Since 1995 Continental has not only used microphone measurements in some points in space but also the technique of spatial transformation of sound fields (STSF) to analyse the noise sources of a tire in great detail in the frequency domain and to calculate the sound in the far field. Clearly the eigenmode of the standing tire could be identified from the intensity measurement. For the rolling tire also a clear vibration pattern could be found. A drawback of the acoustical holography is that only planar views of the sound field can be calculated. After trying the STSF technique (NAH in the frequency domain) and NS-STSF (NAH in the time domain) in planes near the tire the next logical step is to measure around the complete tire and with a Hybrid Analysis-Test Tool (HATT) developed in the context of the European Project In order to calculate the tire vibration with the IBEM (Inverse Boundary Element Method) approach an accurate modelling of the outer surface is essential. In real-life operation the tire is inflated and loaded. Therefore, the surface is different from these in the unloaded case whereas the deformation of the tire depends on the tire design and the service load. Especially at higher frequencies a modelling of the surface without taking the real deformation into account can cause unpredictable errors. An experimental determination and the following modelling of the tire surface would be a very time consuming procedure thus the outer surface of the tire is obtained by a FEM calculation of the loaded tire. The validation of the hybrid calculation is shown on the microphone planes of the measurement areas. The comparison of the experimental and the calculation results at 160 Hz shows a very good agreement. Further validation of the procedure was done on a stationary tire excited by a shaker. With accelerometers the vibration on the surface of the tire was measured and compared with the calculated values from the hybrid approach. As an example the acceleration flux at 114 Hz during random excitation is displayed. At this frequency a vibration pattern with 4 nodes could be identified and the calculated levels agree quite well with the measured accelerations. At a speed of 40 km/h a vibration pattern with 4 nodes was found at 108 Hz. To identify other vibration patterns the IBEM calculation must be done for many frequencies which is very time consuming and not very efficient. Therefore another Method to analyze the measurement results is presented, which works with the wave decomposition of acoustic equivalent source models.


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