## 4 Discussion  
In a field study of homes, parallel indoor recordings of ground-borne vibrations and sounds were carried out for three tramway types in current use. To sufficiently account for the variety of the involved sound characteristics, an integrated psychoacoustic approach was applied. We found compelling support for the accuracy of the issued complaints (especially sleep interference) of the citizens and a poorer acoustical performance of the \"new\" introduced tram (Variobahn) when compared with the \"old\" tram (CityRunner) mainly in use before 2011. In addition, the \"very old\" trams showed lower median values of roughness. However, their number of pass-bys was small compared to the \"new\" trams and did not dominate the soundscape. To assess the potential risk of sleep disturbance the measured maximum indoor sound levels were used to estimate awakening probabilities. Based on the 42 dBA threshold value of the WHO-night noise guidelines [52], we computed the percentage of exceedances (see Table 10 in the Appendix A). The \"new\" trams showed significantly higher threshold exceedances (47%) compared with the \"old\" (20%) and even with the \"very old\" tramways (32%). The exceedances varied from location to location. At one location the threshold was even exceeded in 100 percent of the recorded events during nighttime. As with a large number of exceedances the awakening probability increases, these data support both the issued complaints about sleep disturbance and the citizen's perception of a deterioration after the introduction of the \"new\" trams. Furthermore, dBC-dBA-analysis (Table 1) revealed strong low frequency components: all sites showed differences larger than 15 dB, which is the recommended criterion when the prediction of annoyance is the main aim [53]. Three sites even met the stricter DIN 45680 criteria of 20 dB, which would clearly indicate the inappropriateness of the use of the dBA-weighting in such a case [54]. [PERSON] judged the use of the A-weighting in situations with strong low frequency components as inadequate and leading to incorrect decisions by regulatory authorities [49]. [PERSON] has proposed a 10 dB penalty in the case of relevant low frequency components--depending on the level of the ambient soundscape [55]. Due to the very low background sound levels at most locations we conducted an additional C-weighted analysis of the background to peak noise ratios. The observed ratios at all locations were very high for an urban area (Figure 7) and can result in relevant autonomous cardiovascular reactions during sleep [38; 52; 56]. The analysis indicated also a clear difference between \"old\" and \"new\" trams (Figure 6), which was not distinguishable with the A-weighted approach and further supports not to use A-weighting for this environmental health risk assessment. The citizen's simultaneous exposure to low frequency noise, which also features high sharpness values, raises another question, since those characteristics can be understood as opposed noise phenomena. This question has to be answered by looking at the various noise sources of the complex system tramway. Whereas most of the low frequency noise stems from the rotating wheels on the rail track, the sharp sounds responsible for the high sharpness values are produced by both the pantograph and the corresponding transformer within the vehicle. Overall the psychoacoustic analyses revealed a substantial variability in loudness, roughness and sharpness values across locations and the three tram types. In our view this high variability is one of the main determinant of the citizen complaints. The large variation in time makes the emissions not only well perceptible and intrusive but inhibits also habituation and impairs the adaption of the exposed persons. This view is supported by the older noticeability research, which demonstrated that annoyance was observed to be directly proportional to the detectability of the sounds [57; 58; 59].