A thin, flexible beam shivers from a blow, and a wave runs along it like ripples on water. Along the beam, seven pairs are placed: a sensor feels the bending, and a motor immediately pushes the beam in response. Each pair is like a tiny tuning fork that oscillates in antiphase and quells the tremor at its spot. The wave cannot pass beyond — at certain frequencies, silence falls.
Previously, such zones of silence were judged by the motion of the beam's tip. Here, the focus is on the bending at each point: the sensor measures how strongly the material is curved, and the motor responds so as to reduce that curvature. This more accurately reflects how the wave is damped: not the whole beam tip freezes, but each segment stops bending. Seven sensor-motor pairs form seven links, and each can be tuned individually.
The response strength and the viscosity of the response were changed on the fly. With a weak response, the tremor passes almost unhindered. With a strong response, the beam itself starts oscillating at a new frequency. In between, there is a band where vibrations are damped, like sound in noise-canceling headphones. An experiment on a beam with seven links showed that a zone of silence can be opened at a desired frequency and shifted by changing the settings.
This approach is a step toward structures that adapt to shaking on their own. A bridge, an airplane wing, or an instrument housing could damp vibrations without heavy dampers, simply by changing the response of built-in motors. And just as a tuning fork is tuned to a desired note, each beam segment is tuned to a desired frequency of silence.
Zones of silence for vibrations resemble how noise-canceling headphones quell the hum of an airplane: the speaker emits a sound in antiphase, and the two waves cancel each other out.
In Dan Simmons's novel 'Hyperion', a tree-ship calms vibrations and impacts by adapting its structure — here, the same principle, but in metal and with electronics.