Perhaps the most frequent drone question we get at MAXSUR is, “Which drone is the most quiet?”. Secondly, “How low can the drone fly and still maintain a degree of stealth?”. To date, very few people have answered those questions outside of general vagaries. To provide more objective answers, the MAXSUR team conducted a drone noise level test and the results were very surprising.
Before we get into the details of the test, I want to address an obvious reason why there are little details about noise levels and drones: Most people in the community have a fear of being labeled with the “big-brother” tag. The drones we are testing are common off-the-shelf drones, and are far from the million dollar-plus military drones that are used for covert surveillance. These off-the-shelf drones however are commonly used by public safety and are always used in response to someone or something occurring. In those responses, a life-saving tactic may be to limit detection from a bad guy. These drones are excellent for such events but are very poor at being used for persistent surveillance, like unto military-class drones.
To keep the test as “real-world” as possible, we ditched the normal decibel meter used for noise level testing. After all, there are many more scientific factors involved beyond simple dB levels. The factors we focused on were:
ALTITUDE
Obviously, the higher the drone the better in terms of stealth. However, the higher you go…the less information you’ll be able to obtain about ground information e.g. identity, lp’s, and activity. So, in our test…we determined how low a drone can go without being audibly detected. To start the test for each drone, we go to the maximum legal altitude of 400 feet AGL and lower the drone until we can audibly detect it.
LATERAL DISTANCE
Another obvious factor is how far the drone is away from the target by ground distance. Generally, noise from multi-rotor drones goes straight down. So if the drone can be offset from the target, they’ll be less likely to hear it. In the MAXSUR test, we placed the drones approximately 300 feet away laterally.
AIRCRAFT TYPE
Different classes of drones, especially multi-rotor drones have unique noise signatures. These noise differences come from varying motor RPM levels and propeller pitch. To represent a few major classes of multitotor drones, we tested three very common off-the-shelf units which were the DJI Phantom, DJI Inspire, and the DJI Mavic.
WIND
The wind affects audible drone detection in two ways, first by providing a dampening effect to the listener and secondly…slightly redirecting the sound waves. When conducting the MAXSUR drone noise level test, the wind was gusting at twelve miles per hour.
AMBIENT NOISE
Ambient noise is perhaps the most important factor of all. Any kind of nearby noises common to urban areas will mask drone noises almost completely. In our test, we replicated an urban setting by being adjacent to a busy Texas highway. Additionally, we tested in another more rural area.
TEST RESULTS
| CLASS (not FAA class) |
DRONE |
URBAN SETTING – DETECTED @ |
RURAL SETTING – DETECTED @ |
|
Micro |
DJI Mavic |
336’ |
281’ |
|
Compact |
DJI Phantom 3 |
366’ |
75’ |
|
Mid-Size |
DJI Inspire 1 |
219’ |
132’ |
OPTIONS FOR ENHANCED STEALTH
For using off-the-shelf drones such as the DJI Inspire 1, an easy improvement is to add a zoom camera such as the DJI Z3 and DJI Z30 camera. Both of these cameras offered optical and digital zoom capability. Respective of the model, the Z30 offers 30 times optical zoom and along with 4k video resolution, obtaining facial identity and LP’s are easily obtained several hundred feet away without risk of detection.
Another potential option is a fixed wing aircraft. In general, fixed wing drones require little in terms of propulsion and consequently, are very quiet. For more information about these system, just send us an email to ops@maxsur.com or call us at 1-855-778-6565.
Thanks for reading. We welcome your comments on this article, and please sound-off with your recommendations for other topics that need covered.
Best regards,
Jake Lahmann
COO
MAXSUR