Acoustic sensors play an important role in many areas, such as safety (e.g., sonar arrays), public health (e.g., ultrasonic imaging), surveillance (e.g., acoustic communication and navigation), and industry (e.g., non-destructive damage detection, green buildings). However, conventional acoustic sensors inevitably suffer from some fundamental limitations, which hinder the performance of current acoustic technology. In this talk, various efforts on the development of acoustic sensors that can potentially overcome these limitations will be discussed. First, we will discuss how learning from nature can help overcome the fundamental size constraint of directional acoustic sensors. Two bio-inspired mechanisms will be discussed, including the mechanical coupling found in a parasitic fly Ormia ochracea and the acoustical coupling in lizards. Second, we will discuss how to engineer metamaterials to manipulate acoustic waves at subwavelength scales for overcoming the minimum detection limit. Specifically, we will show how a gradient refractive index (GRIN) acoustic metamaterial with high refractive-index can spatially compress the sound wave and as a result amplify the pressure field. Moreover, we will discuss how GRIN acoustic metamaterials enable the generation of highly directional acoustic radiation. Lastly, we will show other on-going research activities in the group and the vision going forward.
Haijun Liu is an Assistant Professor in the Department of Mechanical Engineering at Temple University. He received his Ph.D. in Mechanical Engineering from University of Maryland, College Park in 2012, and B.S. in Mechanical Engineering and M.S. in Material Science from Tsinghua University in 2002 and 2005, respectively. Before joining Temple in 2015, he was a postdoctoral researcher in the Sensor Science Division at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. With a focus on the fundamental research of sensor science, his research interests include bio-inspired sensing and bio-mechanics, acoustic metamaterials, theoretical and experimental mechanics, shock wave and dynamic pressure measurement, and fiber optic and MEMS sensors. He has published a number of journal articles in Nature Communications, Scientific Reports, Journal of Sound and Vibration, and Applied Physics Letters. His research is funded by Temple infrastructure, Pennsylvania Department of Health, and National Science Foundation.