Actively reducing noise by ionizing air
EPFL scientists show that a thin layer of plasma, created by ionizing air, could be promising as an active sound absorber, with applications in noise control and room acoustics
While this plasma loudspeaker concept is not new, EPFL scientists went ahead and built a demonstration of the plasma transducer, with the aim to study noise reduction. They came up with a new concept, what they call the active “plasmacoustic metalayer” that can be controlled to cancel out noise. Their results are published in Nature Communications.
The scientists were intrigued by the idea of using plasma to reduce noise, since it gets rid of one of the most important aspects of conventional loudspeakers: the membrane. Loudspeakers equipped with membranes, like the ones in your car or at home, are some of the most studied solution for active noise reduction. It’s active because the membrane can be controlled to cancel out different sounds, as opposed to a wall that does the job passively.
The problem with using the conventional loudspeaker as a sound absorber is that its membrane limits the frequency range of operation. For sound absorption, the membrane behaves mechanically, vibrating to cancel out the sound waves in the air. The fact that the membrane is relatively heavy, i.e. the inertia of the membrane, limits its ability to interact efficiently with fast changing sounds or at high frequencies.
“We wanted to reduce the effect of the membrane as much as possible, since it’s heavy. But what can be as light as air? The air itself,” explains Stanislav Sergeev, postdoc at EPFL’s Acoustic Group and first author. “We first ionize the thin layer of air between the electrodes that we call a plasmacoustic metalayer. The same air particles, now electrically charged, can instantaneously respond to external electrical field commands and effectively interact with sound vibrations in the air around the device to cancel them out.”
Sergeev continues, “As expected, the communication between the electrical control system of the plasma and the acoustic environment is much faster than with a membrane.”
“The most fantastic aspect in this concept is that, unlike conventional sound absorbers relying on porous bulk materials or resonant structures, our concept is somehow ethereal. We have unveiled a completely new mechanism of sound absorption, that can be made as thin and light as possible, opening new frontiers in terms of noise control where space and weight matter, especially at low frequencies” says Hervé Lissek, EPFL’s Acoustic Group’s senior scientist.
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