Ionizing air to actively decrease noise levels.

Did you know that wires can be used to ionize air to make a speaker? Simply put, it is possible to generate sound by creating an electric field in a bunch of parallel wires, also known as a plasma transducer, strong enough to ionize air molecules. The charged ions are then accelerated along the magnetic field lines, forcing the remaining deionized air in such a way as to produce sound.

If a loudspeaker can generate sound, it can also absorb it.

While the concept of this plasma loudspeaker is not new, EPFL scientists have gone ahead and built a demonstration of a plasma transducer, with the goal of studying noise reduction. They’ve come up with a new concept, what they call an active, controllable “acoustic plasmonic metal layer” for noise cancellation. Their results have been published in Nature Communications.

Scientists were intrigued by the idea of ​​using plasma to reduce noise, as it does away with one of the most important aspects of traditional loudspeakers: the diaphragm. Diaphragm speakers, such as those in your car or at home, are some of the most studied solutions for active noise reduction. It is active because the membrane can be controlled to cancel out different sounds, unlike the wall which does the job passively.

The problem with using a conventional tweeter as a sound absorber is that its diaphragm limits its operating frequency range. To absorb sound, the membrane acts mechanically, vibrating to cancel out sound waves in the air. The fact that the diaphragm is relatively heavy, i.e. the inertia of the diaphragm, limits its ability to react efficiently to rapidly changing sounds or high frequencies.

“We wanted to reduce the effect of the membrane as much as possible, because it’s heavy. But what can be as light as air? Air itself,” explains Stanislav Sergeyev, a postdoctoral researcher in EPFL’s Acoustic Group and first author. “We first ionize the thin layer of air between the electrodes that we call the acoustic plasmometallic layer. The air molecules themselves, now electrically charged, can respond instantaneously to the commands of the external electric field and actively interact with the acoustic vibrations in the air around the device to cancel out theirs.”

Sergeyev continues: “As expected, the contact between the electrical control system of the plasma and the acoustic environment is much faster than that of the membrane.”

Not only is plasma effective at higher frequencies, but it is also very versatile as it can be tuned to operate at lower frequencies as well. Indeed, scientists have shown that the dynamics of thin layers of air plasma can be controlled to interact with sound at deep subwavelength distances, to effectively respond to noise and cancel it out over a large scale. The fact that their device is active is key, since passive noise reduction technologies are limited in the range of controllable frequencies.

The plasma absorber is also more compact than most conventional solutions. By exploiting the unique physics of plasmometallic wheels, the scientists have experimentally demonstrated perfect sound absorption: “100% of the incoming sound intensity is absorbed by the metallic layer and nothing is reflected back,” says Herve Lissek, EPFL Senior Scientist at the Acoustic Group. They also exhibit tuned acoustic reflection in the several hertz to kilohertz range, with transparent plasma layers only a thousandth of a specific wavelength thick, which is much smaller than conventional noise reduction solutions.

To give an idea of ​​how much pressure a plasma absorber has, consider a low, audible sound frequency of 20 Hz, where the sound wave has a length of 17 meters. The plasma layer would only need a thickness of 17 mm to absorb noise, while most conventional noise reduction solutions, such as absorbing walls, would need to be at least 4 mm thick which often limits their feasibility.

“The most fascinating aspect of this concept is that, unlike traditional sound absorbers that rely on porous bulk materials or resonant structures, our concept is rather ethereal. We have unveiled a completely new sound-absorbing mechanism, which can be as thin and light as possible. , opening new horizons in terms of noise control where space and weight are important, especially at lower frequencies,” says Hervé Lessek.

EPFL has partnered with Sonexos SA, a Swiss-based audio technology company, to develop state-of-the-art active sound absorbers that use the plasmacoustic metalayer concept. Together, they aim to provide new and effective noise reduction solutions in a wide range of applications, including the automotive, consumer, commercial and industrial sectors.

“This strategic collaboration leverages EPFL’s expertise in materials science and acoustics, as well as Sonexos’ proven track record of delivering high-performance acoustic solutions,” explains Mark Donaldson, CEO and Founder of Sonexos.