Noise metrics of the time-varying acoustic far-field of rotors

Rotor noise comprises harmonic features, related to the blade passing frequency, as well as broadband noise. Even though an acoustic spectrum yields a frequency-distribution of energy within an acoustic pressure signal, it does not reveal phase-relations between different frequency components. The latter are of critical importance for the development of accurate prediction- and auralization-algorithms, because they may result in low-frequency modulation: temporal variations in the intensity of higher-frequency rotor noise. Baars et al. recently outlined a methodology to quantify inter-frequency modulation. The current work applies this methodology to a new and comprehensive dataset of a laboratory-scale rotor at advance ratios ranging from J = 0 to 0.6. PIV measurements of the blade-induced flow disturbances complement the acoustic data to elucidate how the vortical flow structures of one blade impact the inflow of the consecutive blade. Findings of the directivity patterns of modulation are described and interpreted using existing knowledge of the specific noise sources of the low Reynolds number (small-scale) rotor.