Methods for assessing and controlling the spatial sound field inside a car

Abstract

The growing demand for lightweight and “greener” vehicles is an open challenge for the NVH engineers in car manufacturer companies to keep produce refined and comfortable vehicles. The internal combustion engines are gradually replaced with electrical machines, changing dramatically the main noise source in a car and, at limits, increasing the total mass of the vehicle, due to added power battery pack. The active noise control techniques have become nowadays, thanks to the low-cost CPU available in audio amplifiers, a valid alternative to control the internal sound field, especially for structure-borne road noise and increase the car occupant’s comfort. In the last 5 years, Virtual Reality (VR) technology has dramatically spread in our lives, thanks to entertainment, video game and social networks industry, opening new opportunities for engineering applications in spatial sound field recording, analysis, and reproduction. In this thesis, the last spatial audio and video techniques, based on dense microphones array and Ambisonics and SPS theories, have been applied to the sound field characterization and assessment in car cabins. The traditional measurement techniques based on binaural microphone recording and headphones playback, have been replaced with dense microphone array spatial recording that allows advanced spatial analysis on 360° colourmap. The spatial sound field analysis technique has been employed in the performance assessment and objective evaluation of an active road noise control system (RNC). The implementation stages of the RNC feedforward system are presented, with a focus on the reference sensor selection. The psychoacoustics parameters, widely employed in automotive industry for objective evaluation of sound quality of cabin noise, have been included in the spatial sound field analysis tool to create Loudness and Sharpness direction-dependent maps. The 360° video and spatial sound field recordings can be played back on loudspeaker rig in an Ambisonics listening room with panoramic screens or on binaural headphones attached to a Head Mounted Display (HMD) benefiting from head-tracking and, even, individualized Head Related Transfer Functions (HRTF). The VR playback applied to car cabin noise allows to make subjective evaluation with a degree of realism unattainable with traditional static binaural approach.