How Built‑In Resonators Quiet Vibrations and Improve Soundproofing

Researchers have determined how built-in resonators handle vibrations under a variety of scenarios. Resonators are devices that help manage vibrations — some vehicles have them to limit the sound emitted from a car’s muffler and some bridges and buildings use them to limit noise and movement from those structures. Resonators use spring-like oscillation to control and change vibrations — some absorb and neutralize them while others amplify and direct them to specific places.

Prior studies examined how to use resonators to control sound that is passed through walls or to reduce the vibrations of moving vehicles. Those studies focused on adding a resonator to an existing structure or vehicle part. The new study found that cutting resonators directly into the wall or vehicle material suppressed the vibrations that could spread.

The researchers cut resonators into rectangular acrylic plates to determine what might happen if resonators were cut directly into a material, rather than added on later. The study did not directly compare cut-in resonators to those that are added on. Cut-in resonators are more streamlined than those that are added on and would be better options for applications where space matters — in the construction of an airplane or a wall, for example.

The researchers held the plates in place using different mechanisms — some were clamped tightly, some were suspended in air, and some were held up by supports. Then they used a mechanized hammer to strike the plates and measured the force of the vibrations created by each plate after the hammer’s impact. They kept one plate unaltered as a control.

They found that vibrations traveled farther on the unaltered plates than on those with built-in resonators. The resonators, their experiments showed, helped the plates flex and absorb the vibrations from the hammer.

The experiments were built to test a design that could have multiple real-world applications and could be used to soundproof walls. It also could be used to build airplane frames that automatically lessen the sound that enters the cabin; for example, a sandwich-like construction consisting of a lightweight panel with a built-in resonator between two panels of sheetrock or layered inside the walls of an airplane. This could be used to reduce vibration of the entire system without reducing the aerodynamics or efficiency since mass is eliminated.

The resonators also were able to absorb vibrations regardless of how the plates were held in place.