'An annoying buzz': The search for stealthier drones

Schmidt is one of a growing number of people living with the noise of drones dispatched by companies including Amazon and Walmart as they seek to speed up home deliveries.

Among them, Amazon Prime Air is in the process of expanding deliveries to nearly 500 cities and towns, delivering 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 popping up too: in some US cities, customers can order flying treats from their local pizza joint or ice cream parlour.

While the steady passing of traffic might blend into the background, the sudden whine of a drone's rotors can pierce the everyday hum.

The sound 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.

"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 points to research showing that people respond strongly to a noise when they consider the source to be "novel, intrusive or frequent". People who benefit from a drone delivery service, for example, may be less bothered by the noise.

However, complaints about the sound of drones are likely to grow, he warns: "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) regulates commercial drones, but it has come under fire for allowing noisy drone delivery operations. The FAA says operators and local community leaders have primary responsibility for addressing any community concerns.

One startup, Greenjets, set out to tackle both "noise level and the frequency of that noise," as Darren James, future technologies lead at the company, puts it.

While standard drones use spinning propellers that slice through open air, creating that high-pitched buzz, Greenjets encloses spinning blades inside a pod, to muffle the sound and direct the noise away from the ground.

Making quieter drones isn't just about keeping the noise down for neighbours who are already sick of airborne deliveries.

Greenjets' low-noise tech has been used by police and harbour security teams in Germany. It has also helped conservationists in Africa, who are seeking to track poachers from above without being detected.

"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 stresses that drone speed, rather than stealthiness, tends to come first for larger drones used in the war between Russia and Ukraine. "Make it faster, not less quiet, seems to be the approach," says Tollast.

For Detroit-based entrepreneur, Greg Sharrow, it was the challenge of broadcasting a classical music performance back in 2012 that set him on the path to quieter drone design.

He needed to film a violinist from a tricky angle and a drone seemed the only solution – 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. Specifically, it was the tips of the blades that he realised were creating the noise as they swiped through the air.

The result was to create a propeller with twisted loop-shaped blades with no points.

It turned out it was easier to test his propeller in water than it was to repeatedly fly a drone, which led Sharrow to initially develop a less noisy boat propeller. Now, however, his company, Sharrow Engineering, is focusing on developing "ultra-quiet" drone rotors.

Companies at the forefront of drone deliveries say they are also working to reduce the noise of their fleets. A spokesperson for Amazon says it has made "measurable sound improvements" to its drones and 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 provides its vehicles for Walmart deliveries, says its propellers are designed to minimise noise. It also varies its flight paths to prevent repeated overflights over any single home or neighbourhood.

For Schmidt, fed up with the noise of drones, the only truly satisfactory solution would be to halt 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.