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Table 4. Comparison of European and US minimum overall sound pressure level requirements for hybrid electric vehicles
On the other hand, associations of the blind still think that there are areas for improvement in the EU legislation. They demand an extension of the speed range, an increase of minimum sound levels, and a mandatory sound when
the vehicle is not moving (EBU, 2019).
The Psychoacoustic Aspect
As pointed out in Regulations for Alert Signals, car manufacturers can individually design their alert signals in compliance with the regulations. The goal of alert signals is to attract sufficient attention to inform pedestrians of the presence of an EV. Accordingly, it was observed that designing sounds based on psychoacoustic principles can double the detection distances relative to a reference vehicle and that artificial sounds based on combustion noise seem to be relatively ineffective (FMVSS, 2016). Studies have also shown that well-designed alert sounds with prominent noise patterns (e.g., roughness) lead to earlier detection of approaching vehicles (Steinbach and Altinsoy, 2020). In particular, dynamic alert signals using level variations or frequency shifts lead to earlier detection by pedestrians compared with alert sounds with a barely noticeable sound change from standstill to low speeds (Steinbach and Altinsoy, 2020).
Listening experiments suggest that the underlying affective structure of AVAS sounds can be described by factors such as ”clarity,” “quality,” and ”power” and that alert sounds can be specifically designed both to increase detection time and to improve the alert sound quality (Matsuda et al., 2019). Obviously, the sound pressure level of the exterior noise of a vehicle is not the sole determiner of its detectability.
This indicates that the assessment of human responses to alert sounds cannot be simply predicted on the basis of overall sound level. In particular, the prediction will be inaccurate if the estimates of noise annoyance do not consider superposition effects of multiple vehicles emitting alert sounds. In the case of superposed alert sounds,
“interesting” psychoacoustic phenomena can occur. For example, it is expected that dissonant sounds will result from the superposition of warning signals differing in pitch, pitch-shifting factor, and noise character and that this might lead to a less harmonious urban soundscape (Laib and Schmidt, 2019). Because car manufacturers can individually design their alert signals, superposition of alert sounds from different brands of EVs might create unexpected and prominent alert signal compositions.
 United States
 Constant speed of 10 km/h
 50 dB(A)
 51 dB(A)
 Constant speed of 20 km/h
 56 dB(A)
 57 dB(A)
 Constant speed of 30 km/h
 62 dB(A)
  47 dB(A)
  48 dB(A)
   European regulations are in United Nations Economic Commission for Europe (UNECE) 138.01. United
States regulation are in FMVSS No. 141. United States requirements are the band sum of two bands with respect to the two-band alert option.
the belief that a high percentage of EVs helps to significantly reduce adverse environmental effects (EAA, 2016).
Large-scale introduction of alert signals for increased pedestrian safety might counteract the goal of environmental noise reduction. However, after the final environmental assessment, the NHTSA concluded that little or no negligible impacts on the human environment are anticipated. It also estimated that the increase of environmental noise due to the safety standard for EVs is negligible (FMVSS, 2016). The NHTSA argued that even if EVs were to reach 50% deployment, a maximum level increase of less than 1.0 dB in urban environments is expected. According to NHTSA, because differences of less than 3 dB are generally considered unnoticeable by humans, the environmental impact is likely to be negligible. Indeed, recent pass-by noise measurements of EVs with or without AVAS support the assumption of only a minor sound pressure level increase due to alert sounds, at least for speeds higher than 10 km/h (Laib and Schmidt, 2019).
However, this assumption probably underestimates the potential noise annoyance effects due to the introduction of warning signals for EVs. Because various regulations require that alert sounds be designed to improve the audibility and detectability of electrified cars, increased noise annoyance might be easy to predict on the basis of the minor sound pressure level increases (Genuit, 2016). Accordingly, Laib and Schmidt (2019) argued that AVAS sounds from multiple vehicles at the same time can lead to a cacophony that could be more annoying than one might predict based on sound pressure level alone.
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