If you have ever flown a drone you instantly become aware of how loud they really are. When you see other people’s drone footage online you very rarely get to hear exactly how loud the drone is. The drones noise can be anywhere from a small high-pitched buzz all of the way to an annoying throbbing which can be heard for quite some distance. In this article we can cover why drones are so loud and exactly what you can do to stop your drone from becoming too loud. Also, we will look at the science behind the latest technology to reduce the noise that drones make as they are flying.
Drones are loud because as the propellers move rapidly through the air, they displace a lot of air quickly introducing pressure spikes and drops as the propeller spins – creating the signature buzzing noise. The rapid motion of the propeller blades and the effort of the electrical motors also creates a lot of noise.
The faster the propellers spend the higher the pitch of the buzz created and the slower the propellers cut through the air the lower the pitch. I have the Mavic air and it has smaller propellers than other DJI drones and there is a noticeable difference in the pitch and sand that they make. Larger drones will have longer propellers and a lower frequency of rotation during flight.
Why are drones so loud? – Here’s the quick answer:
Here are four reasons why drones make so much noise while they are hovering and flying:
- Propeller rotation frequency – as we talked about above, if the propellers whip through the air quickly we get a higher frequency generated. There is not much you can do about the drone requiring a high rotational frequency. That is what keeps it in the air. However, you can choose a drone that is larger and has a slower rotational frequency to avoid the high-pitched “swarm of bees” sound.
- Propeller blades passing over arms – one of the main reasons drones make a high sound is that the propeller blades have to pass over the arms which hold them out beyond the body of the drone. As the propeller blades pass over the arms it temporarily creates a high pressure system over the arm and stops producing pressure under the aircraft. This creates a pressure wave and a sound which is a fairly low frequency.
- The size of the drone – the weight of the drone will dictate how fast the propellers have to spend to generate the don’t thrust that is required for it to hover. If you are buying a heavy drone the propellers will spin faster and will be much louder. Think of a helicopter as being the ultimate loud heavy drone for comparison.
- The length, pitch and depth of the propellers – larger propellers this place more air while they spin that means they produce more thrust and require less of a rotational frequency to keep the drone in the air. The sound is also changed by the angle, or pitch, of the propeller as the pitch determines how much disturbance it causes in the air.
Now you know why drones produce so much noise let’s take a quick look at how noisy drones are and give you the ability to compare that to everyday noises.
How noisy is a drone?
Drones are very in their noise volume considerably. Also, there is a relationship between how close a drone is flown to the observer and how loud the drone is perceived. To understand how noisy a drone is we need to get some reference on the unit of volume which is the decibel.
A decibel is a unit of intensity of sand and is abbreviated to dB. The dB scale is really large because is so sensitive to sand. People with normal range of hearing can hear anything from the light touch on the skin to the roar of a planes engine. The decibel scale is a logarithmic scale. This means that it increases by the power of 10 for each unit increase.
To understand the full range of decibel levels in relation to everyday sounds here is a table of different dB levels.
0 dB is the softest sand the human ear can hear and is almost inaudible. This is like a leaf falling on the ground for example. Any exposure to sand over 140 dB is considered unsafe for human ears and exposure to noises over 85 dB for long periods of time will also damage your hearing.
|dB level||everyday example||drones at this dB level|
|60||quiet traffic noise|
|70||Washing machine working||DJI platinum, DJI spark|
|80||morning alarm clock||Phantom 4 pro, Mavic air, DJI Mavic pro|
|110+||car horn/jet engine from about 100 m|
You can see from the above table that noises can reach unsafe levels very quickly. Reducing the amount of time that you spend listening to very loud noises will protect your hearing. You’ll notice that the drones are between 70 and 80 dB in volume. This is very close to the excessive noise level of 85 dB and a prolonged exposure to these noises will definitely damage your ear. Luckily, drones are made to be flown far away from yourself and therefore it is unlikely that during the course of the flight you will have prolonged exposure to very loud drones.
How can I make my drone quieter?
Due to the way that drones are kept in the air there is nothing we can do to avoid drones displacing air and creating a significant amount of noise. The noise that we here is actually excess energy that is required to move air to create thrust. There are really only two ways in which we can reduce the volume of a drone:
- absorb the sound generated – there are a few products on the market which claim to reduce the noise of a drone while it is hovering. This approach relies on balancing the displacement of the air with absorbing the sound waves generated by a propeller whipping through the air. There are very few drones that are using this approach so you require the use of third-party accessories to achieve absorption of the sound.
- Changing propeller shape and morphology – because the propellers and motors are the primary noise generation parts of a drone there are plenty of manufacturers of drone propellers that are changing the shape and surface textures of the propellers to minimise noise. There are mixed reviews on this approach but if you want to give it a go it is relatively inexpensive to purchase some stealth propellers.
Using these two options as a way of reducing drone noise here are some of the ways in which you can make your drone quieter.
Use more rotors
Adding more rotors to a drone means that each rotor has to displace less air to keep the drone in the air and so each motor has to work a little bit less. In 2015 a team from NASA developed a prototype drone which was developed to silence the annoying drone noise that is generated during flight.
The drone from NASA had eight motors and it means that each motor can work at a slightly different revolutions per minute. This means that there is a generation of smaller harmonics than in a drone with only four propellers and a reduction in the frequency spectrum. This technique is called frequency spectrum spreading and is only possible because there are so many propellers that are precisely controlled by an electronic control unit. This means that when the prototype drone was flown at 30 m you could almost not hear it.
This makes this drone perfect for package delivery!
Bigger slower propeller
This comes down to the simple fact that larger propellers can displace more air than smaller propellers. This means that each motor doesn’t have to spend is fantastic great the same amount of thrust. A bigger propeller will also reduce the frequency of the noise which people have reported is less annoying.
