## 3 Discussion
A possible reason is the high correlation between the loudness (N5) and the TETC index values (R\({}^{2}\) = 0.85). Because the loudness is already considered at the annoyance index, the contribution of the TETC index was small. The TETC index is a useful analysis for the pass-by sounds. Depending on the driving condition, its importance can vary. Relative approach analysis is a powerful tool to identify and characterize signal changes and distinct patterns in temporal and spectral domains. However, it seems that the included psychoacoustical analyses at the annoyance index, such as roughness and fluctuation strength, etc., deliver comparably good results to describe temporal signal changes of pass-by sounds. Therefore, it was decided not to include the relative approach analysis in the annoyance index. In particular, the analyzed driving condition in this study also plays a role in it. The relative approach properties can be adjusted in various ways. Several property variations of the analyses were tried to find the optimum analysis properties in this study but the results were comparable. Depending on future AVAS sound design examples, the role and the consideration of the relative approach, the TETC, or the hearing model, the impulsiveness analysis in the proposed annoyance index can be reevaluated. The developed index delivers better results in estimating the traffic noise annoyance caused by a passenger car than the commonly used A-weighted sound pressure level measures. This result is valid not only for ICE-based car sounds but also for electric cars. Furthermore, the AVAS has been taken into account in this study. The results of the annoyance evaluation experiment show that electric cars with or without an AVAS have the potential to be less annoying than the existing ICE cars. These results are promising for the future because almost all prognoses estimate that the percentage share of electric cars, hydrogen fuel cell cars, or cars based on similar technology will rapidly increase in the near future. It seems that, with this development, the sound pressure level of the overall traffic noise will be 2 to 4 dB lower than the current level, and the perceived annoyance will also be lower [32]. The results of this study clearly show that the development of not only the sound pressure level, but also of the psychoacoustical properties of electric car pass-by sounds (with and without AVAS) should be taken into account for the sound packaging and design decisions regarding the perceived annoyance. In this investigation, analyzed electric car sounds (with and without AVAS) show a promising tendency that their overall psychoacoustical properties are lower and better than conventional car sounds. In this research, we only considered the evaluation of traffic noise annoyance levels based on passenger car sounds. However, the contribution of heavy and utility vehicles to traffic noise is also very important. Investigations have shown that heavy vehicles, such as buses and trucks, play an important role in overall traffic noise [45; 46; 47]. The estimations show that the decrease in the overall traffic noise sound pressure level due to electric cars will be limited and only 1.2 dB if heavy vehicles are not taken into account [33]. The electrification of heavy vehicles will be an important topic in the future because of the corresponding effects on noise emissions. Although reasonable truck electrification may take a very long time, bus electrification is rapidly evolving. A recent study showed that an average electric bus is 8 dB quieter than an ICE bus at a constant speed of 20 km/h and 5 dB quieter at a constant speed of 30 km/h [48]. Both the electrification of cars and buses can cause a significant level of reduction in traffic noise. Therefore, the results of this investigation and particularly the developed index should also be extended for these kinds of vehicles. Heavy vehicles mostly cause loud sounds that have strong low-frequency components [49; 50; 51; 48].