As cities around the world begin to embrace the concept of Urban Air Mobility (UAM), the potential for drones and air taxis to revolutionize transportation has become a growing area of interest. However, one major concern associated with these innovations is the noise pollution they could generate, especially in densely populated urban environments. The sound created by drones, air taxis, and other aerial vehicles during take-off, landing, and cruising could significantly disturb communities. A recent breakthrough, however, has highlighted a potential solution to this problem: porous ground treatments.
In a study published in Scientific Reports, researchers from the University of Bristol revealed that porous ground treatments, such as those made from natural materials like grass or moss, can significantly lower the noise produced by drones and air taxis. The study, titled “Porous ground treatments for propeller noise reduction in ground effect,” demonstrates for the first time how modifying the surface of the ground can mitigate the acoustic disturbances caused by aerial vehicles, especially during their landing phases.
The Study’s Core Findings
The study, led by Dr. Hasan Kamliya Jawahar from the University of Bristol’s Aeroacoustic Group, in collaboration with Professor Mahdi Azarpeyvand, showed that porous surfaces could reduce noise by up to 30 decibels at low to mid frequencies. Additionally, the performance of propellers was enhanced, resulting in increased thrust and power coefficients compared to when the vehicles operated over solid ground surfaces. These findings suggest that treating urban surfaces—such as rooftops, landing pads, and vertiports—with porous materials could help make drone landings quieter, ultimately benefiting communities that might otherwise be disrupted by UAM activities.
Dr. Jawahar explained, “It was already known that ground effects can influence both propeller performance and the noise produced during take-off and landing. However, while noise reduction solutions have been well-documented, there has been a lack of solutions specifically tailored to urban environments. My inspiration for this study came from natural porous materials, such as vegetation, which are known for their noise-damping properties. This led me to explore engineered porous surfaces as a potential solution for reducing noise and improving aerodynamics for UAM.”
The Role of Porous Surfaces in Reducing Noise
The principle behind the noise reduction lies in the properties of the porous surfaces. When a drone’s propeller operates close to a porous surface, such as grass or moss, it interacts with the flow of air near the ground in a different way than it would with a solid surface. Porous materials absorb some of the energy from the airflow that is directed at the ground, reducing the velocity of the tangential wall jet—the high-speed air flow that moves along the ground surface. This interaction is a key factor in mitigating the aerodynamic noise created by the propeller.
Additionally, the porous structure of the surface traps portions of the airflow, preventing its reflection back toward the propeller. This is critical because reflected air, also known as re-ingested airflow, can interfere with propeller operation, creating tonal and broadband noise. By minimizing this re-ingestion, the noise levels generated by the vehicle are significantly reduced.
Dr. Jawahar elaborated further on the role of natural materials in noise reduction: “Vegetation has long been recognized for its noise-dampening qualities. Its complex structure, foliage density, and moisture content contribute to its ability to absorb sound. Vegetation has been widely used for environmental noise reduction, particularly along roadways and in urban green spaces. What we’ve done here is to investigate how similar porous materials can be applied to the emerging field of Urban Air Mobility, a context that has not yet been explored.”
Methodology of the Experiment
To test the effectiveness of porous ground treatments, the team conducted a series of experiments in an anechoic chamber—a soundproof room designed to eliminate reflections and background noise. In these experiments, a pusher propeller (a type of propeller commonly used in aerial vehicles) was mounted above a ground plane, which alternated between solid and porous treatments with different levels of porosity and thickness. Acoustic measurements were taken using microphones placed in both near-field and far-field positions to capture the noise generated by the propeller at various distances. Meanwhile, a six-axis load cell measured the aerodynamic forces at play.
The data collected from these experiments allowed the researchers to calculate how different porous surfaces influenced noise and performance in ground-effect conditions, where the proximity to the ground significantly impacts the behavior of the airflows around the propeller.
Potential Applications for Urban Air Mobility
These findings have major implications for the future of Urban Air Mobility. As cities around the world look to integrate drones and air taxis into their transportation infrastructure, noise reduction will be a critical factor in ensuring public acceptance. If urban surfaces like rooftops, landing pads, and vertiports can be treated with porous materials, the noise produced by drones and air taxis could be dramatically reduced, making these technologies more acceptable to urban communities.
The development of porous landing surfaces could also have a significant impact on the design of future aerial vehicles. By reducing noise and improving aerodynamic performance, these surfaces can help make drones and air taxis more efficient and sustainable. Additionally, reducing noise pollution is crucial for ensuring that urban air mobility can operate in accordance with local noise regulations and environmental standards.
Dr. Jawahar added, “Our research shows that innovative porous landing surfaces can drastically reduce the noise produced by drones and air taxis, opening the door for quieter, more sustainable urban skies. With the rapid growth of UAM, it is vital that we prioritize noise reduction in order to foster greater community acceptance and compliance with urban noise regulations.”
Real-World Applications: Green Spaces and Sustainable Design
Beyond the technical aspects of UAM, there is also an environmental benefit to using porous materials in urban settings. The use of vegetative materials like grass and moss on rooftops and vertiports not only helps to reduce noise but also contributes to sustainable urban design. These green spaces can improve air quality, provide natural insulation, and help with stormwater management, all while reducing the noise pollution associated with aerial transportation.
Moreover, the idea of using natural materials for noise reduction in urban environments fits into a larger trend of creating eco-friendly cities that prioritize sustainability, health, and well-being. Urban planners and architects can take inspiration from this study to develop more harmonious cities where new technologies like drones and air taxis coexist with nature and community life.
Conclusion: A Quieter Future for Urban Air Mobility
The research conducted by Dr. Kamliya Jawahar and his team marks a significant step forward in addressing one of the primary challenges associated with Urban Air Mobility: noise pollution. Their work demonstrates that porous ground treatments, inspired by the natural noise-damping properties of vegetation, can reduce noise levels by up to 30 dB and improve propeller performance. This discovery opens up exciting possibilities for creating quieter, more efficient aerial transportation systems that integrate seamlessly into urban environments.
As UAM continues to grow, the findings of this study could play a pivotal role in shaping how cities design their infrastructure to support these innovations. By using porous materials in the construction of landing zones, vertiports, and even rooftops, it will be possible to minimize noise disturbances, making drone and air taxi operations more viable and less disruptive to city life.
Ultimately, this research offers a promising glimpse into the future of urban mobility—one where technology and nature work together to create quieter, more sustainable cities that are better for both people and the environment.
Reference: Kamliya Jawahar, H. et al. Porous ground treatments for propeller noise reduction in ground effect, Scientific Reports (2025). DOI: 10.1038/s41598-024-82876-9. www.nature.com/articles/s41598-024-82876-9