Sound design for electric vehicles

The interior of an electric vehicle can sound eerily quiet without the noise of a combustion engine, yet also annoying due to unfamiliar, high-frequency sound components. Options for introducing virtual sounds are boundless, but as Soogab Lee and colleagues at Seoul National University show, psychoacoustics and musical harmonic theory can offer guidance. Both loudness and sharpness, which is related to the proportion of high-frequency components, can negatively affect pleasantness. To enhance the dynamic impression, the researchers wanted the sounds to correlate with speed. Harmonious engine sounds, whose frequencies are in simple integer ratios, have been found to be more pleasant, so the Seoul team focused on tones harmonically related to the car interior’s dominant high-frequency component during acceleration. The researchers had 27 volunteers evaluate five combinations of added tones (available online) by rating their overall impression and then describing the sounds in terms of attribute pairs “pleasant–unpleasant,” “calm–dynamic,” “smooth–rough,” “loud–quiet,” “sharp–dull,” and “luxury–cheap.” As shown by the figure’s blue lines, two of the five were rated higher for luxury and pleasantness than the base sound (dashed line) and were more preferred overall. For the combination rated on the left (the fourth audio sample), the original and added tones had the frequency ratio 5:3:1, reminiscent of a clarinet. The other (the third sample), with subharmonics that were three and four octaves below the original, was deemed rougher and louder but more dynamic. That combination was particularly favored by the dozen testers who like their cars “sporty.” (D. Y. Gwak et al., J. Acoust. Soc. Am. 136, EL391, 2014, doi:10.1121/1.4898742 .)