Tonal Noise Reduction Through Ultrasonic Frequency Shifting

This paper presents a method for reducing rotor tonal noise by shifting the first blade passing frequency toward the near-ultrasonic range. The key parameters enabling this frequency shift are an increased blade count and higher rotor revolutions per minute (RPM). Aerodynamic forces are computed using the Blade Element Momentum Theory (BEMT), and tonal noise predictions are performed using the Hanson model. The Hanson model requires only geometric inputs and aerodynamic loading data—the latter derived from BEMT—to estimate tonal noise levels. The study compares sound pressure levels across three RPM settings for a range of blade counts. Results show that for configurations with 25 blades, the Hanson model predicts negative tonal noise levels at RPMs up to 20,000, indicating a significant reduction in noise output. Although sectional thrust and torque coefficients tend to decrease with increasing blade count, the variation is modest, suggesting that the noise reduction is not directly tied to these aerodynamic coefficients. The primary reason for the noise reduction is the inefficient sound propagation characteristics of high-frequency noise. Overall, the findings highlight that increasing the number of blades is critical for effective frequency shifting and subsequent reduction of tonal noise, offering a promising strategy for quieter rotor design.