  As the presence of flying robots continues to grow in both commercial and
private sectors, it necessitates an understanding of appropriate methods for
nonverbal interaction with humans. While visual cues, such as gestures
incorporated into trajectories, are more apparent and thoroughly researched,
acoustic cues have remained unexplored, despite their potential to enhance
human-drone interaction. Given that additional audiovisual and sensory
equipment is not always desired or practicable, and flight noise often masks
potential acoustic communication in rotary-wing drones, such as through a
loudspeaker, the rotors themselves offer potential for nonverbal communication.
In this paper, quadrotor trajectories are augmented by acoustic information
that does not visually affect the flight, but adds audible information that
significantly facilitates distinctiveness. A user study (N=192) demonstrates
that sonically augmenting the trajectories of two aerial gestures makes them
more easily distinguishable. This enhancement contributes to human-drone
interaction through onboard means, particularly in situations where the human
cannot see or look at the drone.
