Diffuse energy transport and coda-wave interferometry for resonant transmission between reverberant structures

Approximate analytic and numerical theories are developed with which to model, and compare with laboratory measurements, the diffuse ultrasonic field transmitted from one elastic body to another through a resonant coupling. Particular focus is on the sensitivity of the transmitted field to perturbations in the natural frequency of the coupling and the manifestation of such perturbations as time delays in the second body. The immediate application is to measurements of slow dynamic elastic nonlinearity. It is found that the developed approximate theories do a good job of predicting the time dependence of the mean square transmitted signal and signal spectra. They also predict the time delays, both their erratic character and means. Analytic dependence of these time shifts on the fractional frequency perturbation df/f is derived.