Effect of frequency shifting of additional sound for electric vehicles on acceleration impression

Drivers get rich information concerning the vehicle condition and status by sounds from any parts of the vehicle they are driving. Electric vehicles are generally quieter than most of conventional engine vehicles. The quietness of the propulsion system might make drivers difficult to realize or not enough to feel the feedback information, e.g. how fast the vehicle is, how much accelerating it is, and so on. There is a demand to design additional sounds on these vehicles, however, there are not sufficient knowledge how to design the sound to make drivers easier to understand vehicle situation or status. When the additional sound for quiet vehicles, which is independent from vehicle propulsion mechanism, is designed, it is needed to understand about the relationship between frequency shifting and acceleration impression. To understand the relationship, a subjective rating experiment using a driving simulator that was developed to examine the driving sound impression of electric vehicles as well as a passive rating experiment using audio-visual stimuli and Scheffe's paired comparison method were conducted. In both active and passive experiment, a view from a vehicle cabin was presented to the participants. The audio stimuli were mixtures of three one-third-octave band noises, of which frequency shifting were independently controlled. The result showed that frequency shifting could affect to vehicle acceleration impression. It was also shown that the impression could be affected by not only the range of frequency shifting but also the absolute frequency of shifting band.