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It can be used for different and even complex configurations and may help to integrate this very special noise source in environmental modelling for noise calculation.Introduction The radiation from tunnel-mouths has been studied in some papers theoretically and practically /1 9/. In the following the procedure is used as a part of an integral approach to calculate the noise radiated from tunnel openings. These techniques have been standardized /13/ and in the meantime after about 8 years of experience with the application of these strategies they form an experimentally well and computer based method that can be used to solve many of these special problems.
These techniques allow to take into account the sound diffracted at the tunnel edges by an adequate directivity of the tunnel mouth source.Problem definition Roads and railways are guided through tunnels to prevent sensitive living environments from noise exposure or to cross under hilly terrain to avoid costly mountain roads. The application of such ray tracing techniques offers the advantage that even complex tunnel constructions with partially absorbing covered surfaces can be treated. It is shown that the application of ray tracing techniques provide exactly the same results, if this time averaging is applied. As it is shown later only based on these conditions the sound field in a tunnel is diffuse this is not the case if the sound distribution inside the tunnel produced by a moving car or train shall be predicted.
It can be taken into account if this part ds would be included in the calculation. The first part is radiated from the part ds of the track inside the tunnel that can be seen from the receiver and propagates without diffraction. Related to a given receiver position near the tunnel opening three different sound parts have to be taken into account. But the sound radiated from the tunnel mouth into the environment may produce a level increase that can not be neglected.
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With rectangular cross section( )baU += 2 (1) with half circle circumference with radius rRU )2( += (2) If N traffic lines or tracks with emission values LE,n are inside the tunnel, one emission value is calculated from( )dBL NEEE LLLEmission ,2,1, 1,01,01,0 10.1010lg10 ++= (3)Figure 4 Railway tunnel with one and road tunnel with two ductsWith example figure 4 the emission values of the railway tracks have to be added according to equation (3), while the emission value of each road has to be assigned to the relevant duct. The tunnel is characterized by its cross section a x b if rectangular - or by the circumference U of the open cross section and by the energetic sum of the emission values of all tracks or traffic lines inside. According to national or other calculation guidelines these emission values are produced by the software applied from the acoustically relevant parameters like traffic flows, speeds and road surfaces. It depends on the absorption of these surfaces up to what orders these reflections play a role.Figure 3 Reflected sound radiated from inside the tunnel to the receiver.How to take a tunnel opening in noise prediction calculations into account Roads and railways are characterized by an emission value that may be different for different time intervals e. It can be determined by subdividing the track in small parts and by applying a usual diffraction calculation.Receiver Figure 2 The direct sound from inside the tunnel diffracted at the tunnel edgeThe third level contribution is caused by sound that is reflected at the inner surfaces.
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