Sound quality evaluation of electric cars : preferences and influence of the test environment

Abstract

Many are the upcoming noise, vibration and harshness (NVH) challenges regarding electric vehicles (EV), not least due to absence of the masking combustion engine noise. In general, and vibration levels in the car's interior are significantly lower for EV's. However, tonal sounds caused by electromagnetic forces in the electric motor are in varying degrees present and provide audible feedback of operation during driving. The key question for how to target these sounds is to gain an understanding about what quality means for EV's. As an initial study, investigations were carried out to determine how the test environment influences the evaluation of the perception in an EV. Headphone evaluation with playback of binaural head recordings in both listening lab and a vehicle demonstrator sound car were compared to the real perception in a production-like electric car. A modified established scorecard for internal combustion engine (ICE) powertrain quality evaluations was used to rate the different characteristics during a full throttle acceleration from zero up to 100 km/h. The results of this study show that there were no significant differences in quality judgement in vehicle compared to laboratory. In addition to this, the subjects were divided into two groups where each group was categorized by a specific evaluation order. The purpose was to investigate the effect on rating of having co-driven the vehicle before evaluating in laboratory, compared to not having a previous in-car experience before evaluating in laboratory. The main finding was that the group of subjects who assessed the in-car condition prior to the two laboratory conditions had smaller variance in within-subject differences between the in-car and the listening room assessments. Analysis was also done on the preferences of the quality regarding the tonal components from an accelerating EV. Increasing levels of high frequency tonal components and decreasing mid frequency tonal components from the electric motor and transmission gave high rankings on sharpness, annoyance, toughness/aggressiveness and powerfulness. Lowering the same high frequency components and increasing the mid frequency components yielded high rankings in overall satisfaction. The findings from these studies should be considered in the design and evaluation stage of complete vehicle target sounds for EV's.