Assessment and control of tonal components in electric vehicles

Abstract: New challenges arise for noise, vibration and harshness (NVH) engineers when electric motors are introduced into cars traditionally powered by internal combustion engines (ICE). The noise signature from an electric traction motor is characterized by speed-dependent high frequency tonal components due to electro-magnetic excitation. Despite the relatively low overall levels, those tonal components can be prominent and contribute to perceived annoyance for the occupants inside the car. With the rapid increase of various types of hybrid/electric vehicles (EV), new methods are required to secure a desired sound quality in future electric vehicle launches. The objective of this thesis was to explore the audible perception of the electric powertrain and develop and validate experimental methods for assessment of noise transmission. The thesis is based upon seven papers. Four of the papers are oriented around tools and techniques for sound quality assessments of EV-noise. Due to the specific character, there is a proven need for engineers to identify relevant psycho-acoustic metrics for quantifying the subjective perceptions of an accelerating EV. The relationship between the metric prominence ratio (PR) and perceived annoyance was explored and requirement criteria were proposed. PR is appropriate for quantifying individual tonal components, well frequency separated from each other. For more complex sounds, e.g. with multiple proximate prominent tones, the suitability of this metric is questioned. Therefore, the harmonic content of a large number of EVs on the market was examined in order to bring further facts to the on-going discussion on sound quality quantification of EV specific noise. Furthermore, the assessment environment’s role as well as the impact of experiencing the sound sensation in the real environment for sound quality evaluations were studied. Finally, perception of the tonal components related to different DC/AC conversion techniques including randomized pulse-width modulation was explored. The mainmechanical system properties that are responsible for the airborne radiation and transfer of electric powertrain induced noise were studied. The acoustic transfer functions’ (ATF) spatial resolution with respect to estimated interior noise was investigated for theoretically derived and measured surface vibrations. ATFs and surface velocities are also central for the presented in-situ method for determination of radiated sound power from the stator housing due to a dominant order. By acquiring the operational deflection shape due to an electro-magnetic radial force wave, a simplified expression for estimation of the radiation efficiency was derived. The two papers highlight the advantages of the rotational symmetric force excitation acting on the stator housing which enables sound power to be used as an acoustic source characterization, which perhaps is the most adequate measure for system target setting directed towards external suppliers. The final contribution of methods for handling EV-specific noise concerns structure-borne sound. The method of in-situ estimation of blocked forces, a theoretically independent source characterization, is evaluated for transfer path analysis (TPA) of a double isolated electric rear axle drive. The method has two main advantages; it is substantially faster compared to the conventional TPA method and the blocked forces allow for accurate prediction of the acoustic response when the same source is being integrated with a different receiving structure. The prerequisite for the second is that the source activity remains invariant and this was further studied. In summary, the work reported in this thesis is intended to support engineers to succeed with the acoustic refinement and thereby also to contribute to increasing competitiveness of hybrid/electric vehicles. Keywords: Electric motor acoustic noise, sound quality, sound power, NVH, vehicle acoustics, blocked force