'An annoying buzz': The search for stealthier drones

‘An annoying buzz’: The hunt for quieter drones

Nature enthusiast Ashley Schmidt no longer spends much time in her garden in North Garland, a suburb of Dallas.

She has also stopped cycling to the nearby parks where she used to enjoy taking close-up pictures of bugs, lizards and toads.

The reason is that she cannot tolerate the noise of delivery drones flying overhead and dropping off her neighbours’ online purchases.

“They emit this very annoying buzz. It triggers my fight or flight response,” says the 44-year-old administrative assistant.

“You can hear the drones on approach and you can hear them inside your home when they pass directly overhead and then you hear them as they leave,” she says.

Schmidt is among a rising number of people living with the sound of drones sent out by companies including Amazon and Walmart as they try to speed up home deliveries.

Delivery drone services, once a novelty, are becoming part of daily life for many people in the US.

Amazon Prime Air, among them, is expanding deliveries to nearly 500 cities and towns, carrying items that fit into a space the size of a shoe box. It has also launched to customers in and around Darlington in the north of England.

Smaller drone delivery services are also appearing: in some US cities, customers can order flying treats from their local pizza shop or ice cream parlour.

Even in places where noise is already a constant feature, the sound of drones appears to irritate in a distinct way.

Whereas the steady flow of traffic may fade into the background, the abrupt whine of a drone’s rotors can cut through the ordinary hum.

The noise tends to be “particularly noticeable, even when overall sound levels are relatively low,” says Antonio Torija Martinez, professor in acoustic engineering and psychoacoustics at the University of Salford.

Unlike planes, drones fly at low altitudes and may pass directly over residential areas.

“They can expose communities that have not previously experienced significant aircraft noise, and who may therefore be more sensitive to changes in their local soundscape,” says Torija Martinez.

He cites research indicating that people react strongly to a noise when they see its source as “novel, intrusive or frequent”. People who use a drone delivery service, for instance, may be less troubled by the sound.

Still, he warns that complaints about drone noise are likely to increase: “At the moment we are seeing some of these problems localised in some areas, but… as the drone sector expands, more and more of these complaints will happen.”

In the US, the Federal Aviation Administration (FAA) oversees commercial drones, but it has faced criticism for permitting noisy drone delivery operations. The FAA says operators and local community leaders have the main responsibility for dealing with any community concerns.

Could better technology soften the irritating buzz?

One startup, Greenjets, aimed to address both “noise level and the frequency of that noise,” as Darren James, future technologies lead at the company, puts it.

The London-based firm has created a propulsion system it says can cut drone noise by at least two thirds.

Where standard drones use spinning propellers that cut through open air and create that high-pitched buzz, Greenjets encloses spinning blades inside a pod to dampen the sound and send the noise away from the ground.

Its system also reduces noise through slower rotor speed and by using materials that cut vibration noise.

Making drones quieter is not only about reducing the annoyance for neighbours already tired of airborne deliveries.

Greenjets’ low-noise technology has been used by police and harbour security teams in Germany. It has also assisted conservationists in Africa, who want to track poachers from above without being noticed.

Drone operators on the battlefield also have strong reasons to keep noise down, so as not to alert the enemy to the approach of their unmanned vehicles.

“There are some efforts to reduce the sound of drones. The US Special Forces are interested, at an experimental stage,” says Robert Tollast, a research fellow at Whitehall think tank The Royal United Services Institute (RUSI).

But he says that for larger drones used in the war between Russia and Ukraine, speed usually comes before stealth. “Make it faster, not less quiet, seems to be the approach,” says Tollast.

For Detroit-based entrepreneur Greg Sharrow, the push toward quieter drone design began with the challenge of broadcasting a classical music performance in 2012.

He needed to film a violinist from a difficult angle, and a drone seemed like the only answer — except that it would be far too noisy.

As a starting point, “I knew that most of the noise from the drone was coming from the propeller, not the motor,” he says. More specifically, he realised the blade tips were producing the noise as they sliced through the air.

“I asked myself a very simple question, what would a tipless propeller look like and how would it function?”

The answer was a propeller with twisted loop-shaped blades and no points.

It turned out to be easier to test the propeller in water than to keep flying a drone repeatedly, which led Sharrow to first develop a quieter boat propeller. Now, though, his company, Sharrow Engineering, is concentrating on “ultra-quiet” drone rotors.

“We have validated that the technology works in drone applications and that it is dramatically quieter,” says Sharrow.

Companies leading drone deliveries say they are also trying to cut the noise from their fleets. A spokesperson for Amazon says it has made “measurable sound improvements” to its drones and that its latest designs use custom-designed propellers “that produce almost half the perceived sound of the prior generation, with a milder tone”.

Drone maker Wing, which supplies its vehicles for Walmart deliveries, says its propellers are designed to reduce noise. It also changes its flight paths to avoid repeated overflights over any one home or neighbourhood.

For Schmidt, who is fed up with drone noise, the only truly acceptable solution would be to stop drone deliveries in her area. “I would like them to completely stop until they figure out how their services stop being a nuisance,” she says.