In this commentary by Tomas Pribanic, Founder and CEO of Undefined Technologies, he explores the problem of noise pollution and urban restrictions for drone delivery.

The company is a Florida-based tech startup developing its first-ever silent eVTOL drone powered by ion propulsion. In April, it had a successful first mission flight.

Three years ago, Bonython, a township in southern Canberra, Australia, was the site of a new drone delivery service deployed by Google spin-off Wing. Initially, residents cheered the last-mile food delivery.

The company even planned to expand its service, but the trial caused anger among the residents. Some locals soon became bothered by the continued noise of the drones and complained.

Residents cited concerns about the impact of drone noise on wildlife. Others cited migraines and worried the noise would worsen their medical condition. “With the windows closed, even with double glazing, you can hear the drones,” one resident told ABC News.

The issue reached a boiling point when several residents began threatening to shoot the drones out of the sky. As a result, Wing had to modify the blades used on its drones and began randomising routes from the depot and to individual homes so that no one area was bearing the brunt of a drone noise corridor.

Australia’s experience shows the rapid deployment of drones without proactive noise mapping or modeling can incite significant community backlash and regulatory response. Could this scenario repeat in the US? The answer would probably be ‘yes.’

As major retailers accelerate the deployment of drones in urban areas, we will see higher demand for autonomously operated drones that must comply with federal and county regulations and ordinances. 

Walmart recently announced its significant investment in DroneUp, the nationwide on-demand drone delivery provider, while Amazon is building their own unmanned aircraft. In 2020, Amazon’s PrimeAir fleet received FAA approval to commercially operate drones.

Noise Matters

As drone technology takes flight, the world will be getting much louder – as if ‘neighborhoods were filled with leaf blowers, lawnmowers and chainsaws.’ Drones’ distinctive and irregular buzzing sound remains an unresolved issue.

Research conducted by NASA in 2017 and a follow-up analysis published in 2018 found that drone noise ranks highest in terms of annoyance. According to the study, the noise made by road traffic was “systematically judged to be less annoying” than the high-pitched buzzing made by drones.

Participants were not told the noise they were hearing was from drones. Still, all listeners consistently rank drone noise as more annoying than noise from airplanes or delivery vehicles due to psycho-acoustical properties not easily modelled or captured by traditional acoustical metrics.

A year later, the National Academies of Sciences, Engineering, and Medicine was approached by NASA to evaluate aerial mobility’s benefits and disruptive effects, including drones. Their preliminary findings, released in the spring of 2020, call for further research into public annoyance associated with drone noise.

No Regulations

Despite the different noise ordinances established by counties in the US, no clear regulatory framework exists at the federal level to monitor any noise in urban areas.

This year, congressional lawmakers already announced plans to re-introduce the Helicopter Safety Act on Capitol Hill to ban non-essential helicopter flights over New York City. The measure was originally introduced in 2019, and aims to mitigate helicopter traffic and noise pollution.

Noise mapping could bring more clarity to the situation. In the United States, noise mapping is only common at a project-level scale for specific sound sources such as airports, manufacturing, commercial facilities, and highways since no law or regulation requires monitoring regional noise.

However, at a larger scale, regional noise maps are more common in Europe due to EU Directive 2002/49/EC, which requires regional noise maps in areas with high population densities.

There are just a few regional noise maps in the US. For example, the National Park Service (NPS) has collected data from its parks across the United States. The NPS has paired these data with several environmental, human activity, and topography maps to model anthropogenic noise and natural sound. The Volpe Center also released its National Transportation Noise Map in 2017, representing the first national transportation noise dataset available to the public.

Health & Well-being Impact

Scientists estimate that a single drone can make an area 8 to 12 times louder than it is now. Serious commercial drones have eight or more propellers (Wing has 14; Amazon’s Octocopter has eight propellers spinning at thousands of revolutions per minute, physically beating the air to generate lift and movement). The heavier the load, the harder the drone must work, the more air gets beaten, the louder the sound emitted.

Without innovation and regulation, the buzzing of drones may soon fill the city and suburban skies – adding to the din in many places and disturbing the peace of even those wealthy suburbs whose residents can afford the convenience of rapid home delivery. Even neighbourhoods that have managed to avoid being under airport flight paths will find themselves surrounded by the buzz.

Commercial drones could turn a quiet day into something different. It’s not just loudness. Drones have relatively small propellers, which don’t move much air, but move rapidly. The speed of the spinning equates to its frequency.

Adding a payload to a drone means the propellers must put more energy into the air by spinning faster – making a louder and higher-pitched sound. The frequencies they generate are, in fact, the same frequencies people are most sensitive to.

“Imagine tens or even hundreds of drones buzzing around your neighborhood, delivering packages to homes and businesses,” says Garth Paine, Associate Professor of Digital Sound and Interactive Media, Arizona State University, in his article Drones to deliver constant buzzing noise, and packages.

“Next, imagine the round-the-clock hives of aerial activity that warehouses and distribution centers will become, in addition to their existing burden on local roads.”

The amount of time a person is exposed to different sound levels matters, too. The US Occupational Safety and Health Administration says workers exposed to 85 decibels or louder for eight hours or more may suffer hearing damage or loss. The Federal Aviation Administration says that residential areas should not have aircraft noise averaging above 65dB in a 24-hour period.

Near-Silent Solution

Uber Elevate, even in 2016, predicted that “new noise exposure and annoyance from these vehicles could limit the success of integrating UAM into the transportation system.” To reduce the impact of drones’ noise, planning for their operations requires an innovative approach and an important consideration for regulators.

Ion propulsion technologies could make a difference since it offers unique opportunities in generating silent thrust.  Most VTOL (vertical takeoff and landing) drones rely on high-speed rotors to generate enough thrust to take off and stay in the air.

Meanwhile, ion propulsion is entirely electric and produces zero carbon emissions. The process uses a high-voltage electric field to ionize the air molecules in the atmosphere; the air molecules naturally return to their original state after ionization.

Currently, ion propulsion is already used to keep communication satellites properly oriented and to propel deep-space probes. This technology propels faster and for greater distances. It’s also less expensive than any other propulsion technology currently available.

On Earth, much larger amounts of the thrust are needed to overcome both gravity and the atmosphere. If ion propulsion is made viable for eVTOL operating in the earth’s atmosphere, it would be a game-changer in urban cargo delivery.

The technology will inevitably affect industries vertically and horizontally across sectors, enhancing the addressable market dramatically. The challenge is fascinating and achievable from many other perspectives, including creating, storage, inversion, and thermodynamically efficient transmission and dissipation of energy.