You can buy aftermarket propellers pretty much every type of drone and this can impact the amount of noise generated. Increasing the diameter of the propellers will also increase the thrust factor in creating vertical lift more effectively. Larger propellers with reduced RPM will stress the motors and cause increased demand on the batteries and other internal components – this means that there is always a payoff when you swap out the propellers for a different type.
Sand down the surface
When you buy a commercially available drone you quite often have logos and other things and embossed into the propeller.
If you have bought a drone with moulding marks or other logo marks you can quickly remove the excess plastic by using a 800 or 600 sandpaper and correcting the shape by gently rubbing on the surface. Typically the plastic used in propellers is a reinforced nylon composite is not easy to damage. If you’re worried about sanding down a propeller and throwing your drone off balance you should simply rub the same number of times on each propeller. Alternatively, you can just rub for the same amount of time on each propeller.
Buy Low noise propellers
There are plenty of aftermarket low noise propellers. If you go to any retailer (such as Amazon) and type in low noise propellers and your model of drone you will find a range of low noise, quick release upgrade propellers for your model. They tend to work in a couple of ways
- they reduce revolutions per minute
- stealth propellers have a very smooth surface.
Typical outcomes of using stealth propellers are a reduction in noise by about 3.5 dB, by reducing the revolutions per minute by about 20%. An added side-effect of super-efficient and smooth blades are that you are likely to get a longer flight time with up to 14% more hovering time.
The good thing about using this approach is that there are no setting adjustments to make to your drone to obtain optimum performance. No setting changes need to be made. All you have to do is take off the old propellers and put on the new stealth propellers and the DJI Mavic air will make all of the adjustments. If there has been a software update you can play with the gain to deliver a better performance.
you can also purchase what is known as a shroud. A company called dotterel has produced a noise reduction and safety shroud for drone propellers. They reduce the noise by dampening the interaction of the air with the body, arms and other parts of the drone.
A safety shroud reduces the noise by firstly absorbing the sand and then also reflecting any residual noise up and away from people on the ground. There are also specific frequencies absorbed in the core of the shroud as it is made of acoustic materials that absorb some frequencies particularly well.
How science is making drones quieter
Ever since drones have been flying people have asked how can you make a drone quieter? There are plenty of commercial and technical reasons of why you may want a drone to be quieter. Perhaps you will be flying over animals, perhaps you will be flying over people who cannot be disturbed – perhaps you are even recording audio for a movie. Whatever the reason sciences started looking into the ways that we can reduce drone noise levels and we will cover some of the research here.
Noise scattering by drone fuselage
In 2019, a study by a Chinese research group looked at the design of quiet drones by understanding the physical aspects of drone noise generation and propagation. In their paper they present a framework to evaluate the scattering effect of the rotor noise of a flying drone using different modelling and mathematical techniques.
By understanding the effect of the body on rotor noise and the scattered noise generated from the reflected noise they have presented a way of analysing exactly what needs to happen to reduce the drone noise from different drone body types. In their paper, they use a drone body type that is very similar to the DJI phantom series. This is what the drone noise pattern looks like:
You can see that there is a load amount of noise reflected upwards and very little which is reflected downwards during a flight.
Designing a quiet propeller for an unmanned air vehicle
In 2009 scientists from Israel note that designing a quiet and efficient propeller is a demanding task because these two goals often lead to contradictory design trends.
The study looked at a multidisciplinary design optimisation process for the design of a quiet propeller. This design is subject to power, structural, and side constraints but they were able to design a quiet propeller for an electric mini unmanned air vehicle. The design variables include things like blades geometry, blade cone angle, propeller radius, the number of blades, and the rotational speed of the motor. The motor characteristics were also found to have a very important influence on the optimisation of the propeller and it should therefore be taken into account when designing the silent propeller.
They conclude the following:
- that the main mechanism of reducing propeller noise involves a reduction of the propellers rotational speed.
- Increasing the number of blades reduces the noise signature. The reduction in noise can get up to 10 dB quieter.
- The noise of the drone relates to how close the drone is above the observer and optimisation for a minimum noise at the point directly above the flyer should be prioritised in the design.
- The results of the paper emphasise the importance of a multidisciplinary approach to the design of a quiet propeller. It is important to address all of the disciplines simultaneously whilst designing a quiet propeller.
This article goes to show how hard it really is to create a drone which is truly quiet.
Acoustically tested stock and modified propellers
A recent paper published in 2021 looked at the stock DJI phantom to propellers and compare them to a stop propeller which was modified with a trailing edge notch.
A comparison of the two propellers showed the research is that the trailing edge notch and stop propellers have a similar sound pressure level at all distance measured. The trailing edge notch of the propeller did seem to shift the frequency higher and was the only significant difference between the two types of propellers.
This effect seems to be reduced as you increase the amount of propellers. A DJI hexacopter was also flown with the stop and trailing edge notch propellers. The hectic was 20 dB noisier than the phantom to but there was no discernible difference in sound pressure level between the two propellers.
This means that any significant changes to the trailing edge of the propeller are unlikely to cause a significant increase or decrease of noise. Interesting!
Will drones ever be silent?
Given the way that drones stay in the air I don’t think that drones will ever become silent. This is because they rely on the displacement of a significant amount of air which inherently will cause noise. Just think about a fan or other fast-moving object and there will always be noise.
Drone technology will need to come on in an unimaginable leap if drones were to become completely silent. However, there are scientists working on reducing the amount of noise and we can even get a drone to the point of being almost inaudible as it flies about 30 m in the air. That is incredible considering the amount of potential applications in an urban setting that drones may be able to fill.
They may not be silent but there will be very close!