We've come a long way since the days of pouring wax into our ears to block out siren songs. A team of researchers at Hong Kong University of Science and Technology have developed a sound-cancelling system that eliminates 99.7 percent of noise, no matter how quiet.
Typically, passive sound deadening technologies have relied on materials that simple absorb sound waves (and usually only along a narrow band of frequencies). But even the most absorbent material tends to scatter some of the sound incoming sound waves. As such, this new system absorbs incoming sounds not once but twice. It uses a pair of "impedance-matched" resonators. These are devices that naturally vibrate at a specific frequency and, in the case of "impedance-matching", that frequency is equal to that of the the background medium (whatever the resonator is mounted to).
The first resonator eliminates a majority of the incoming sound waves. However at very low energy levels (ie very quiet sounds), even the best resonator tends to scatter a little bit of the sound at its own frequency. That's where the second resonator comes in -- it's tuned precisely to the first resonator's frequency, allowing it create destructive interference for any sound the first resonator scatters. This single-layer system builds and improves upon the team's earlier work, published last year in Nature. That study fit a soft absorbent layer atop a hard reflective one and separated them with a thin layer of air. The idea was that any sound that got through the soft layer would bounce off the reflective layer and cancel out any incoming sound waves.
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