MSGID: 1:317/3 64781f28
PID: hpt/lnx 1.9.0-cur 2019-01-08
TID: hpt/lnx 1.9.0-cur 2019-01-08
 Actively reducing noise by ionizing air 

  Date:
      May 31, 2023
  Source:
      Ecole Polytechnique Fe'de'rale de Lausanne
  Summary:
      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.


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FULL STORY
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Did you know that wires can be used to ionize air to make a
loudspeaker? Simply put, it's possible to generate sound by creating an
electric field in a set of parallel wires, aka a plasma transducer, strong
enough to ionize the air particles. The charged ions are then accelerated
along the magnetic field lines, pushing the residual non-ionized air in
a way to produce sound.

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

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."  Not only is the
plasma efficient at high frequencies, but it is also versatile since it
can be tuned to work at low frequencies as well. Indeed, the scientists
show that the dynamics of thin layers of air plasma can be controlled
to interact with sound over deep-subwavelength distances, to actively
respond to noise and cancel it out over a broad bandwidth. The fact that
their device is active is key, since passive noise reduction technologies
are limited in the band of frequencies that can be controlled.

The plasma absorber is also more compact that most conventional solutions.

Exploiting the unique physics of plasmacoustic metalayers, the scientists
experimentally demonstrate perfect sound absorption: "100% of the
incoming sound intensity is absorbed by the metalayer and nothing is
reflected back," says EPFL's Acoustic Group's senior scientist Herve'
Lissek. They also show tunable acoustic reflection from several Hz to
the kHz range, with transparent plasma layers of thicknesses down to
only a thousandth of a given wavelength, much smaller than conventional
noise reduction solutions.

To give an idea of how much more compact the plasma absorber is, consider
a low, audible sound frequency of 20 Hz, where the sonic wavelength is
17m meters long. The plasma layer would only need to be 17 mm thick to
absorb the noise, whereas most conventional noise reduction solutions,
like absorbing walls, would need to be at least 4 m thick which often
limits its feasibility.

"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 Herve' Lissek.

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

"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," explains Mark Donaldson,
CEO and Founder of Sonexos.

    * RELATED_TOPICS
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==========================================================================
Story Source: Materials provided by
Ecole_Polytechnique_Fe'de'rale_de_Lausanne. Original written by Hillary
Sanctuary. Note: Content may be edited for style and length.


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Journal Reference:
   1. Stanislav Sergeev, Romain Fleury, Herve' Lissek. Ultrabroadband
   sound
      control with deep-subwavelength plasmacoustic metalayers. Nature
      Communications, 2023; 14 (1) DOI: 10.1038/s41467-023-38522-5
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Link to news story:
https://www.sciencedaily.com/releases/2023/05/230531102009.htm

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