How to Reduce Drone Noise From Propellers

By Lauren Nagel

Reducing drone noise is a priority for many drone developers and operators. Purchasing low noise propellers or a low noise drone is one way to achieve that outcome, but there’s almost always room for further improvement.

What more can you do, as a drone developer or operator, to reduce your drone’s noise?

In this article, we look at how propellers contribute to drone noise, and how the noise level differs between balanced and imbalanced props. We finish with a practical recommendation on how to reduce noise coming from your drone's propellers.

Table of Contents

1. Propeller Damage and Noise

2. Low Noise Drone Propeller Performance

3. How to Reduce Drone Noise

Figure 1: Quadcopter drone in flight

1. Propeller Damage and Noise

Intuitively, one assumes that imbalanced propellers produce a greater amount of noise than balanced propellers. However, for small imbalances not significantly impeding the flight of the UAV, the results are mixed.

For example, in Pechan (2015), the authors observed the opposite effect. Propellers with various imperfections such as protuberances, notched blades, and mass imbalances were tested for noise production and rotated at various RPMs from 3000 to 5500 RPM.

In the majority of their experiments, the propellers with protuberances or notches showed, on average, equal or higher noise production (LZmax) compared to the undamaged propeller.

Figure 2: The highest detected noise level (LZmax) of damaged (B1 - B5), imbalanced (B6), and smooth propellers (Pechan, 2015)

In the case of the imbalanced propeller, which had material removed from one blade via sanding, the results showed that the balanced propeller actually produced more noise at higher frequencies at 5000 RPM. 

Figure 3: Noise spectra of smooth and imbalanced propellers at 3000 (left) and 5000 RPM (right) (Pechan, 2015)

The authors do not specify how much material was removed from the blade, so it is difficult to interpret these findings. The propellers that were damaged by addition or removal of material were also not characterized in detail, and likely represent an imbalanced state themselves.

The authors discuss the difference between tonal and broadband noise, which they differentiate as follows: “Tonal noise is related to the aerodynamics and kinematics of the blade in uniform flow, [...] thickness, loading and quadrupole noise. Broadband noise is more complex and is generated by the interaction between the flow and various components of the blade”.

The tonal noise levels represent most of the contribution to the total noise, while the broadband noise represents only a small portion. They note that a major contributor to tonal noise is thickness noise, determined by the thickness of the propeller blade.

It may be hypothesized that by reducing the thickness of the imbalanced propeller’s blade through sanding, they may have reduced the thickness noise and lowered the overall tonal noise production.

If you need to add or remove weight from propellers for balancing or other purposes, there are safe ways to do so.

2. Low Noise Drone Propeller Performance

A third study performed by Semke (2021), sought to characterize how damage influences the acceleration, acoustic levels and flight performance of UAVs. Testing was performed using a DJI Phantom 4 Pro with four unique sets of propellers; Two of the sets had a low-noise design and the other two sets had a standard design. In each set, a centimeter of material was removed from the tip of one of the propellers to introduce an imbalance, while the other was unmodified.

The results demonstrated that for two of the standard blades, the imbalanced propellers showed a 4 dB noise level increase compared to the unmodified propellers. In contrast, the low-noise propellers showed little to no difference between the damaged and undamaged conditions.

Figure 4:Sound pressure level in dB of UAVs with undamaged and damaged propellers (Semke, 2021)

They also studied the vibration of the aircraft using an on-board accelerometer. The results parallel the acoustic measurements: all damaged aircraft exhibited increased vibration, but the increase was less pronounced in UAVs with low-noise propellers.

Figure 5: Acceleration increase between UAVs with undamaged and damaged propellers (Semke, 2021)

Each of these studies take for granted that the UAV’s propellers and motors were well balanced prior to the experiments. It would be interesting to see further studies of these effects with the following additions: 1) known propeller mass, 2) known mass or magnitude of the imbalance, 3) a measurement of the propeller’s balance against an established standard.

The takeaway from these studies is that small imbalances in drone propellers can result in increased noise production. Large imbalances will likely result in even more noise, becoming more noticeable with larger propellers or propellers operating at very high RPM.

3. How to Reduce Drone Noise

Studies such as these bring up several questions about the relationship between propeller balancing and noise production. Overall, it seems that having well balanced propellers can help you reduce drone noise.

There are many static balancers on the market that can help you achieve a basic balance of your prop, but for more precise balancing, and minimizing those last few dB, dynamic balancing is essential.

At Tyto Robotics, we have developed a dynamic propeller balancing system that allows users to quantify and correct imbalances in their propellers in just a few spins.

We use the ISO 21940-11 standard as the basis for the balancing, so you can quickly balance your propellers to internationally recognized standards.

The system pairs with a sound sensor to measure the noise produced by your propeller at different states of imbalance.

Click here to learn more and start balancing.

Figure 6:Adding weights to the blades of a propeller to achieve equal mass distribution

Conclusion

We hope that this article has provided some insight into the relationship between propeller balancing and drone noise production. If you have any further ideas or questions, let us know in the comments.

To learn more about how imbalanced propellers can impact your drone's performance, download our white paper: Consequences of Propeller Imbalance on UAV Performance:

Download White Paper