Sound Waves Focused Like A Camera Lens

A metamaterial capable for the first time of manipulating a variety of acoustic waves with one simple device has been designed and computationally tested. The device holds promise for applications across various acoustic fields, from medical ultrasound to higher-sensitivity surface acoustic wave sensors and higher Q factor resonators.

Man-made optical metamaterials have been used over the past decade for a variety of applications such as cloaking and perfect lenses. The basic principles of optical metamaterials apply to acoustic metamaterials: Artificial structures are created in patterns that bend the acoustic wave onto a single point, and then refocus the wave into a wider or narrower beam, depending on the direction of travel through the proposed acoustic beam aperture modifier.

A team of researchers at Penn State Materials Research Institute built the acoustic beam aperture modifier on gradient-index phononic crystals — in this case, an array of steel pins embedded in epoxy in a particular pattern. The steel pins, or obstacles, slow down the acoustic wave speed so that they can be bent into curved rays.

The acoustic beam aperture modifier can effectively shrink or expand the aperture of an acoustic beam with minimum energy loss and waveform distortion. With such an acoustic lens, the need for a series of expensive transducers of different sizes is eliminated. (Image: Sz-Chin Steven Lin, Penn State)

Although other types of acoustic metamaterials could focus and defocus an acoustic beam to achieve beam aperture modification, the Penn State device is small in size and offers high energy conservation, according to Sz-Chin Steven Lin, a postdoctoral scholar at Penn State and lead author of the paper, which appeared in The Journal of Applied Physics.

For the last several years, Lin has been working to apply optics concepts such as gradient-index (GRIN) lensing, to the phononic crystals. He has applied his GRIN concept to different fields such as optofluidics and nanophotonics to obtain optical lenses.

Currently, scientists and surgeons need to have transducers of multiple sizes to produce acoustic waves with different apertures. With the new device, the desired aperture can easily be attained by changing the modifier attached to the transducer.

The device will benefit almost all current sonic and ultrasonic applications, including evaluations and imaging. It could also provide more accurate and efficient high-intensity focused ultrasound therapies, a noninvasive heat-based technique targeted at a variety of cancers and neurological disorders.

The team currently is working on a prototype based on this design.

For more information, visit: www.psu.edu