Determination of Radiated Sound Power from an Electric Rear Axle Drive In-Situ and its Contribution to Interior Noise

System and component target setting for noise and vibration are important activities within automotive product development. New challenges arise when electric motors are introduced into cars traditionally powered by combustion engines. The emitted noise from an electric traction motor for hybrid electric vehicles is characterized by high frequency tonal components from the dominating magnetic harmonics which can be perceived as annoying. Sound power is frequently used for quantifying the airborne noise from rotating electrical machines. This paper describes the process of determining the radiated sound power from an automobile electric rear axle drive in-situ and its contribution to interior cabin noise for a prominent rotor order. The sound power was calculated by combining the average stator surface vibration velocity together with an estimate of the radiation efficiency. To model the radiation efficiency, the vibration shape of the cylindrically shaped stator shell was determined by means of operational deflection shapes. The acoustic transfer functions between the source and receiver positions close to a passenger's ear, were defined as the ratio of sound pressure and sound power. By combining the acoustic transfer functions with the calculated operational sound power, the airborne interior noise can be predicted. The calculated interior noise levels were found to be in accordance, within approximately 3 dB, with the measured airborne noise from the stator shell. This indicated that the method is valid and could be helpful for calculating allowed radiated power from interior noise targets and acoustic transfer functions.