Plasma Acoustic Meta-layers: A New Frontier in Noise Reduction

Noise pollution is an often underappreciated environmental issue with direct impacts on public health. The World Health Organization warns that exposure to noise levels above 65 decibels—comparable to a loud conversation—poses health risks. When noise reaches 75 decibels, akin to the din of city traffic, these risks escalate, disrupting sleep and potentially leading to cardiovascular and metabolic disorders. At a thundering 120 decibels, the volume of a rock concert, noise can cause immediate pain. As urbanisation accelerates, the intrusion of noise pollution into our lives becomes more pervasive, often going unnoticed as we grow accustomed to the constant hum of city life.

Noise reduction techniques, which involve both passive and active methods, aim to mitigate this risk. However, challenges persist. Passive techniques are often bulky and tend to operate at higher frequencies, while active methods face limitations in terms of their bandwidth and stability.

Researchers at Acoustic Group headed by Hervé Lissek within EPFL's Signal Processing Laboratory have made a significant breakthrough by developing a plasma-based acoustic sound absorber (PASA) which forms a plasmacoustic metalayer to facilitate deep-subwavelength ultrabroadband sound control. This novel technology first ionises a thin layer of air, which is then modulated according to a prescribed control law to establish a target acoustic impedance by way of a separate controlling voltage. By setting the acoustic impedance for absorption, the metalayer is able to absorb incident sound energy, thus improving the acoustics of the space in which the PASA is deployed. However, further development is required before the solution can be considered for everyday use.

Enter Sonexos, led by CEO Mark Donaldson, a world-renowned expert in active control. With a vision to bridge the gap between laboratory innovation and market-ready solutions, Sonexos is paving the way in commercialising this exciting new technology through a licensing agreement recently established with EPFL’s Technology Transfer Office, and supported by it's enable program.

We spoke with Mark about employing this technology in electric vehicles, which represents the goal of an Innosuisse project recently secured by the company and the EPFL. The aim of this project is to provide a revolutionary solution for noise reduction in vehicle cabins, replacing heavy sound-absorbing materials with PASA devices. Such an approach is expected to lower noise by up to 5 dB across a relatively wide frequency range and, unlike current active noise-cancelling methods that focus on reducing noise at the passengers’ ears, operates globally throughout the entire cabin.

PASA technology boasts several benefits: its compact size, its ability to absorb low frequencies, and its independent functioning without the need for extensive computing resources. It's also worth noting that this technology is transparent to airflow, making it suitable for use in vehicle AC systems and exhaust sound control solutions.

Donaldson emphasises the technology's versatility and potential to transform the acoustics of various enclosed spaces, including aircraft cabins, apartments, recording studios, and home theatres, for example. This technology has the potential to revolutionize the way we think about and experience sound control. Although challenges remain, such as manufacturing the plasma layers and fine-tuning its amplification and control systems, Sonexos is making steady progress.

In summary, plasma acoustic meta-layers represent a significant step forward in combating noise pollution and demonstrate the impactful partnership between academic research and industry. This innovation from EPFL, further developed and commercialized by Sonexos, could significantly enhance our quality of life across multiple environments.