Sources of Propeller Noise
Propeller and rotor noise comprises two categories: tonal (discrete frequency) and broadband. Tonal noise occurs at the blade passing frequency and its harmonics, generated by steady and unsteady aerodynamic loading and by the periodic displacement of air (thickness noise). Broadband noise arises from turbulent boundary layers on the blades, blade-wake and blade-vortex interactions, and trailing-edge scattering. The Ffowcs Williams-Hawkings equation provides the theoretical foundation for predicting both components.
Blade Passing Frequency
The fundamental frequency of propeller noise is B·N/60 — blade count times revolutions per second. A 4-blade propeller at 2400 RPM produces a 160 Hz fundamental. Higher harmonics carry significant energy, especially when inflow is non-uniform (due to wing upwash, nacelle blockage, or atmospheric turbulence). Increasing blade count at constant thrust raises BPF, which can be beneficial since higher frequencies attenuate faster during atmospheric propagation.
Tip Speed and Compressibility
Tip speed is the single most important parameter controlling propeller noise. Below Mach 0.7, loading noise dominates and scales moderately with tip speed. Above Mach 0.85, thickness noise grows explosively due to transonic compressibility effects — local shocks form on the blade surface, generating impulsive high-amplitude pressure waves. Helicopter blade-slap during high-speed forward flight is a dramatic example of this phenomenon.
Urban Air Mobility Challenges
The emerging eVTOL (electric vertical takeoff and landing) industry faces intense acoustic constraints for urban operations. Multiple small rotors operating at high RPM in complex installation environments produce dense tonal spectra and significant interaction noise. Active research focuses on distributed propulsion acoustic optimization, blade design for minimum noise at fixed thrust, and operational trajectory shaping to minimize community noise exposure.