Study On Transformation Acoustics And Acoustic Zero-Refractive-Index Materials

Recently, to manipulate electromagnetic and acoustic field is one of the most popular areas of scientific research. Two ways that scientists pay lots of attention to are controlling sound waves by transformation acoustics and zero-refractive-index (ZRI) materials. Based on transformation acoustics, we can get the analytical expression of medium parameters theoretically, thus corresponding devices can be designed. So the transformation theory is very powerful for us to design the propagating trajectory of sound waves. Meanwhile, ZRI material, due to its unique parameter value, also has very peculiar properties in manipulation of sound wave field. Although it is difficult for conventional materials to meet the design requirements, with the help of artificial metamaterials, controlling of sound wave field is entirely realizable. Numerous devices designed by using transformation acoustics and ZRI materials to achieve many novel effects such as cloaking, tunneling of bent et al have widely potential applications and can make outstanding contributions to the infrastructure construction, national defense and people’s living.Based on transformation acoustics and ZRI materials, this thesis discusses the theoretical results and corresponding designs for acoustic cloaking, acoustic black hole, acoustic tunneling effect of bents et al by using effective medium theory and finite element simulation. We also experimentally observe the acoustic cloaking effect and tunneling effect of bents in a near zero index phononic crystal. The main jobs are as following:1. Two different ways for acoustic cloak, cylindrical shell cloak and carpet cloak, were studied, and medium parameters for each cases were calculated. By analysis of motion equation, we found that the motion equation form of fluid sound was invariant after coordinate transformation, only the effective parameters (effective mass density and bulk modulus) would change. Then by using the finite element software COMSOL Multiphysics, we showed the simulation results of acoustic cloaking for both cases.2. By analogy of the electromagnetic black hole, an acoustic black hole with high absorption efficiency was studied. The shell of black hole guides the incident sound wave into its inner core spirally without backscattering, thus improving the absorption capacity greatly of the absorptive core. Also, a two-dimensional artificial structure has been designed to achieve the same effect. By using effective medium theory, we successfully proposed an artificial acoustic black hole to achieve broadband omnidirectional absorption in water. The max absorption efficiency can be 98.6%. In addition, due to the fact that artificial structure did not depend on resonance, it is suitable to work at a rather wide bandwidth.3. A phononic-crystal-based ZRI material was proposed and investigated theoretically. The phononic crystal (PC) was composed of square steel rods in a square lattice with the filling ratio of 0.79. From the dispersion curves, the second band is very flat at the frequencies near 0.5443c/a. This flat branch was originated from the zeroth Fabry-Perot (FP) resonance, which leading to the negative values of effective mass density and bulk modulus. When working frequency was slightly higher than the FP resonant frequency, both the effective mass density and reciprocal of bulk modulus would be very close to zero, exhibiting the ZRI effect.4. The propagation properties of sound waves in the ZRI phononic crystal were experimentally investigated. Because the PC exhibits a zero refractive index, so the phase of sound waves did not experience any change when travelling through the PC, giving rise to many fascinating effects. By simulation and experiment, we explicitly demonstrated the acoustic cloaking effect and tunneling effect of a 90° bent in the ZRI phononic crystal.5. Slow sound effect in the ZRI phononic crystal was studied. Thanks to the existence of the flat band, the phase velocity was very large while group velocity was very small, thus slow sound effect occurred. We experimentally detected the delay of sound wave in a pulse experiment and calculated the velocity in PC was about 0.158 time of that in air.