  A complete treatment regarding frustrated total internal reflection (FTIR) of
ultrasonic waves is presented and validated against experiments, providing a
theoretical explanation for the physics behind this phenomenon. Two different
approaches are used to develop a theoretical model capable of studying
transmission in fluid-coupled elastic plates. One is the multiple reflections
approach (analogous to the study of FTIR in electromagnetic/optical waves),
which is shown to have limited applicability for incident angles beyond the
first critical angle. This prompted us to address the problem using the second,
potentials-based approach, which is established and validated against
experimental data with correct predictions for a thin air-coupled steel sheet.
A relation between the transmitted power and the dispersion curves for guided
waves in the plate is established, highlighting the two fundamental causes of
FTIR in such systems. First, a thin plate, when compared to the wavelength of
the wave incident on it, will always be subjected to the effects of FTIR. This
is because the evanescent wave created inside the plate will never assume
negligible values, thus always allowing transmission to the other side. Second,
and more surprisingly, the excitation of the fundamental antisymmetric mode
$A_0$ of the plate is shown to be a direct enabler of FTIR, even for thicker
plates.
