Tones In Noise: A study of the perception of motor noise in an electric vehicle

Abstract: Electric and hybrid vehicles are becoming a more and more frequent addition in car manufacturers’ fleets of vehicles. Noise and vibrations in these vehicles differ from the noise and vibrations produced in a combustion engine car. Sound quality of combustion engines have previously been researched but little information is to be found on the sound quality of electric vehicles. In this study the perception of the tonal components produced by the electric motor in a pure electric car is investigated. The tones were rated on annoyance and well sounding scales by 30 subjects in a listening test and the threshold of when the tones become audible was investigated. Coupled to the subjects’ ratings is a measure called prominence ratio. Prominence ratio is used in the IT industry to classify the prominence of tonal components in IT-products. Sound files used in the listening test were recorded in a Volvo C30 electric on smooth and rough asphalt at constant speeds of 50 km/h, 80 km/h and 0-100 km/h acceleration. The recorded files were filtered to suppress existing tonal components. Synthesized tones, created to represent the tones produced by the electric motor, were added to the filtered sound files. The synthesized tones were generated in two programs, either Pure Data or Head Acoustics Artemis. The results from the listening test show that the presence of tones in the interior sound of a car leads to an increase in annoyance compared to when the tones are not heard. In the study, tones below 1 kHz were harder to detect than tones above 1 kHz. The low frequency tones were detected by more than 50% of the subjects at values of prominence ratio between 1-4 dB and the higher tones were detected at 0 dB. The results also reveal that frequencies above 1 kHz were rated as more annoying than tones below at a significance level of 95%.