Why Are Drones So Noisy?

Last updated January 21st, 2020
A kid shouting into a mic

A study carried out by NASA discovered that the buzzing sound that drones are known for is far more annoying to people than the rumble of cars and trucks.

And this is a fact that companies with ambitions of drone delivery, like UPS, Amazon, Uber, and Domino’s Pizza, are going to have to take note of.

After all, their delivery schemes are going to reshape the soundscape of cities and suburban areas forever—and that might be for the worse.

For the study, scientists at NASA played recorded audio samples of a car, a truck, two vans of different sizes, and drones for 38 willing subjects.

Subjects were not told what the sources of the sounds were. They were then asked to rate the sounds on a scale ranging from “not at all annoying” to “extremely annoying”.

The results of the study showed that the sound of “road vehicles seem to be systematically judged to be less annoying” than drones.

The drones were recorded at various speeds and different altitudes. But the sound at higher altitudes made almost no difference in the level of annoyance associated with it.

You may be wondering about the reason for so much hate.

One reason researchers at NASA put forward suggests that people are simply used to the sounds of road vehicles. They also likely use them more to get around so they are more likely to tolerate the noise which road vehicles make.

In other words, we have become numbed to the noise which they make even as the level of noise pollution in cities worsens.

This issue with noise pollution is likely to be a factor that will affect how far reaching drone deliveries will be. Very few people are going to be comfortable with the idea that their late night drone delivery will likely disturb their neighbors.

Why Are Drones So Loud?

When we talk of loud, we mean when the drone is beside you. Because if a drone is 250 feet away, you won’t hear anything at all.

Long story short, that loud, annoying buzz that drives you crazy when you are beside a drone is caused by the rotors slicing through the air.

On their own, electric motors are very quiet when they work. If you ever activate your drone without the rotors, all you are going to hear is a slight whine as the electric motors spin.

The role of the propeller is to convert their kinetic energy to thrust or lift force strong enough to lift the drone off into the air.

But anybody with a basic understanding of physics and mechanics knows that this process of changing or transducing one form of energy to another is never perfectly efficient.

Some of the kinetic energy is lost in the process to produce undesirable things such as heat and noise.

So a typical airliner might have rotors that are 80% efficient — meaning 80% of the kinetic energy gets transformed to the lift force for takeoff.

A multirotor drone might have a propeller efficiency of 60% to 70%, meaning that only 60% to 70% of the kinetic energy in the propeller gets transformed into lift force. The remaining 30% to 60% is dissipated or lost as noise or heat.

Now you know where the sound you hear comes from. And if you hear two propellers that are spinning at about the same rate, and yet one sounds louder than the other, you should know that the louder one is the less efficient one between the two.

More Efficient Drones Make Less Noise

It goes without much saying that if you want a drone that makes less noise, you need a propeller that is more efficient at transducing kinetic energy to thrust energy.

You want a propeller that allocates less energy to noise creation.

The main factors that affect propeller efficiency include blade count, diameter, pitch shape, and the operating revolutions-per-minute (RPM).

In theory, single blade propellers are the most efficient, but in practice they are hard to implement. That’s why less efficient, 2-blade propellers are more easy to find.

Smaller propellers are also less efficient than larger ones for a host of reasons that include compressibility, viscosity, and so on.

In your search for an efficient, less noisy drone, these are factors you should bring together in order to make a decision.

Are Silent Propellers Better?

In our quest for drones that are less noisy — or outright silent, could we be unknowingly shooting ourselves in the leg and clamoring for aerial devices that perform worse than the ones we now have?

In other words, are silent drones more efficient?

The buzzing of propellers might drive you crazy, but are silent propellers necessarily a better option?

There are a host of new wave propellers that are being rolled out which are touted as low noise propellers. And the only way to answer correctly would be to consider what the differences and similarities between the original propellers and the new low noise propellers are.

Both sets weigh about the same. But the aerodynamic design of the new low noise propellers is different from that of the other.

The low noise propellers are also thinner and about 3mm longer than the original propeller blades.

While all these adjustments make for less noise, they do come with a few downsides.

First of all, they cause slight vibrations that can be noticed through the camera.

They are also more liable to break since they are much thinner. This lightweight nature also means they are less resistant to interference from outside sources.

Perhaps, you would rather not be bothered by all these. If that is the case, you may want to consider getting a drone that is manufactured to be quiet from the onset.

”Silent” drones exist and are very popular among drone enthusiasts.

One of the best examples of such silent drones is the Force1 F100 which many believe to be among the quietest drones in the world. This is a brushless drone with brushless motors and independent electronic speed controllers for better performance.

It is also fairly affordable.

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