EPFL researchers have developed a 100% effective, ultra-thin active noise cancelling system that uses an ionized air plasma propulsion system instead of speakers. A 17-mm-thick (0.6-in) layer can block 20 Hz noise as well as a 4-m-thick (13-ft) wall.
If you know how active noise cancellation works, then skip ahead. Essentially, the sound waves we hear are pressure waves in the air around us. Speaker cones are big, lightweight membranes designed to push air around in precise patterns to create those pressure waves, either in the form of pleasant music, or whatever it is the kids are listening to these days.
Active noise cancellation (ANC) is the idea of measuring those pressure waves with a microphone, then generating the exact same pressure waves in reverse, and playing them through a speaker. Pressure is taken away from the positive pressure peaks, and added to the negative pressure troughs, and the new wave more or less scrubs the old wave out of existence.
ANC works well in headphones and cars, where the acoustic environment is reasonably controlled and the listening points are reasonably easy to locate. It also works well at the room scale – although, as you can see in the video below, if you want to cancel out low-frequency, long-wavelength bass sounds, you're going to need a wall full of speakers to do it.
But what if your speakers could be super-slim sheets instead of stacks of heavy cones? The EPFL Acoustic Group team decided to test the noise-cancelling capabilities of plasma-based ionic speakers, which are thin, lightweight, simple and cheap to build.
These speakers work like ionic propulsion systems, using an electric field to ionize ambient air into a plasma, creating positively and negatively charged particles. These ions are then accelerated magnetically, and they push against the ambient air to create pressure waves. By varying the voltage applied, you can instantly vary how much air is being pushed.
The EPFL team released the video below in 2020, showing a plasma-based loudspeaker built using perforated metal plates with wire loops a short distance behind them.
These things aren't going to challenge a nice set of speaker cones for fidelity. But they're super-responsive to changes in voltage, since there's no relatively heavy speaker membrane to move before you can start moving air.
“We wanted to reduce the effect of the membrane as much as possible, since it’s heavy,” explains Stanislav Sergeev, postdoc at EPFL’s Acoustic Group and first author on a new study published in Nature Communications. “But what can be as light as air? The air itself. 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.”
The research team found that yes, these plasmacoustic speakers were exceptionally responsive and efficient at cancelling out high frequencies. But they also demonstrated they could be totally effective at killing low-frequency sounds – which is interesting, since bass reproduction was not one of these ionic speakers' strengths in the 2020 video.
As an example, 20 Hz is roughly the low-frequency limit of the human hearing apparatus. This note is so low that its wavelength in air is 17 m (56 ft). If you wanted to deaden a 20-Hz sound wave using conventional noise reduction foams or sound-absorbing walls, you'd need to have something about 4 m (13 ft) thick. The plasmacoustic metalayer system could completely cancel that wave at just one-thousandth of the wavelength in thickness, or 17 mm (0.6 in).
"One hundred percent of the incoming sound intensity is absorbed by the metalayer and nothing is reflected back," says EPFL’s Acoustic Group’s senior scientist Hervé Lissek. "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."
EPFL has licensed the plasma-based technology to Swiss company Sonexos, which is moving to commercialize it across a wide range of applications.
"We are tremendously excited about the immense potential of plasma technology in reducing noise and enhancing the acoustics of various interior environments, including vehicle and aircraft cabins, office spaces, and homes," says Sonexos CEO Mark Donaldson in a press release. "As we lead the development and commercialization efforts, we are privileged to collaborate with the exceptional team of engineers at EPFL."
We'll be fascinated to see how this concept fares on the path to market, since it looks like an exceptionally cheap, effective, reliable and convenient way to eliminate noise. Obviously, things become much more complex in the real world than in the laboratory, but this kind of thing has the potential to change lives for the better.
The research is open access at Nature Communications.
