Did you know that you can make a speaker by ionizing the air using wires? Simply put, creating an electric field strong enough to ionize air molecules in a network of parallel wires or a plasma transducer can result in the production of sound. The charged ions are then propelled along the magnetic field lines, pushing the remaining unionized air in a way that creates sound. A speaker can both produce and absorb sound.
The researchers developed a new idea they call an active “plasma acoustic metalayer” that can be tuned to reduce noise. Although the idea of a plasma speaker is not new, EPFL researchers nevertheless built a demonstration of a plasma transducer to research noise reduction.
Scientists were intrigued by the idea of using plasma to reduce noise because it gets rid of one of the most important aspects of conventional speakers: the diaphragm. Speakers equipped with diaphragms, such as those in your car or home, are some of the most studied solutions for active noise reduction. It is active because the membrane can be controlled to cancel out different sounds, unlike a wall that does this job passively.
The problem with using a conventional speaker as a sound absorber is that its diaphragm limits the frequency range of operation. To absorb sound, the membrane behaves mechanically and vibrates to disrupt 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 effectively with fast-changing sounds or at high frequencies.
“We wanted to reduce the effect of the membrane as much as possible because it is heavy. But what can be as light as air? The air itself,” explained Stanislav Sergeev, a postdoc in EPFL’s Acoustic Group and first author. “First, we ionize a thin layer of air between the electrodes, which we call the plasma-acoustic meta layer.
Sergeev added: “As expected, the communication between the electrical control system of the plasma and the acoustic environment is much faster than with the membrane.”
Not only is plasma effective at high frequencies, it is also versatile as it can be tuned to low frequencies as well. Indeed, scientists show that the dynamics of thin layers of air plasma can be controlled to interact with sound at deep subwavelength distances to actively respond to and cancel noise over a broad band. The fact that their device is active is key because passive noise reduction technologies are limited in the range of frequencies that can be controlled.
The plasma absorber is also more compact than most conventional solutions. Using the unique physics of ‘plasmacus metalayers’, scientists experimentally demonstrate perfect sound absorption: “100% of the intensity of the incoming sound is absorbed by the metalayer and nothing is reflected back,” says Herve Lissek of EPFL’s Acoustic Group.
To get an idea of how much more compact a plasma absorber is, consider a low audible sound frequency of 20 Hz, where the sound wave length is 17 m meters. The plasma layer would only need to be 17mm thick to absorb the noise, whereas most common noise reduction solutions such as absorptive walls would need to be at least 4m thick, often limiting its feasibility.
“The most fantastic aspect of this concept is that unlike conventional silencers that rely on porous bulk materials or resonant structures, our concept is somehow ethereal. We have unveiled a completely new sound absorption mechanism that can be as thin and light as possible, opening new frontiers in noise control where space and weight matter, especially at low frequencies,” said Herve Lissek.
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