Acoustic and psychoacoustic characterisation of small-scale contra-rotating propellers

This paper reports an experimental investigation of noise radiation and psychoacoustic evaluation of contra-rotating propellers with different blade counts over multiple emission angles and thrust settings. A series of aeroacoustic and psychoacoustic tests were performed with a contra-rotating propeller rig at the University of Salford, UK, where both propellers' blade count and their axial separation z were varied at static conditions and constant thrust. The sounds were analysed using conventional acoustic metrics, such as Sound Pressure Level (SPL) and Sound Power Level for tonal, broadband, and overall noise, and using the psychoacoustic Sound Quality Metrics (SQMs) Loudness, Fluctuation Strength, Roughness, Tonality, and Impulsiveness. Configurations with axial separation-to-diameter ratio z/D between 0.2-0.4 exhibited a balance between tonal and broadband noise generation mechanisms, resulting in reduced noise levels and reduced annoyance for on-axis observers. The number of blades played a secondary role on determining noise levels: higher blade counts resulted in lower overall noise levels at larger axial separations, although at shorter distances (z/D < 0.2) increasing the blade count led to higher noise levels. The SQMs Loudness, Fluctuation Strength, and Tonality were found to also reach a minimum at z/D = 0.3, although increasing the number of propeller blades led to a decrease in Loudness but not in Fluctuation Strength and Tonality. The dominant noise generation mechanisms observed here present a directivity null perpendicular to the propellers' axis, and no clear trends were observed for acoustic and psychoacoustic metrics at this emission angle. A listening experiment was set up to validate the results of the acoustic and psychoacoustic analysis, and its results suggest that the reported noise annoyance is driven by Loudness, Fluctuation Strength and Tonality metrics (p < 0.01) at on-axis radiation, while at 90 degrees emission angle, Loudness (p < 0.01) and Tonality (p < 0.05) are the main contributors to noise annoyance.