It is fascinating to discover that wires can be utilized to ionize air, resulting in a unique type of loudspeaker. This method, known as a plasma transducer, involves creating an electric field between parallel wires, which ionizes the surrounding air particles. The charged ions are then propelled along magnetic field lines, causing the remaining non-ionized air to move and generate sound.
Interestingly, if a loudspeaker can produce sound, it can also absorb it.
While the concept of a plasma loudspeaker is not new, scientists at EPFL have recently developed a practical demonstration of the plasma transducer and explored its potential for noise reduction. They introduced a novel concept called the active “plasmacoustic metalayer,” which can be controlled to actively counteract and eliminate noise. Their findings have been published in Nature Communications.
The researchers were intrigued by the idea of using plasma to mitigate noise, as it eliminates one of the crucial components of conventional loudspeakers: the membrane. Traditional loudspeakers, such as those found in cars or homes, employ membranes that are extensively studied for active noise reduction. The “active” aspect refers to the membrane’s ability to be controlled and cancel out specific sounds, as opposed to passive noise reduction achieved by walls or other stationary objects.
The challenge with using conventional loudspeakers as sound absorbers lies in their membranes, which limit their operational frequency range. Membranes function mechanically, vibrating to counteract sound waves in the air. However, their relatively heavy nature—manifested in the membrane’s inertia—restricts their effectiveness in efficiently interacting with rapidly changing sounds or high frequencies.
Stanislav Sergeev, a postdoc at EPFL’s Acoustic Group and the study’s first author, explains the motivation behind their research: “Our goal was to minimize the impact of the membrane, considering its weight. We sought a solution as lightweight as possible, and what could be lighter than air itself? Hence, we ionize the thin air layer between the electrodes, creating what we term a ‘plasmacoustic metalayer.’ By electrically charging the air particles, they can instantaneously respond to external electrical field commands and effectively interact with sound vibrations in the surrounding air, actively canceling them out.”
Continuing his explanation, Sergeev adds, “The communication between the plasma’s electrical control system and the acoustic environment surpasses our expectations in terms of speed compared to a membrane-based system.”
Not only does plasma exhibit efficiency in handling high frequencies, but it also showcases remarkable versatility by being adaptable to low frequencies. The researchers demonstrate that they can control the dynamics of thin air plasma layers to effectively interact with sound waves over incredibly small distances, canceling out noise across a wide range of frequencies. The active nature of their device is crucial since passive noise reduction techniques have limitations in terms of the frequency bands they can effectively control.
Furthermore, the plasma absorber offers a more compact solution compared to traditional methods. By exploiting the unique physics of plasmacoustic metalayers, the scientists achieve perfect sound absorption, ensuring that 100% of the incoming sound intensity is absorbed by the metalayer without any reflection. They also achieve tunable acoustic reflection from several Hz to the kHz range, using transparent plasma layers that are only a thousandth of the wavelength in thickness, significantly smaller than conventional noise reduction approaches.
To illustrate the compactness of the plasma absorber, let’s consider a low audible frequency of 20 Hz, where the wavelength is 17 meters long. In this case, the plasma layer would only need to be 17 mm thick to absorb the sound, whereas most traditional noise reduction solutions, like absorbing walls, would require a thickness of at least 4 meters, often making them impractical.
Lissek, a senior scientist at EPFL’s Acoustic Group, enthuses, “The most fascinating aspect of this concept is that, unlike conventional sound absorbers that rely on bulky porous materials or resonant structures, our concept is ethereal in a sense. We have unveiled an entirely new mechanism of sound absorption that can be incredibly thin and lightweight, opening up new possibilities for noise control in scenarios where space and weight are crucial, especially at low frequencies.”
EPFL has entered into a strategic partnership with Sonexos SA, a Swiss audio technology company, to develop state-of-the-art active sound absorbers based on the plasmacoustic metalayer concept. Together, they aim to deliver innovative and efficient noise reduction solutions for a wide range of applications, including automotive, consumer electronics, commercial spaces, and industrial settings.
Mark Donaldson, CEO and Founder of Sonexos, highlights the collaborative effort, stating, “This strategic collaboration combines EPFL’s expertise in material science and acoustics with Sonexos’ proven track record in delivering high-performance audio solutions.”