Metasurface absorbs sound, outputs electricity
One man’s music can be another man’s noise. Projects involving audio need to ensure that the sound does not disturb other, nearby parties. These concerns may be particularly pertinent for projects such as hotels where soundproofing is an important part of the installation. Unfortunately soundproofing involves the usage of thick absorbent materials which may not always be the most aesthetically pleasing.
Physicists from the Hong Kong University of Science and Technology however may have a more apt solution to the aforementioned problem. They have developed a thin metasurface which not only helps absorb sound but can also be used to generate electricity.
Current sound absorption materials must have a thickness comparable to the wavelength of the sound they are trying to damp. Low frequency sounds, hence having high wavelengths, require the thickest sheets of soundproofing. The new metasurface can however be tuned to absorb any frequency the user desires without the need to modify the thickness.
The metasurface utilizes a “decorated membrane resonator” (DMR), which resembles a drum membrane embedded into a solid support, to provide sound absorption while remaining thin. The membrane’s elastic modulus is low so the harmonics of the metasurface responds to the wavelengths of sounds travelling through the air. A reflecting backing then sandwiches the sealed gas layer.
Researchers at the Hong Kong University of Science and Technology experimented with a membrane that was 9cm across, had a thickness of 0.2mm and a 2cm diameter platelet in the center. The metasurface exhibited resonance at audible wavelengths such that there is near total absorption of sound, and dissipation of the energy along the lossy membrane. Effective sound absorption is achieved with the help of “impedance matching” with the sound waves.
Most interestingly though, the researchers found that if the vibrating platelet in the membrane system was attached to an energy generation system the sound absorbed could be used to generate electricity. At present the metasurface is able to provide a sound to electricity conversion efficiency of 23 percent.
The drawback of the metasurface is that each DMR type absorbs only a limited range of frequency. In terms of professional AV installations, this would mean that multiple different metasurafces would need to be used to provide sound proofing across a range of frequencies. However if only specific sound frequencies need to be eliminated the metasurafce is a viable option.