The Sound of Silence: A Preliminary Investigation into the Effects of Blade Row Spacing in Counter-Rotating Propellers on the Sound Intensity in the Near-Field

Abstract

Video recording drones offer users unprecedented film-making opportunities with their smooth aerial shots and subject-tracking capabilities. Their uses include cinematography, land-surveying, building inspection, and surveillance. Current video recording drones have a significant shortcoming, however, when it comes to uses such as cinematography - they lack audio recording capabilities. Drones are not equipped to record audio because any audio they do record is dominated by the noise the propellers generate. This issue may be exacerbated by a potential new trend in the drone industry - the use of counter-rotating propellers. The experiments in this paper test the influence that the vertical spacing between the counter-rotating propellers has on the noise intensity generated by the propellers in the surrounding three-dimensional field. In Method 1, a first set of trials involved recording the sound intensity at certain locations where a microphone may be placed as a function of the propeller spacing. In Method 2, two counter-rotating propellers were spaced by a selected distance and the greater near-field acoustic profile was measured. Three different types of propellers were used for these experiments: the 9455S, the 9450, and the 8331F. It was found that propeller spacing was strongly correlated with sound intensity at some locations, while at other locations the correlation was not very significant. Additionally, the acoustic patterns of counter-rotating propellers in the greater near-field revealed additional candidate locations for the placement of a microphone where destructive interference decreased the sound intensity, sometimes significantly. Drone designers may use the interference patterns found in these experiments as guidelines in their design of an audio recording drone equipped with counter-rotating propellers.