Tackling drone noise pollution

A team at the University of Bristol has discovered that porous surfaces can reduce drone/UAM landing noise. They tell Vertiports about their findings

Our ever-evolving urban landscape means the prospect of swarms of drones delivering parcels and futuristic air taxis shuttling passengers above congested roads has become less the stuff of science fiction and more an impending reality. While these emerging forms of urban air mobility (UAM) promise to ease traffic woes and provide swift on-demand transport, they also bring with them a formidable challenge: noise. Now, groundbreaking research led by Dr Hasan Kamliya Jawahar at the University of Bristol in the UK has demonstrated how substituting solid ground surfaces with porous, vegetation-like materials can drastically curb drone and air taxi noise, offering a bold new solution to one of the biggest hurdles facing widespread adoption of UAM systems.

The study, recently published in Nature Scientific Reports, has proven that ground materials such as grass, foam or specially engineered porous composites can reduce certain noise frequencies by up to 30 decibels (dB) compared to conventional solid ground. This not only helps improve the overall efficiency
of propellers, but also holds the promise of creating “soundless city skies”. With major cities worldwide already grappling with traffic congestion and noise pollution, these results stand poised to guide policy, influence vertiport design and drive further research in advanced aerial transportation.

A promising future

The concept of UAM encompasses a wide spectrum of aerial vehicles, from small unmanned drones delivering urgent medical supplies to large, electrically powered aircraft transporting commuters across town. The envisioned fleets of electric vertical take-off and landing (eVTOL) aircraft have the potential to alleviate ground-based congestion and slash travel times dramatically.

Yet, as leading authorities and aviation experts acknowledge, noise has emerged as a crucial obstacle to public acceptance. While people are eager for faster, greener transportation, they are also wary of any technology that could worsen noise pollution, particularly in crowded urban environments. Urban planners are also bound by strict noise regulations, making it imperative to devise strategies to minimise or absorb the intense sound generated by the spinning propellers required to keep drones or air taxis airborne.

In existing UAM prototypes, the buzzing or droning noise of propellers during take-off and landing is especially intrusive. These stages of flight bring propellers close to the ground, intensifying noise due to direct mechanical emissions and reflections off solid surfaces. Landings on rooftops, elevated platforms or ground-level vertiports can affect entire neighbourhoods when repeated dozens or even hundreds of times per day.

The science of noise reduction

Researchers have long understood that ‘ground effect’ – the aerodynamic phenomenon that occurs when an aircraft or rotorcraft operates close to a solid surface – significantly alters noise emissions. If a drone or rotor is hovering near the ground, recirculating airflow, turbulent vortex structures and acoustic reflections collectively boost noise levels, particularly at low and mid frequencies. While numerous studies have investigated the ground effect on aerodynamic efficiency, fewer have focused on developing practical noise mitigation strategies tailored to the specific challenges of urban environments.

It is within this context that the University of Bristol’s aeroacoustic group embarked on a set of experiments. Led by Dr Kamliya Jawahar, the team’s work zeroed in on the pressing need to address noise when aerial vehicles are ‘in ground effect’ (IGE). This phase of flight usually happens within one to two propeller diameters from the ground surface – precisely the zone where passenger pick-ups and parcel deliveries are most likely to occur in future cities.

Dr Kamliya Jawahar explained: “It was known that ground effects influence propeller performance and noise, particularly during take-off and landing. While noise issues are well-documented, solutions tailored to urban environments are limited.”

The power of porous media

The idea to try porous materials came from observations in environmental acoustics. Quiet forests, landscaped parks and thick roadside foliage all stand as informal testaments to how vegetation can naturally dampen and diffuse unwanted noise.

Dr Kamliya Jawahar said: “I drew inspiration from natural porous materials, such as vegetation, known for their noise-damping properties. This led to exploring engineered porous surfaces as a potential solution to reduce noise and improve aerodynamics. Vegetation is known to function as a natural porous medium, where its structural complexity and material properties such as foliage density and moisture content contribute to its noise absorption capabilities. It has been widely used in environmental noise reduction strategies such as roadside barriers and urban green spaces, but this is the first time it is being investigated in the context of UAM.”

