Band gap materials are a new class of materials that possesses internal structures similar to those of lattices found in natural crystals. Waves that fall into a certain frequency range cannot pass through such materials, forming a gap in the transmission spectrum. Materials having such effects on sound waves are called the phononic materials or sonic crystals. They have many direct applications in noise control and sensor designs. The same principle can be applied to other types of waves for more exciting applications. For example, their cousin for electromagnetic waves and lights, called the photonic materials, has been hailed by scientists as having the promise of technical impacts of similar magnitude as semiconductors.
In this research, the investigator and his colleagues explore the physics and mechanisms for the design of tunable phononic materials. In such materials, the frequency gap where wave propagation is prohibited can be adjusted by the end user, making the materials more adaptive. The tunable sonic crystals used in this research consist of coated cylinders embedded in a host medium. Using a soft material or fluid as the coating, the gap can be adjusted by altering the locations of the solid cylinders.
Rigorous theories for elastic wave propagation in such phononic materials are developed. These theories include: wave propagation in layered cylinders; wave propagation in the presence of an infinite number of cylinders lined up periodically along a straight line; and a methodology for decomposing a problem involving a larger number of cylinders into several problems involving smaller numbers of cylinders. Based on these theories, a computer simulation system is developed for full-scale deterministic analyses such that every minute details of the material structure can be accounted for. This system is then used to perform extensive and systematic simulations, from which a set of design rules is devised. Simulation results are also experimentally validated.
This research also supports an outreach program called Sonic Playground --- a life-size demonstration model of sonic crystal based on the findings of this research. Visitors can walk into the sonic crystal to experience this exciting new phenomenon first hand. It is designed and built entirely by students for the purpose of bringing the excitement of science exploration and new discoveries to larger audience, especially the younger generations. Through this program, engineering and arts students are united under a common goal of attracting K-12 students into science and engineering disciplines.
