Drone technology: Start

‘An annoying buzz’: The search for stealthier drones

Amazon Prime Air is expanding to 500 US towns and cities

Amazon An Amazon drone flies over an US city. In the background are mountains and a blue sky. — Amazon Prime Air is expanding to 500 US towns and cities

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 <a 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 <a she says.

Amazon A worker kneels next to an Amazon drone and loads a package into its cargo bay. The drone is large - as big as the worker. — Amazon says it is working to reduce the noise of its drone fleet

Schmidt is one of a growing number of people living with the noise of drones dispatched by <a 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 them 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 A regulates commercial drones, but it has come under fire for allowing noisy drone delivery operations. The FAA says operators and local
Greenjets

Greenjets encloses some of the blades on its drone inside a pod

Might better <a 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.