A team of researchers at Penn State has designed and computationally tested a type of manmade metamaterial that is capable, for the first time ever, of manipulating a range of acoustic waves with one simple device. The new invention will be an advantage to nearly all current sonic and ultrasonic applications, for example, ultrasonic non-destructive evaluations and ultrasonic imaging.
In addition, the device should provide more accurate and efficient high-intensity focused ultrasound (HIFU) therapies, a non-invasive heat-based technique targeted at various cancers and neurological disorders.
Optical metamaterials have been extensively studied in the past decade for applications such as cloaking and perfect lenses. The essentials 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 acoustic wave into a wider or narrower beam, depending on the direction of travel through the proposed acoustic beam aperture modifier.
Acoustic Beam Aperture Modifier
The acoustic beam aperture modifier is built on butt-jointed gradient-index phononic crystals (GRIN PCs), in the present case consisting of an array of cylindrical steel pins embedded in epoxy in a specific pattern. The steel pins act as obstacles slowing down the acoustic wave speed in order to bend the acoustic waves into curved rays.
In accordance with post-doctoral scholar and the paper’s lead author, Sz-Chin Steven Lin, while other types of acoustic metamaterials also could focus and defocus an acoustic beam to achieve beam aperture modification (though before this no such beam modifier has been proposed), their device possesses the advantage of small size and high energy conservation.
Currently, researchers and surgeons must have many transducers of different sizes to produce acoustic waves with different apertures. This is analogous to having to swap out lenses on a camera to change the lens’s aperture.
With this invention, by changing the modifier attached to the transducer the desired aperture can be easily attained, the way a photographer uses a zoom lens.
The study is the first design concept for an acoustic beam aperture modifier to appear in the scientific literature, and no acoustic beam modifier device is available in the market. Consequently, the authors anticipate their device could have wide applications across several important acoustic fields, from medical ultrasound to higher sensitivity surface acoustic wave sensors to higher Q factor resonators.
The team is currently making a prototype based on the concept.
Study: Design of acoustic beam aperture modifier using gradient-index phononic crystals