Reflection coefficient measurement using a finite-difference-injection technique

Experimentally determining the acoustic reflection properties of materials requires accurate knowledge of the incident and reflected wave field at the reflecting interface. Consequently, a number of methods have been proposed for measuring the reflection coefficient, but most are limited to measuring only the normal incidence reflection coefficient or assume plane wave conditions. Here, we derive the reflection coefficient from pressure measurements by using a finite-difference wave field injection technique, which is applicable for a wide range of incidence angles and does not rely on the common plane wave assumption. It requires measurements at three planes parallel to the reflector and addresses two key objectives, namely, (1) the recorded wave field is separated into its incident and reflected components without the need of time-windowing, and (2) the separated components are re-datumed to the reflecting interface. The latter step comprises a forward propagation in time of the incident wave and a time-reversed backward propagation of the reflected wave. We experimentally test the methodology on laboratory data of the reflection from the free surface recorded in water and demonstrate its applicability to accurately measure the reflection coefficient for incidence angles up to 60°.