Researchers at NASA have released the results of a study determining how annoying the noise of a small unmanned aircraft system (sUAS) really is to people. By having human testers compare the noise generated by consumer drones to that of cars, the researchers did, indeed, find a greater annoyance level for sUAS, but they plan to do more extensive research and testing in order to take other factors into consideration.
Entitled “Initial Investigation into the Psychoacoustic Properties of Small Unmanned Aerial System Noise,” the technical report is authored by Andrew Christian and Randolph Cabell, aerospace technologist and branch head, respectively, of the Structural Acoustics Branch (StAB) at the NASA Langley Research Center in Hampton, Va. The StAB is tasked specifically with researching aircraft noises and the effects on their environment.
Because the noise of sUAS is not like that of other aircraft and because there are not yet other studies of this kind, the researchers set out to record the noises of various types of drones, perform a “psychoacoustic test” on people and analyze the data.
At Oliver Farms in Smithfield, Va., in September 2016, the team deployed a Drone America DaX8 octocopter, a Stingray 500 variable-pitch quadcopter and a DJI Phantom 2 fixed-pitch quadcopter (with variable-pitch referring to the different noise of the drone according to which blades were affixed to it). Later in the year, the researchers went to Cleveland National Forest near San Diego to test an Endurance sUAS from Straight Up Imaging (SUI). For both tests, microphones were placed on tripods below the flight path.
Lastly, in January of this year, NASA recorded noises made by vehicles: a passenger hatchback, utility van, diesel-powered box truck and step van.
NASA notes that the tests also incorporated “auralizations” – i.e., computer-generated noises – of a quadcopter and “small civil aviation plane.” They were included “for comparison with previous human subject tests from which they were taken and in anticipation of follow-on-testing using auralizations of sUAS,” the research paper explains.
In February, back at NASA’s Langley Exterior Effects Room, “a small, acoustically treated auditorium,” the human testers – between 18 to 50 years of age – listened in random order to the 103 “non-unique sounds” during four 15-minute sessions.
The 38 subjects, who were not told the sources of the sounds, were asked to rate each noise as “not at all annoying, slightly annoying, moderately annoying, very annoying or extremely annoying.”
The researchers took into account factors such as a “startle-related penalty for noises that rise in intensity too rapidly.” In turn, the sounds incorporated two-second fading.
According to NASA, “common comments during informal conversations after the test were that the subjects were typically not aware that the non-road noises came from drones and that the fact that some were flying overhead and some were presented as drive-bys did not significantly impact their judgments. This suggests that the subjects were queuing off of qualitative differences between the sample sounds.”
Also following the test, NASA says some subjects discussed how they found the higher-pitched noises to be more annoying rather than the lower-pitched noises, and vice versa, and noises “that appeared to linger” or were described as “patterned” were also generally found to be more annoying.
After analyzing the data, the researchers state that there “may be a systematic difference between the annoyance response generated by the noise of the sUAS and the road vehicles included in this study.”
“It is unknown as of now whether this difference can be accounted for by other factors or whether it is being generated by qualitative differences between the sound of road vehicles and sUAS,” the paper says. For example, the researchers say they must take into account the familiarity of a car noise as opposed to a drone noise.
Notably, though NASA says the idea has not yet been “completely objectively explored,” the test found an “insignificant change” in annoyance level according to the height of the drones. For example, when the SUI Endurance drone was flown at 20, 30, 50 and 100 meters at 5 meters per second, the annoyance level didn’t change all that much.
“Alleviating annoyance from sUAS operations may not be simply a matter of flying higher,” the researchers point out.
NASA also makes it clear that its study is not a “comprehensive examination of noise from either sUAS or road vehicles”; instead, it was conducted to “demonstrate the extensibility of tools and facilities that NASA already possesses to the realm of sUAS noise.”
“Therefore, it is unwise to attempt to generalize the results of this study beyond those stated in the discussion and beyond the limited set of vehicles and conditions tested,” the research paper notes.
Following more analysis on this test, the researchers are planning another round later this year.