## 4 Discussion
This in turn leads to a larger disturbance of these sensors, which ultimately results in a larger deviation of the chord method from the METAS reference. During isolated measurements, sensors \(1\) and \(4\) even reached the edge of the measuring range and generated an error signal, which in turn also negatively influences the corresponding chord methods. For reduction of the negative influence of displacements or tilting on the measurement results, it would be conceivable to add additional accelerometers and a gyroscope in the form of an IMU (inertial measurement unit) to the setup. With additional information about the applied disturbances, the measurement data could be corrected. Applying a vibration to the sensor plate significantly worsens the result of the single sensor. This can be observed already for low applied amplitudes. The chord method is not negatively affected. This statement is valid regardless of the applied frequency. Applying a filter to the data set to remove the influence of vibration from the signals would be possible. However, the frequency of the vibration is usually not known, and depends on the speed of the train, for example. On the train, different vibration frequencies would act on the structure at the same time. In this study, only one frequency was applied at a time, but it can be assumed that vibrations would not influence the measurement result in the case of a chord method. The use of four sensors provides several advantages over the use of fewer sensors. Most importantly, the strengths of the individual chord methods can be combined to achieve a better result. For longer wavelengths, this setup would offer the advantage to evaluate six additional two-point methods. The optimized acoustic roughness approach can improve accuracy. The inherent redundancy of a four-sensor arrangement provides robustness, as it can be used to check plausibility of the obtained results by comparing the chord methods. This could be especially relevant in the event of a sensor failure. Considering future developments of the setup and methods, an alternative sensor arrangement with different sensor distances could still lead to further improvement in measurement quality. A symmetrical chord method (\(a=0.5\)) could be added to the arrangement, and thus complement the asymmetric chord methods. The method selection per each one-third octave band is currently limiting the advantage of the optimized acoustic roughness approach. Instead, the selection of the chord method could be done for each narrow band individually, which could lead to a further improvement. The test stand in the laboratory does not take into account two effects, which could be relevant for a measurement on the train. One is the hunting oscillation of the train, which, according to [PERSON] [29], could lead to lateral displacements in the range of up to \(4.25\) mm. The effect of rail vibration, which according to [PERSON] et al. [30] also depends on the characteristics of the vehicle, is an external disturbance, which could not be investigated with the above-described test bench. The entire test stand could be vibrated. However, since the measurement setup is directly mechanically connected via the linear guide, no distinction could be made in this scenario between the vibration of the measurement platform and the vibration of the rail. This should therefore be tested under real conditions. The identical applies to rail deflections due to the contact force of the wheelset. The described laboratory environment in which the tests were carried out did not allow measurement at higher velocities, due to the limited length of the test rail. For further investigations, the velocity should be increased significantly to practical values on a train. A higher velocity has an influence on the data point distance, which significantly increases. EN 15610 [9] specifies a maximum value of 1 mm.