Speed-dependent directivity patterns of road-traffic vehicles.

Vehicle exterior noise is composed of speed-dependent contributions from tire/road interaction, powertrain, and aeroacoustic noise, each contributing to overall sound emission by characteristic directivity patterns. This work focuses on identifying the spatial and speed-dependent directivity patterns of a broad range of road-traffic vehicles (including electric, hybrid, and combustion-powered vehicles) by utilizing time-variant wave backpropagation on microphone array signals. The derived directivity patterns are compared to the analytical Harmonoise model and further analyzed for their spectral properties in third-octave bands, as well as their speed-dependent sound power. Besides drivetrain technology, the results highlight the crucial influence of driving conditions, particularly the engine's rotational speed (related to the engaged gear), on the directivity patterns. Being more symmetric in the median plane, asymmetric and more directional patterns are observed in the sagittal plane. At the same speed, the sound power level of individual cars varies in a range between 10 and up to 20 dBSWL, converging at higher speeds. The results are published in an accompanying open-access database containing directivity patterns and sound power in the speed range between 10 and 120 km/h, using standardized Spatially Oriented Format for Acoustics and OpenDAFF data formats for noise mapping and auralization applications.