Air conditioners, cars and other machinery may soon be free of vibrations and thus much quieter than they are now, thanks to new so-called adaptive phononic crystals.

A group of researchers has demonstrated that by changing an electrical parameter of such a material, it is possible to get it to modify its mechanical properties and to program the way sound propagates through it, canceling out vibration.

The advance is a move toward mechanical components with freely programmable properties, and could lead to much quieter consumer appliances. It could also help develop vibration-free microscopes and optical precision instruments.

Controlling vibrations

When an alternating force is applied to one point of a solid, the surrounding areas are also affected, creating a wave that propagates through the object. When these waves reach the boundaries, under certain conditions they get reflected onto themselves, creating so-called standing waves.

These structural vibrations create sound, with the loudness dependent on the frequency and amplitude of the waves. For example, a drum makes sound when its membrane vibrates.

In their recent work, the researchers from the Swiss Federal Laboratories for Materials Science and Technology (Empa), ETH Zürich and the Georgia Institute of Technology tried to control the way the waves travel through a solid to make that object vibration-free and, hence, soundproof.

To do so, a team led by material engineer Andrea Bergamini of Empa arranged 10 small aluminum cylinders on an aluminum plate just a millimeter thick in a periodic manner.

This type of structure has been around for some time and is called a phononic crystal — an artificially created material made by arranging certain elements to control the flow of sound.

The size and specific position of the cylinders block certain frequencies from being transmitted through the material, by interacting with and scattering the sound waves.

Andrea Bergamini, of Swiss Federal Laboratories for Materials Science and Technology, holds a model of an adaptable phononic crystal

Andrea Bergamini, of Swiss Federal Laboratories for Materials Science and Technology, holds a model of an adaptable phononic crystal that could lead to much quieter consumer appliances. (Photo: Empa)

Piezo springs

Typically in phononic crystals, the cylinders are attached directly to the plate with an adhesive. But Bergamini's team inserted tiny disks made from piezoelectric material between the plate and the cylinders.

Piezoelectric material generates electricity in response to mechanical stress, and vice versa. So one can modify the material's mechanical properties simply by changing certain electrical parameters.

"The piezoelectric disks that we used are a ceramic material with metal contacts on either side, between the sheet and the cylinders," Bergamini said. "These disks can be stimulated electronically to spontaneously change their thickness."

The researchers found that by controlling the properties of an electrical circuit connected to the disks, they were able to weaken and eventually almost rupture the link between the plate and the cylinders. And if the link became very weak, the effect was as if the cylinders were no longer attached to the plate.

In that case, the cylinders were no longer able to scatter the sound propagating through the plate and, hence, no longer able to block it.

"The funny thing that we showed is that if we play with our piezos in the right way, we can effectively disconnect — mechanically — the plate and the cylinders at a certain frequency," Bergamini said.

The analogy of the system is the "base isolation" technique used in many modern skyscrapers to make them resistant to earthquakes. The building rests on springs designed so that at certain frequencies they will not transmit force, preventing the building from shaking when an earthquake happens.

"The equivalent of such 'springs' in our case are the piezoelectric disks. Their advantage is that by changing properties of the electrical circuit the crystal is connected to — for instance, by changing how the electrical current flows through the circuit — we can change their stiffness," Bergamini said.

This way, a typical phononic crystal turns into an adaptive phononic crystal — a material with adaptable properties.


In this experiment, the electrical circuit was a real analog one, and the scientists controlled the current flowing through it manually, by turning a knob. But they say in the future a microchip would be pre-programmed to make the material change its properties when needed.

"It is a step towards so-called 'programmable material.' Such materials — made from metal, plastic and even ceramics — may include some electronics," Bergamini said. "For special applications and requirements, having an electronic layer in the material may prove advantageous."

An embedded computing device would control whether and how waves are allowed to propagate in the aluminum plate by changing some electrical parameter of a circuit. The circuit would then change the stiffness of the piezoelectric element to get it to "disconnect" the two things that are attached to its two sides — to the point when no mechanical stress could be transmitted from the plate to the cylinders, making the material vibration-free.

In other words, think quieter cars.

"Nowadays, the interior of our cars is often lined with soft, absorbing materials that help damp the vibration of the structure and absorb sound. If we were successful at keeping the structure from vibrating at acoustic frequencies - anything that your ear can hear, something between 20-50Hz to a few kHz - then we could expect cars to be quieter," Bergamini said.

Physicist Fabrizio Scarpa of the University of Bristol in the U.K., who was not involved in the study, called the concept "really novel."

"The concept has potential far-reaching implications, because it shifts the way metamaterials have been designed so far. It may be possible to develop true adaptive optics based on modifying the dispersive properties of a metamaterial in an active way," Scarpa said.

The challenge is to verify whether the approach would work with other materials besides just piezoelectrics, he added. But nevertheless, "this work really opens an exciting field of exploration for researchers working in smart and metamaterials," Scarpa said.

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