Panel transmission measurements: the influence of the non plane wave nature of the incident field

For reasons of cost and practicality, laboratory measurements of the acoustic transmission and reflection properties of materials for use in underwater applications are typically performed on samples of limited dimensions - and with the source and receiver separated by relatively short distances - resulting in a non planar measurement field. The influence of this on the resulting measurements is investigated in this paper. In particular, for low frequency measurements the influence of the evanescent wave contributions can become significant. In this paper two alternative approaches are used to evaluate the transmission properties. The first method decomposes the incident spherical wave into its plane wave components and integrates the resulting transmitted waves numerically to evaluate the transmitted field. The second approach uses an asymptotic expansion of the field in terms of wave front curvature: bounds are then placed on the error in this expansion at low frequency using thin plate theory. Results are compared and contrasted for measurements in the frequency range 1 to 60 kHz for panels of simple elastic materials (steel and Perspex (polymethylmethacrylate)). In addition the nature and significance of the modes of the panel for evanescent waves are considered. The consequences for laboratory measurements are also outlined