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.