Physical Sciences
July 10, 2021

How to make noisy drones a little less irritating

Unmanned drones hold the potential to transform many aspects of our society for the better. So far, however, their perception by the public has been held back by the annoying noise they make. In his research, Dr Antonio Torija Martinez at the University of Salford in Manchester studies how the impact of drone noises on communities can be reduced – both by diverting their flight paths away from quieter areas, and optimising vehicle design. His team’s work could soon set the stage for the widespread use of drones in our everyday lives, without their noise significantly compromising our health and wellbeing.

In the space of just a few years, remote-controlled drones have come to captivate our imagination in many ways – with popular applications including easy-to-use platforms for aerial photography. Yet many researchers believe that the public use of drones has so far only scratched the surface of their full potential. As materials become more advanced, electric motors more compact and powerful, and communications systems more sophisticated, leaders in the industry are now advocating their use in a growing number of useful applications – ranging from automated package delivery, to fast-response emergency services, and even people transportation.

One major barrier to the rollout of these facilities is the irritating noise that drones make as they fly. A bit like a giant buzzing insect, their sounds are distinctively loud, fluctuating, and high-pitched. It is hardly surprising that many people caught under their low-altitude flight paths find them annoying, and possibly even detrimental to their health and wellbeing. Current evidence suggests that without the technological innovations required to reduce the impact of this noise, public support for the widespread use of drones in populated areas will inevitably remain low. The European Union Aviation Safety Agency has recently reported safety, security and noise as the top three concerns for drones. In his research, Dr Antonio Torija Martinez explores how this problem can be addressed.

Test rig used to understand the variation of sound quality metrics with different configurations of a contra-rotating propeller (i.e. varying axial distance between rotors). This data helps to understand the optimal configuration of this rotating system for minor psychoacoustic impact. The test rig was assembled by Dr Chaitanya Paruchuri of the University of Southampton.

Accounting for human responses
When planning new products like a drone, it is often crucial for designers or engineers to consider how users will respond to them. Whether it is a piece of furniture designed for comfort, or computer software designed for ease of use, human experiences of a final product cannot simply be accounted for as an afterthought. Any subsequent additions or modifications to the design are more difficult, more expensive and likely much less efficient. Instead, aspects which improve our responses to products must be engrained into their designs, right from the start of the production process.

Negative impacts on the public could be reduced by simply concentrating the flight paths of drones along busy main roads.

For Dr Torija Martinez, the principles of drone design should not be any different. “By integrating human responses into the design process, the most undesirable noises can be avoided in the earliest stages of vehicle development,” he says. “For instance, if drone manufacturers incorporate these strategies into their designs, they might just build machines that are not only efficient, but also just that little bit less irritating.” Until now, drone manufacturers have not possessed the tools to optimise their vehicles to ensure that the sound they produce is less annoying. Dr Torija Martinez’s proposal, to radically change the processes used to design and operate drones, offers significant benefits in efficiently achieving a reduction in noise impact.

Assessing annoyance in urban soundscapes
Before assessing the design process itself, Dr Torija Martinez and colleagues first set out to explore public perceptions of the sounds made by existing drone designs. “Some of my most recent research has been focused on understanding how communities will respond to drone noise, depending on how the drone is flying and where,” he explains. “The outcomes of this research can inform the operation of drones, with reduced impact on communities’ health and wellbeing.”

In a 2019 study, Dr Torija Martinez and his colleagues asked participants to report on their perceived levels of loudness, annoyance, and pleasantness within different urban soundscapes, when drone noise was either present or absent. Using virtual reality headsets, they presented the participants with a range of different urban environments: from quiet streets to busy main roads. They found that while the noise generated by the drone did not significantly increase the annoyance reported by the participants in areas with high levels of road traffic, it became far more noticeable within quieter soundscapes, such as residential areas and parks.

Values of psychoacoustic annoyance with varied axial spacing between the contra-rotating rotors. The results indicate that the optimal spacing (relative to propeller diameter) for minor psychoacoustic annoyance is approximately 0.2-0.4 z/D. Source: Torija, A.J., Chaitanya, P. and Li, Z. (2021).

Taking together these results alongside the potential issues with safety and security, the researchers suggested that negative impacts on the public could be reduced by simply concentrating the flight paths of drones along busy main roads. These new paths could then serve as arteries – allowing drones to travel to quieter areas, mostly without irritating the people under their flightpaths. Yet although innovative, this proposal still did not address the root of the problem in the drone design process.

Optimal propeller designs
A variety of drone designs exist, including different sizes, number and positioning of propellers. One of the designs that provides significant advantages for aerodynamic performance and vehicle’s safety is a vehicle with so-called contra-rotating propellers. Here, two sets of propellers are positioned in front of each other (usually with a very small spacing) and rotate in opposite directions. In a well-designed contra-rotating propeller, high volumes of air are pushed through the blades, with little energy expended by the aircraft’s engine. However, if noise is not factored into the design process, these propellors are notoriously loud and potentially annoying.

