Physicists at Hong Kong University of Science and Technology have experimentally demonstrated a system capable of perfect sound absorption. Acoustic waves go in, nothing comes back out. At all.
As often as we may like to imagine quietude, it's not a particularly natural phenomenon. An acoustic insulator may do a pretty good job of soaking up a small range of sounds, but some of that energy is always going to be reflected back, even if at very tiny scales. A material may be a highly complex composite of many different layers of many different viscosities, with each one tuned to absorb some wavelength of sound—what's called a gradient-index material—but imagining that material with all of the layers required to capture all of the different acoustic wavelengths is not very reasonable.
But there are other approaches.
In a paper published in the Applied Physics Letters this week, the Hong Kong researchers describe a rather more active absorption system (but not totally active) composed of two resonators, both of which are tuned to the same frequency, which is itself "impedance-matched" to the background medium, e.g. the open air or whatever surrounding the acoustic absorber. The result is sound-killing destructive interference.
To unpack that a little, you can imagine a resonator as a thing that naturally vibrates at some frequency. This is important because at sub-wavelength scales, it becomes increasingly difficult for any material to dissipate waves at such low energies. Dissipation comes as a result of the material's "give" as the thing to be dissipated (a sound wave) hits it. The resonator provides this "give." Makes sense.
There's a bit more to it. Working against the material's absorption potential is the resonator's natural coupling to the acoustic radiation of the surrounding medium. The sound waves want to scatter, but at now at the frequency of the resonator. The second resonator is ready for this and is tuned to just the right frequency to create destructive interference. The waves that would have scattered are canceled out.
The work builds on experiments described in a study published last year in Nature Materials by some members of the same group. The set-up then was to use a thin absorbing material in conjunction with a hard reflecting layer, with a super-thin pad of air in between. The idea was that waves would leak through the weakly absorbing material, bounce off the reflective surface, and then collide with the incoming waves in such a way to create interference and neutralize the sound.
The earlier idea also depended on dueling resonances, but the current version manages the same thing without the layer of air and bonus reflective surface. It's single-layer.
The end result of the new and improved version is 99.7 percent total silence, according to the current study. Which is pretty quiet.