Metamaterials with zero, or even negative refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at sub-wavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields that can be used, for example, to hide or cloak objects from incident acoustic energy.
Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures, and have led to the development of dynamically reconfigurable, loss-compensating, and parity/time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures, and converting laboratory experiments into useful devices.
A super-material has been invented that bends, shapes, and focuses sound waves that pass through it. Finely shaped sound fields are used in medical imaging and therapy, as well as in a wide range of consumer products such as audio spotlights and ultrasonic haptics.
The collaborative research team assembled a metamaterial layer out of small bricks that each coil up space. The space coiling bricks act to slow down the sound, meaning that incoming sound waves can be transformed into any required sound field.
The new metamaterial layers could be used in many applications. Large versions could be used to direct or focus sound to a particular location and form an audio hotspot. Much smaller versions could be used to focus high-intensity ultrasound to destroy tumors deep within the body. Here, a metamaterial layer could be tailor-made to fit the body of a patient, and tuned to focus the ultrasound waves where they are needed most. In both cases, the layer could be fitted to existing loudspeaker technology, and be made rapidly and cheaply.
The metamaterial bricks can be 3D-printed and then assembled together to form any sound field. This phenomenon can be achieved with only a small number of different bricks. The goal is to create acoustic devices that manipulate sound with the same ease and flexibility with which LCDs and projectors manipulate light. The research could enable new acoustic devices combining diffraction, scattering, and refraction, and lead to future development of fully digital spatial sound modulators that can be controlled in real time with minimal resources.
For more information, contact Sriram Subramanian, University of Sussex, at