  Acoustic spectra of rotor noise yield frequency-distributions of energy
within pressure time series. However, they are unable to reveal phase-relations
between different frequency components, while the latter play a role in
low-frequency intensity modulation of higher-frequency rotor noise. Baars et
al. (AIAA Paper 2021-0713) outlined a methodology to quantify inter-frequency
modulation, which in the current work is applied to a comprehensive acoustic
dataset of a rotor operating at low Reynolds number at advance ratios ranging
from $J = 0$ to $0.61$. The findings strengthen earlier observations for the
case of a hovering rotor, in which the modulation of the high-frequency noise
is strongest at angles of $\theta \approx -20^\circ$ (below the rotor plane).
For the non-zero advance ratios, modulation becomes dominant in the sector
$-45^\circ \lesssim \theta \lesssim 0^\circ$, and is maximum in strength for
the highest advance ratio tested ($J = 0.61$). Intensity-modulation of
high-frequency noise is primarily the consequence of a far-field observer
experiencing a cyclic sweep through the noise directivity patterns of the
relatively directive trailing-edge/shedding noise component. This noise becomes
more intense with increasing J and is associated with the broadband features of
the (partially) separated flow over the rotor blades.
