Brief Summary

Scientists at École Polytechnique Fédérale de Lausanne have developed an active sound absorber which can absorb frequencies down to 20Hz and whose thickness at 17mm is only 1000th the wavelength of the audio being absorbed.

Going Deeper

When tackling room acoustics it’s an awkward fact that the lower the frequency the harder it becomes to address issues. Whether dealing with acoustic treatment or controlling leakage, problems at high frequencies are far easier to fix than lower down, and at the extreme low end the physical requirements of traditional acoustic treatment become unmanageable for most people.

When it comes to acoustic treatment, bass trapping is important, for more information on how bass interacts with rooms check out our article Small Studios And Bass - Know Your Enemy. Using traditional passive bass trapping the treatment involved has either to be thick and occupy a significant amount of valuable studio space, or tuned to a specific frequency. The holy grail of acoustic treatment is something thin, which absorbs sound from the highest to the lowest frequencies. This hadn’t yet proved possible but an intriguing new technology suggests that it might be with us soon.

Active Acoustics

Most of us have experienced noise cancelling headphones. Rather than blocking out sound, like traditional ear defenders, they create an exact opposite of sound reaching the listener’s ears, resulting in near silence at the eardrum. This active approach has improved enormously in the last few years and a similar approach can be taken to sounds in rooms. In my article Active Acoustics - What Are They And How Do They Work? I describe examples of how active acoustics can be used to change the reverberant character of a performance venue or to trap bass in a studio. However there is always a tradeoff between the area of a traditional pistonic driver required to produce deep bass, and overcoming the inertia introduced by the mass of a large driver to make it reproduce high frequencies. This is where ionic solutions have an advantage.

The videos below give a good overview of this technology

Scientists at École Polytechnique Fédérale de Lausanne have demonstrated a transducer which overcomes the mass issue by dispensing with driver diaphragms altogether. As Stanislav Seergev of EPFL put it:

“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”. “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 video below starts with a demonstration of the transducer but after this, not very impressive, demo, the construction and operating principle is explained. Jump to 3.30 to find this section.

Using a relatively simple and inexpensive assembly using a grid of high voltage wires behind a grille, one hundred percent of the sound is absorbed and nothing is reflected back. This plasma based solution is so compact that an absorber of only 17mm thick is effective down to 20Hz. An equivalent passive absorber would need to be as much as 4m thick!

Partnership And Commercial Development

A partnership between EPFL and Swiss based audio technology company Sonexos has been announced to develop sound absorber products using this technology. Although the potential for this technology in professional audio are clear to see, the applications in the much larger potential markets of automotive and industrial applications are where we’d anticipate early implementations to emerge.

Plasma transducers aren’t entirely new. Esoteric plasma based drivers have been around for years. Perhaps it is is the area of cancelling out sound rather than projecting it that they will finally find an application where the benefits outweigh the disadvantages.