Page 24 - Winter 2020

ALERTING PEDESTRIANS BY SOUND
requirements, it must use an AVAS at any speed up 20 km/h. Apart from minimum overall sound levels for certain speeds, minimumlevelsofatleasttwoone-thirdoctave-bandlevels must be exceeded, as in the US standard. Moreover, the regulation requires that at least one band from the vehicle exceed the defined minimum levels, below or within the
1,600 Hz (midband frequency) one-third octave band.
In addition, and unlike the US NHTSA regulation, a “frequency shift” must be implemented to signify acceleration and deceleration so that the frequency content of the AVAS sound is a function of the vehicle speed. To do this, at least one tone within the specified frequency range must vary proportionally with speed by an average of at least 0.8% per 1 km/h in the speed range from 5 to 20 km/h inclusive when driving in the forward direction. A detailed specification of what can be considered as a tone within the AVAS signal is not given. However, it is specified that a sound-analysis system that is capable of spectral analysis at a sampling rate and with a frequency resolution sufficient to differentiate between the frequencies of the various test conditions must be used (UNECE, 2017). The major issues of the requirements stated in FMVSS No. 141 and UNECE R138.01 are presented in Table 3.
The UNECE regulation defines a maximum overall sound level of 75 dB(A) at a 2-m distance for vehicles equipped
with an AVAS. FMVSS No. 141 does not directly specify a maximum sound pressure level to avoid “unnecessary” loud alert sounds. Overall, the resulting minimum overall level requirements comparing the US and UNECE regulations are quite similar, although the US standard is slightly more conservative (see Table 4).
Car manufacturers are allowed to design their own AVAS as long as they meet the legal requirements. Thus, the broad frequency range and flexible choice of possible one-third octave bands in the regulations are intended to provide manufacturers with the flexibility to design alert sounds that are acceptable to their customers (FMVSS, 2016). FMVSS No. 141 was recently proposed to be amended to allow manufacturers to install a number of driver- selectable pedestrian alert sounds in each EV (FMVSS, 2019), and this is also permitted in UNECE R138.01.
The Conflict of Goals
Environmental noise protection hoped for reduced road traffic noise in urban areas due to an increasing percentage of EVs. Without a doubt, the electrification of vehicle powertrains brings a potential to reduce exterior noise in the low-speed range, a condition particularly relevant for cities. This hope was reflected in the white paper from the European Commission (2011) demanding cutting in half the use of “conventionally fueled” cars in urban transport by 2030 and phasing them out in cities by 2050. This is based on
Table 3. Comparison of European and United States regulations on minimum sound requirements hybrid electric vehicles
Europe
United States
Speed range (forward motion)
Up to 20 km/h (±1 km/h)
Up to 30 km/h (+2km/h)
Reverse
6 km/h (±2 km/h)
0 km/h (stationary)
Minimum third-octave levels for nonadjacent bands
Mandatory
Mandatory
Frequency range
160 Hz to 5,000 Hz
4 Nonadjacent one- third octave bands spanning no fewer than 9 bands from 315 Hz to 5,000 Hz
2 Nonadjacent one-third octave bands from 315 Hz to 3,150 Hz
Sound while vehicle is stationary
Not mandatory
Mandatory
Pitch shifting
Mandatory
Not mandatory
European regulations are in United Nations Economic Commission for Europe (UNECE) 138.01. United States Regulation are in FMVSS No. 141.
24 Acoustics Today • Winter 2020