Fortunately, this noise can be reduced if both rotating elements of the propellor are spaced at just the right distance apart. In a further study, Dr Torija Martinez and his colleagues explored the impact of this rotor spacing on various features of sounds which can negatively impact the perceptual responses to propeller noise. The researchers used a series of sound quality metrics to define the optimal separation distance between the contra-rotating propellers. The different sound quality metrics are good indicators of how the human auditory system responds to different sound features (e.g. loudness, pitch, etc).

The experiment represented a step forward from previous studies, which are based on more conventional approaches relying solely on sound power levels. “The use of sound quality metrics can account for how we perceive key features of sound, like how loud it is. This allows us to inform the design of these vehicles so that their noise annoyance is substantially reduced,” Dr Torija Martinez explains. From their results, the team could establish an optimal relationship between propeller blade diameters, and the spacing between their contra-rotating rotors – ultimately minimising the annoyance of the noise they produced.

Test rig of a conventional ducted fan use for acoustic testing within the InCEPTion project. The test rig was kindly provided by the University of Cambridge’s Whittle Lab.

A new type of noise assessment
These studies have now enabled Dr Torija Martinez and his team to continue the development of models that can be used by designers to smartly assess how annoying a particular propeller design will be. By integrating this analysis into the earliest part of the propeller design process, the researchers hope that the noise annoyance of people under the flightpaths of drones will be at the forefront of future drone designs, from the very beginning of the production process. However, more work is currently being done by the research team to connect sound quality models directly to design parameters.

These innovations could be instrumental in ensuring minimal impacts of drone (and other novel aerial vehicles) noise to public health and wellbeing.

In their upcoming research, the team will continue to develop their approach for other novel aerial vehicles through the Integrated Flight Control, Energy Storage and Propulsion Technologies for Electric Aircraft (InCEPTion) project. Co-funded by Innovative UK, and the UK’s Aerospace Technology Institute, this project will draw Dr Torija Martinez’s findings together with the latest advances in all-electric aircraft engineering.

“Here, we will implement cutting-edge sound quality models for the low noise optimisation of an all-electric fan propulsor, and therefore will go beyond the traditional approach on aircraft noise assessment,” Dr Torija Martinez describes. Through these efforts, the researchers hope to contribute to a large industry consortium to produce a quiet, compact, and highly efficient propulsion module, suitable for use in a wide variety of applications.

Anechoic chamber: Image of the University of Salford’s Anechoic chamber during sound perception testing.

New generations of flying vehicles
Ultimately, the aim of Dr Torija Martinez within the InCEPTion project is to integrate their state-of-the-art sound quality models into the design of an all-electric fan propulsor. The expectation is that these models can be used to aid the design of vehicles of different shapes and sizes. In the not-so-distant future, these aerial vehicles could be used in areas as diverse as emergency responses, heavy cargo delivery, and taxis for transporting commuters. If achieved, these new aerial vehicles could free up a lot of space on the road; both reducing travel times and cutting emissions of air pollution and CO2.

The transportation sector is rapidly evolving towards more electric concepts. This offers an opportunity for a paradigm shift to do things differently, improving the methods to assess and intervene, minimising noise impacts on communities. As they work to make the noisy drones of today sound less annoying, the innovations made by Dr Torija Martinez and his research team could soon be instrumental in ensuring minimal impacts of drone (and other novel aerial vehicles) noise to public health and wellbeing as these applications are rolled out. “My vision is to develop a suite of methods, metrics, and procedures to optimise the design and operation of novel aerial vehicles for lesser community noise impact,” he concludes. “Through this, we could transform the way transportation noise issues are addressed.”

Personal Response

Are you confident that reductions in drone noise annoyance will allow for applications like unmanned package delivery within the next few years?

<>This is certainly a difficult question to answer. In principle, evidence suggests that there is a positive initial attitude to drones and other novel aerial vehicles. Very recently, the European Union Aviation Safety Agency reported findings from their study on the societal acceptance of Urban Air Mobility operations. The report shows that there is strong support for the use of drones for applications of public interest, i.e. when operations are perceived to be useful (e.g. delivery of medical supplies), and that there are clear benefits identified to drone use: faster, cleaner and extended connectivity.

However, noise is one of the top three concerns. Tackling the noise issues of these vehicles alone does not guarantee the wider adoption of drone applications. There are other concerns to deal with, for example safety and security. However, what is indisputable is that public acceptance of drone operations will not be assured until the issues of noise are appropriately addressed.

This feature article was created with the approval of the research team featured. This is a collaborative production, supported by those featured to aid free of charge, global distribution.

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