Near-Field Pressure Distributions to Enhance Sound Transmission into Multi-Layer Materials

The large impedance difference between air and most solids prevents significant energy transfer from incident acoustic waves across the air-material interface. Refraction also plays a role in preventing acoustic transmission, as the wave speed difference between air and solid materials results in an increase of the resulting propagation angles, creating near-field pressure distributions in the solid material. By utilizing evanescent pressure distributions, which decay normal to the usual direction of propagation and are represented as plane waves propagating with complex angles, energy propagation through the interface can be increased in the subsonic region of wave propagation: i.e., where waves typically do not propagate into a material with any effectiveness. By using an array of sources, it is possible to produce evanescent pressure distributions in the solid. The way in which the characteristics of this array of sources affect the efficiency of the generation of evanescent pressure distributions are explored. Because high impedance materials can be paired to a lower impedance materials of interest to impede acoustical energy transmission, the wave propagation through multi-layer materials must be considered to give an full accounting of power transmission into structures. A model for wave propagation in multi-layer systems of solids and fluids was developed using wave potentials in each layer, allowing for coupling between material types and calculation of inter-layer states. By using wavenumber-frequency analysis, it is possible to target specific components in the multi-layer system and understand the particular wavetypes that cause energy propagation into the system