The technique, which could be adapted to provide noise reduction in a range of applications including automotive and industry, was developed by a team at the Swiss Federal Institute of Technology in Lausanne (EPFL).
It is possible to generate sound by creating an electric field in a set of parallel wires, known as a plasma transducer, which ionises air particles. The charged ions are accelerated along the magnetic field lines, pushing the residual non-ionised air in a way that produces sound.
While the plasma loudspeaker concept is not new, the EPFL scientists built a demonstration of the plasma transducer, with the aim to study noise reduction – if a loudspeaker can generate sound, it can also absorb it. They came up with a new concept called the ‘active plasmacoustic metalayer’, which can be controlled to cancel out noise.
The team was interested in using plasma to reduce noise as it removes one of the most important aspects of conventional loudspeakers: the membrane. Loudspeakers equipped with membranes are some of the most studied solutions for active noise reduction, which is 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 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,” said Stanislav Sergeev, a postdoc researcher in EPFL’s Acoustic Group and first author of a paper on the work.
“We first ionise 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.
“As expected, the communication between the electrical control system of the plasma and the acoustic environment is much faster than with a membrane.”
The plasma works efficiently at high frequencies, the researchers said. It can also be tuned to work at low frequencies as well, unlike passive noise reduction technologies that are limited in the band of frequencies that can be controlled.
The absorber is also more compact than most conventional solutions. Exploiting the unique physics of plasmacoustic metalayers, the scientists said they experimentally demonstrated perfect sound absorption. “100% of the incoming sound intensity is absorbed by the metalayer and nothing is reflected back,” said senior scientist Hervé Lissek.
For a low, audible sound frequency of 20Hz with a wavelength of 17m, the plasma layer would only need to be 17mm thick to absorb the noise. Most conventional noise reduction solutions, like absorbing walls, would need to be at least 4m thick.
“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,” said Lissek.
EPFL has partnered with Sonexos SA, a Swiss-based audio technology company, to develop active sound absorbers that use the plasmacoustic metalayer concept. They aim to provide ‘novel and efficient’ solutions for reducing noise in a wide range of applications.
“This strategic collaboration leverages EPFL's expertise in material science and acoustics, as well as Sonexos' proven track record in delivering high performance audio solutions,” said Mark Donaldson, CEO and founder of Sonexos.
Results of the research were published in Nature Communications.
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