Adapting a propeller noise model for aircraft at cruising altitudes

Abstract

An existing propeller harmonic noise model is adapted to incorporate the effects of a stratified atmosphere so that predictions can be made of the ground-level noise for an aircraft flying at cruising altitudes. Specifically, the speed of sound and the atmospheric absorption are both made functions of altitude. The effects of refraction and ground reflection are examined, but deemed small enough to exclude from the initial model adaptation. The adapted model is applied to long-range acoustic data for a turboprop aircraft at three different cruising altitudes. The results show that a relatively good fit can be found between the model and the experimental data at the Doppler-shifted blade passing frequency within a range of source radiation angles from 30° to 110° with respect to the aircraft's direction of travel. The results also highlight the influence of molecular oxygen on the atmospheric absorption of the propeller harmonics. Recommendations are made for further improvements to the model.