The experiment

To investigate whether porous media could dampen noise for drones and air taxis, the researchers set up a series of experiments in an anechoic chamber. Their test rig included a commercially available propeller with a 10in diameter that was mounted in a ‘pusher’ configuration, similar to what might be found on advanced drones or small eVTOL prototypes. It was suspended in the centre of the facility to isolate the noise sources and ensure minimal interference. A flat, rigid plate positioned at varying distances from the propeller served as the ‘ground’. This allowed the researchers to replicate different heights during take-off and landing, covering both IGE and out-of-ground effect (OGE) scenarios.

Three types of porous foams, varying in thickness and pore densities, were applied to the ground plane. This included a relatively coarse 45PPI, a fine 75PPI and a thicker 75PPI variant, reflecting a gradient of thicknesses and impedance.

A polar arrangement of microphones captured acoustic data in both ‘near-field’ and ‘far-field’ regions. Near-field microphones were flush-mounted on the plane directly under and radially out from the propeller hub, while far-field microphones were positioned around the chamber to analyse noise in multiple directions and angles. To measure any concurrent impact on aerodynamic performance, a six-axis load cell recorded thrust, torque, and other relevant forces. With drones, small changes in propeller thrust can significantly influence flight efficiency, flight time and battery consumption.

By systematically comparing solid ground conditions with porous surface treatments at different distances, the team was able to quantify exactly how different surfaces shift airflow patterns, reduce reflected turbulence and, in doing so, cut down both tonal and broadband noise.

Drawing conclusions

The first finding was that the use of porous materials could deliver noise reduction across the spectrum. In the near-field region, where noise is most critical for community disturbance (eg, directly under a vertiport or rooftop landing pad), the porous treatments reduced low to mid frequency noise by as much as 30dB. At the same time, overall sound pressure levels in select higher-frequency bands also dropped substantially. This suggests that both the throbbing hum of propeller blades and the whine of higher harmonics can be mitigated.

The team also noticed the materials’ stabilising effects on turbulence. High-coherence metrics used to measure the stability and similarity of pressure signals revealed that porous surfaces reduce random chaotic fluctuations in the flow. Essentially, these surfaces organise the air patterns just enough to prevent intense, chaotic backwash from re-entering the propeller blades. This consistent reduction in disturbed airflows helps reduce noise at the source.

Furthermore, contrary to concerns that a porous surface might sap aerodynamic efficiency, the study found that propeller thrust and power coefficients actually increased in certain scenarios. By modifying the ground-jet flow and partially reflecting beneficial pressure waves, the porous surfaces effectively contributed to an air-cushion phenomenon that could boost lift. The biggest improvements were noted in the extreme in ground effect phase, roughly within one diameter of the propeller to the ground. This is precisely when a drone or air taxi would be about to land on a rooftop or a designated vertiport, a scenario with the greatest risk of noise nuisance for surrounding areas.

Implications for urban planning

As UAM operators look to popularise short-distance, on-demand flights, public acceptance will hinge on the perceived noise footprint. The University of Bristol’s findings point to a simple, yet profoundly impactful measure: the adoption of natural or artificial porous surfaces whenever possible on rooftops, landing pads or vertiports.

Rooftop gardens, moss, synthetic foams or specialised building materials could serve dual roles, both dampening the audible roar of propellers and beautifying an urban skyline often criticised for concrete uniformity. In noise-sensitive neighbourhoods, such as near hospitals, schools or residential high-rises, this approach addresses a need that acoustic insulation alone typically struggles to meet outdoors.

Such a multi-pronged solution could dovetail with broader sustainability goals, providing not only noise relief but also improving local air quality and rainwater absorption, as well as mitigating the urban heat-island effect. Indeed, the study’s alignment with Sustainable Development Goals, particularly SDG 11 (Sustainable Cities and Communities) and SDG 9 (Industry, Innovation and Infrastructure) underscores the research’s relevance beyond mere engineering applications.

Dr Kamliya Jawahar said: “Our research demonstrates that innovative porous landing surfaces can drastically reduce noise from drones and air taxis, paving the way for quieter and more sustainable urban skies. We hope these findings encourage city planners, aerospace companies and environmental policy-advisers to collaborate on adopting porous materials for vertiports and roofs, ensuring UAM’s integration remains as unobtrusive as possible.”