Transformation acoustics (also called coordinate transformation) is a mathematical tool to obtain a transformed space from the original one. It provides a systematic method to control the propagation of sound waves and offers a way to design the extraordinary metamaterial devices. And the beam shifter is one of these devices. The first beam shifter and beam divider were presented in 2008. No matter the incoming beam incidented either normally or obliquely on the interface, the entire beam would be translated in another direction or divided into two beams. Based on the transformation optics, a new model to rotate the direction of optical beam propagation by 90^o appeared in 2008.

Researchers from the Institute of Acoustics, Chinese Academy of Sciences (IACAS) focus on a kind of beam shifter designed by transformation acoustics, namely, a two-dimensional model of acoustical right-angle bend in fan shape for sound beam. The quarter circular model is comprised of metamaterials with anistropic refractive index. Due to the gradual decrease of the refractive index from inner to external along the radius of the model, the sound velocity increase with the increase of radius, leading to a capability of bending a sound propagating beam through a right angle with no reflection loss.

This right-angle bend should be a broadband model. The upper limit of the effective frequency depends on the width between two layers. Researchers from IACAS only illustrate that the performance of this model is excellent from 3500Hz to 4500Hz. However, the range of the effective frequency could be wider. This model bends a sound propagating beam without reflection. It may have potential for practical applications in various areas such as sound absorption in pipeline and detection in industry.

Fig.1 A snapshot of the sound field (Image by BI). 

References:
BI Yafeng, HU Wenlin, YANG Jun. A Model of Acoustical Right-angle Bend (With Metamaterials). Proceedings of the 21st International Congress on Sound and Vibration, ISSN2329-3675/ISBN 978-83-62652-66-2, Interanational Institute of Acoustics and Vibration (IIAV), SS08-936, 2014. 

Contact:
YANG Jun
Key Laboratory of Noise and Vibration Research, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China
E-mail: jyang@mail.ioa.ac.cn