Noisy electric vehicles—but not too noisy

At city speeds, electric cars and hybrids in electric mode are very quiet. That silent operation is a boon to urban dwellers plagued by noise pollution, but it poses a threat to pedestrians, particularly if they are visually impaired. So groups worldwide are exploring how to best equip electric vehicles with warning sounds. One such group, a part of the European Union–funded eVADER (electric vehicle alert for detection and emergency response) project, is working specifically to produce a readily perceived signal that does not generate excessive noise; the group presented its first experimental results at the June meeting of the Acoustical Society of America in Montreal. As part of its experiment, the team, led by Etienne Parizet (INSA Lyon), simulated the sounds of nine electric vehicles whose alerts, based on a 300-Hz sinusoidal tone, differed in their harmonic richness, purity of frequency, and consistency of amplitude. It also used the sounds of two controls: an unalarmed electric car and a diesel car 7 dB louder. A total of 91 subjects, 33 visually impaired, listened to an urban soundscape that included one of the vehicles approaching at 20 km/h; the subjects were tasked to strike a computer key when they perceived the oncoming car. Two of the nine alarmed electric cars were identified as readily as the diesel, even though they were less than 1 dB louder than the control electric car. Amplitude variation and a relatively narrow bandwidth, the eVADER group reported, were key elements that allowed warning signals to be easily perceived. (E. Parizet et al., Proc. Mtg. Acoust. 19, 040033, 2013, doi:10.1121/1.4800182 ; see also E. Parizet et al., http://www.acoustics.org/press/165th/2aNSa5_Parizet.html .)