EPFL researchers have developed a 100% effective ultra-thin active noise cancellation system that uses an ionized air plasma propulsion system instead of speakers. A 17 mm (0.6 in) layer blocks noise up to 20 Hz, as well as a 4 m (13 ft) thick wall.
If you know how active noise cancellation works, skip ahead. Essentially, the sound waves we hear are pressure waves in the air around us. Speaker cones are large, lightweight membranes designed to push air in precise patterns to create pressure waves, whether in the form of pleasing music or whatever the kids are listening to right now.
The idea of Active Noise Cancellation (ANC) is to measure these pressure waves with a microphone, then generate the exact same pressure waves in reverse and play them through a speaker. The pressure is taken away from the positive pressure peaks and added to the negative pressure troughs, the new waves more or less canceling out the old ones.
ANC works well in headphones and cars, the acoustic environment is well controlled and the listening point is fairly easy to locate. It also works well at room range – though, as you can see in the video below, if you want to eliminate low-frequency, long-wavelength bass, you’re going to need an entire wall of speakers to do it a little.
2012 Soundrelief Prototype
But what if your speakers could be an ultra-thin sheet instead of a bunch of bulky cones? The EPFL Acoustics Group decided to test the noise-canceling capabilities of plasma-based ionic speakers, which are thin, light, simple and inexpensive to manufacture.
These speakers work like ion propulsion systems, using an electric field to ionize the surrounding air into a plasma, which creates positively and negatively charged particles. These ions are then magnetically accelerated and push against the surrounding air creating pressure waves. By changing the voltage applied, you can instantly change the amount of air being pushed.
The EPFL team released the video below in 2020, showing a plasma-based speaker made using a perforated metal plate with coils not far behind it.
2020 plasma speakers
These things don’t challenge the fidelity of a good set of speaker cones. But they are very sensitive to voltage changes because there is no relatively heavy speaker membrane to move before it starts moving the air.
“We wanted to reduce the influence of the membrane as much as possible, because it is heavy,” explains Stanislav Sergeev, a postdoc in the acoustics group at EPFL, also published in natural communication. “But what can be as light as air? Air itself. We first ionize a thin layer of air between the electrodes, which we call a plasmonic acoustic metal layer. Now the same charged air particles can instantly respond to an external electric field command and effectively interacts with sound vibrations in the air around the device, canceling them out.”
Yes, these plasmonic speakers are responsive and efficient at canceling high frequencies, the research team found. But they’ve also shown that they’re completely effective at canceling low-frequency sounds — which is interesting, because bass reproduction isn’t one of these ionic speakers’ strong suits in the 2020 video.
For example, 20 Hz is roughly the low frequency limit of the human auditory apparatus. The note has a very low wavelength, 17 m (56 ft) in air. If you wanted to attenuate 20 Hz sound waves with traditional noise-reducing foam or sound-absorbing walls, you’d need about 4 m (13 ft) of material. Plasmon-acoustic metalayer systems can completely cancel this wave at a thickness of just one-thousandth of the wavelength, or 17 millimeters (0.6 inches).
“100% of the incoming sound intensity is absorbed by the metal layer and nothing is reflected back,” says Hervé Lissek, a senior scientist in the acoustics group at EPFL. “The most fantastic aspect of this concept is that, unlike traditional sound absorbers that rely on porous bulk materials or resonant structures, our concept is somewhat ethereal. We have introduced a completely new sound absorption mechanism that can be made like Possibly, opening up new frontiers in noise control where space and weight are important, especially at low frequencies.”
The Federal Institute of Technology in Lausanne (EPFL) has licensed the plasma-based technology to Swiss company Sonexos, which is working to commercialize it for a wide range of applications.
“We are excited by the enormous potential of plasma technology to reduce noise and enhance the acoustics of a variety of interior environments, including vehicle and aircraft cabins, office spaces and homes,” Sonexos CEO Mark Donaldson said in a release. Very excited.” “We are honored to be working with the exceptional team of engineers at EPFL as we lead the development and commercialization efforts.”
We’re interested to see how this concept fares in the market, as it looks like a very cheap, effective, reliable and convenient way to cancel noise. Obviously, things are much more complicated in the real world than in the lab, but this kind of thing has the potential to make life better.
The research is open access natural communication.