What Is The Sound Of No Sounds Happening?

Researchers in Hong Kong achieve near-perfect silence

Figure 2. Scatterings and absorption coefficient of the flat panel composite absorber

Figure 2. Scatterings and absorption coefficient of the flat panel composite absorber

Applied Physics Letters 107, 104104 (2015)

Think, for a moment, about the quietest quiet experience. Perhaps a still night in the desert, far from any of the sounds of civilization, or maybe a hike in a cave or catacomb, where the body of the Earth itself provides a strong barrier against intruding sounds. Whatever you imagined, it's likely nowhere near as quiet as a lab at the Department of Physics of Hong Kong University of Science and Technology, where Min Yang et al. created almost perfect silence. Using a pair of resonators, their experiment absorbed 99.7 percent of ambient sound.

Besides a librarian shhhhh-ing a room full of students, there are a few ways to create quiet. One is a material that absorbs sound waves rather than reflecting them, similar to the way stealth aircraft absorb radar waves. There are finite limits on how much sound can be absorbed materially, so instead the Hong Kong researchers used a pair of resonators. The first of these resonators vibrates at a frequency designed to scatter the incoming sound waves. A second resonator then vibrates at a frequency designed to dissipate the scattered sound. The end result: sound on one side of the resonators gets caught and destroyed, creating almost perfect silence on the other side.

From the paper:

Almost perfect absorption for sounds with wavelength at least 10 times larger than the absorber has been observed in both cases, with excellent agreement between theory and experiment. In addition, we would like to note that, owing to the similarity between acoustic and electromagnetic waves, the present proposed total absorption mechanism by degenerate mirror-symmetric and anti-symmetric resonances should be valid for electromagnetic waves as well, with specific polarizations.

Sounds cool, right? Incorrect. It barely sounds at all.