference dip is shifted towards higher frequencies with increasing distance between source and receiver Fig 4. Vertical noise map, point source, hard ground, NORD2000 S PL re free field, db d = 50 m d = 200 m In Fig. 5 Hs = 2 m, Hr = 2 m and d = 50 m. The flow resistivity is the parameter. The dip is shifted towards lower frequencies with increasing softness of the ground Fre quen cy, Hz Fig 2. Ground effect. Parameter: Distance between source and receiver. In Fig. 3 Hs = 2 m, d = 50 m and σ = kns/m 4. The parameter is the receiver height. The interference dip is shifted towards higher frequencies with decreasing receiver height. However, the depth of the interference dips varies with frequency in a very complex manner. This is the main reason why noise maps calculated with NORD2000 in general show very irregular contour lines. One example is shown in Fig 4. This figure shows a simple set up, an point S PL re free fi eld, db σ = 500 σ = Frequenc y, Hz Fig 5. Ground effect. Parameter: Flow resistivity in kns/m 4

4 (assuming wind speed 5 m/s and receiver height 2 m). This was expected since GPM is a downwind method. On the upwind side however, considerable discrepancies occur. At 200 m distance from the source GPM returns a result that is 6-7 db higher than the result of NORD2000. At 300 m the difference is more than 10 db. Especially on the downwind side the contour lines are irregular. This must be explained by the ground effect and the fact that the wind has a different effect on the direct and the reflected sound. SCREENING The screening effect in NORD2000 is based on existing theory although this theory has been modified in certain aspects. In order to avoid on/off effects for receivers near the border of the barrier shadow the height of the screen is evaluated using a Fresnel-zone approach. The side diffraction around vertical edges is also taken into account but will be no further discussed in this paper. A big difference to the old methods appears when weather effects are combined with screening. Fig. 12, 13 and 14 show vertical noise map for a point source between two screens. The source height is 1 m and the screen top is 10 m above the terrain (flat and hard). Fig. 12 is done with GPM and Fig. 13 with NORD2000 assuming a westerly wind with 5 m/s. Fig 12 Vertical noise map, point source between two parallel and non-reflective screens. GPM. Fig 10. Horizontal difference map, GPM NORD2000. Hs = Hr = 2m. σ = kns/m 4. 5 m/s wind from west. Fig. 11 is a vertical difference map of the situation in Fig. 10. From Fig. 11 it can be seen that the large discrepancies between GPM and NORD2000 appear only close to the ground (in the shadow zone ). The source is in the lower right corner of the picture, only the upwind side of the situation is shown. Fig 13 Vertical noise map, point source between two parallel and non-reflective screens. NORD2000, westerly wind 5 m/s. Comparisons between Fig. 12 and 13 clearly demonstrate two facts. First, the contour lines produced by NORD2000 are not as smooth as we are used to from the old methods, here GPM. This is caused by the ground effect. Secondly, GPM overestimates the level on both sides of the screen, especially on the upwind side. This is even clearer in Fig. 14 that shows the difference between the results in Fig. 12 and 13. Fig 11. Vertical difference map, upwind side, GPM NORD2000. Hs = Hr = 2m. σ = kns/m 4. 5 m/s wind from west. Fig 14 Vertical difference map, Fig 12 Fig 13.

5 Please note this: For conventional reasons GPM normally restricts the screening effect to 20 db. Since NORD2000 has no such limit (so far) this restriction has been omitted here. Screens REFLECTIONS NORD2000 handles reflections by introducing a mirror source. This is not different form the existing Nordic methods. Fig. 15 shows how the position of the mirror source is found. To ensure continuity when the reflection point moves from inside to outside the building again a Fresnelzone approach is used in NORD2000. This is new. The set-up in Fig. 15 would cause no reflection according to the old methods since the reflection point falls outside the building. In NORD2000 the reflected energy is proportional to the active part of the mirror. Source Fig. 16 Top view of the set-up Fresnel ellipsoid S Fresnel zone M i r r o r A c t i v e I n a c t i v e R * S Fig. 15 Illustration of the active and inactive part of a mirror Fig 17. Horizontal noise map showing only the contribution from the first reflection. Hs = Hr = 1m. σ = kns/m 4. GPM. The effect of this Fresnel-zone approach is illustrated in Fig. 17 and 18. A plan of the set-up is shown in Fig 16. The set-up includes a point source and two (very high) screens. The screens are vertical and the surfaces are fully reflecting. The ground is flat and hard and the source is 1 m above the terrain. The shown noise maps are valid for receivers 1 m above the terrain. The noise maps in Fig. 17 and 18 only includes the contribution from the first reflection, i. e. the direct sound is omitted. The size of the Fresnel zones are frequency dependant, and therefore the size of the active and inactive part of the reflectors also depends on the frequency in question. To make the effect more clear in the figures only the octave 63 Hz is shown. Fig 18. Horizontal noise map showing only the contribution from the first reflection. Hs = Hr = 1m. σ = kns/m 4. NORD2000.

6 From Fig. 17 and 18 it is quite clear that NORD2000 generates a much smoother shift from reflection zone to free field. This fact contributes to more natural looking noise maps with NORD2000, especially in areas where the direct transmission is shielded. CONCLUSION From the test cases shown in this paper it can be concluded that the capability of NORD2000 offers several advantages when it comes to noise mapping. Weather conditions can be taken into consideration and screens/reflectors are handled in a way that will lead to more reliable and natural looking noise maps than if the old methods were used. On the other hand the complex ground effect will contribute to irregular contour lines. This problem however, will mainly occur in situations with one or a few point sources. With line sources such as roads it will be a minor problem. Note: this paper is revised on September 3, REFERENCES [1] B. Plovsing, J. Kragh, Nord2000. Comprehensive Outdoor Sound Propagation Model. Part 1: Propagation in an Atmosphere without Significant Refraction, DELTA Acoustics & Vibration Report AV 1849/00, Lyngby 2000 and part 2: Propagation in an Atmosphere with Refraction, DELTA Acoustics & Vibration Report AV 1851/00, Lyngby [2] H.G. Jonasson, S.Å.Storeheier: Nord New Nordic Prediction Method for Road Traffic Noise, SP Swedish National Testing and Research Institute, SP Report 2001:10, Borås 2001 [3] H.G. Jonasson, S.Å.Storeheier: Nord New Nordic Prediction Method for Rail Traffic Noise, Swedish National Testing and Research Institute, SP Report 2001:11, Borås 2001

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