'An annoying buzz': The search for stealthier drones

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

She's also given up on cycling to the local parks where she previously enjoyed taking close-up photos of the bugs, lizards and toads.

It's all because she can't stand the sound of the delivery drones that fly overhead, depositing her neighbours' online orders.

"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 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.

Delivery drone services, not long ago a novelty, are becoming part of everyday life for many people in the US.

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.

Even in cities where noise is already a steady part of life, the sound of drones seems to irk in a particular way.

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.

Unlike planes, drones operate at low altitudes and may fly 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 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.

Might better technology muffle the irritating buzz?

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.

The London-based business has designed a propulsion system it claims can reduce drone noise by at least two thirds.

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.

Its system also quietens things down through slower rotor speed and by using materials that reduce vibration noise.

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.

Battlefield drone operators also have good reason to keep the noise down, to avoid alerting the enemy of 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 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.