Is this thing on? That’s likely what Hong Kong University of Science and Technology scientists thought, shortly after they’d developed a new system that absorbs 99.7 per cent of all the sound that hits it.
Many systems use sonic insulators to deaden sound: materials which absorb sound, typically over a small range of frequencies. By combining different insulators into a composite, it’s possible to absorb a large range of sounds — but it’s difficult to create such a material that absorbs all the the frequencies. It would just be too big and complex. That means that there’s a limit to the amount of sound they can absorb.
Instead, the new technique uses a pair of resonators, built as a single layer and designed to naturally vibrate at the frequency of whatever they’re attached to (every object has a natural frequency, a function of its physical and material properties). Sound is largely cancelled out by one of the resonators: it provides a route for sounds to dissipate through, made possible by tuning its natural frequency to that of the thing it’s attached to.
But at the quietest moments, even that absorbing resonator can scatter some sound at its own frequency. That’s where the second resonator comes in: tuned to match the frequency of the first resonator, it can be used to create destructive interference that cancels out any noise that it produces. Together, the pair manage to absorb 99.7 per cent of all the noise that hits them, whatever the volume.
Image by Guian Bolisay under Creative Commons licence