Exposition of Noise Fraction Adjustments for Integrated Noise Modeling of Rotorcraft

The noise fraction adjustment is one of several adjustments to noise-power-distance data that are used to estimate sound exposure in integrated noise modeling. The noise fraction adjustment relates the sound exposure due to an infinitely long flyover to that generated by a finite segment using a 4th-power dipole model. This paper elucidates the origins and derivation of this adjustment. Alternative formulations of the noise fraction adjustment are offered that may be better suited for different helicopter and urban air mobility vehicles and their different operational modes, i.e., departure, overflight, and approach. These different methods can alter noise fraction estimates by several decibels.