Low-Frequency Energy Transmission across Material Interfaces using Incident Evanescent Waves

Transmission of airborne sound into higher-impedance materials is of interest in a range of applications. Sonic booms, for example, may adversely affect marine life, if incident on the ocean surface, or may produce underground pressure waves potentially capable of impacting the integrity of existing structures, if incident on the ground surface. Energy transmission into higher-impedance materials is generally limited by significant reflection and refraction at the material interface, and by the critical angle criteria. However, unlike classical waves, spatially-decaying, or evanescent, incident waves can transmit energy at angles beyond the critical angle. When a decaying component is introduced into the incident trace wavenumber, the interaction at the interface produces a nonzero propagating component of the transmitted surface normal wavenumber, so energy is transmitted across the interface for all oblique incident angles. With the aim of investigating energy transmission using incident evanescent waves, a model for pressure and intensity transmission across the fluid-fluid and fluid-solid interfaces has been developed. Numerical results are given for common interfaces that include the air-water interface and typical air-solid interfaces, where the effects of the incident wave parameters and interface material properties are considered as well. For the air-solid interfaces, conditions can be tuned such that no reflected wave is generated at the interface, owing to impedance matching between the incident and transmitted waves, which yields considerable transmission increases over classical incident waves