Construction of interpolated source from dynamic flights (UAV/AAM Special Issue)

A previous comparison of far-field predictions from equivalent acoustic sources generated from dynamic flight measurements of small unmanned aircraft systems demonstrated that a single pass over an acoustic array was insufficient to accurately predict far-field acoustic levels. In effort to reduce the uncertainty in the current study, measurements of multiple passes of dynamic flight within finite engine speed/airspeed bins were combined. From these measurements, equivalent acoustic sources were decomposed using spherical harmonics fitted to levels depropagated on a spherical surface. Two source descriptions, one at a low speed and one at high speed, represent end points on the noise-power-distance curve, which was similar to a technique demonstrated in a previous study of static measurements. Interpolation of spherical harmonic coefficients between the two sources permitted the determination of intermediate speed conditions. The resulting equivalent acoustic sources removed more than 10 dB of uncertainty from the dynamic flight definitions.