ULTRASONIC INTERFERENCES IN POLYMER PLATES

Reflection and transmission of an ultrasonic beam by a homogeneous pol ymer plate (PMMA) immersed in water are studied experimentally and the oretically by the interference method. The elementary waves which emer ge from the plate after 0, 1, or 2 internal reflections on the lower i nterface are considered individually, and their amplitude and phase sh ift are calculated. The interferences between these elementary waves p rovide the resulting waves reflected and transmitted by the plate. Thi s method is valid because only a small number of these elementary wave s have a significant amplitude when they emerge, due to the relatively high wave attenuation of polymer materials. For a 2-mm-thick PMMA pla te, for example, only two internal reflections on the lower face must be taken into account for the reflection by the plate, and one for tra nsmission. Perfect agreement with results from the classical global me thod confirms the validity of the interference method. This treatment enables us to better understand the physical significance of the refle ction and transmission curves by the plates. Moreover, this method fac ilitates the calculation of accurate ultrasonic parameter values of th e plate (velocities and attenuation coefficients of the longitudinal a nd transverse waves), by adjustment of the theoretical to the experime ntal reflection and transmission curves. (C) 1998 Acoustical Society o f America. [S0001-4966(98)05308-9].