Adjustable acoustic knife edge for anisotropic and dark-field acoustic imaging

A new effective method for acoustic anisotropy and dark-field imaging in reflection ultrasonic focusing devices is proposed. Part of the spherical acoustic wave propagating to and coming from the object is stopped by an ultrasonically opaque acoustic knife edge (AKE) set at the aperture plane of the focusing lens or transducer. This AKE provides the focusing device with directional sensitivity and the capacity for dark-field imaging. Sensitivity of the AKE system to the elastic anisotropy of the leaky surface acoustic wave (LSAW) velocity was demonstrated using an X-cut LiNbO₃ crystal. Dark-field imaging of cracks in Si₃N₄ ceramics and the structure of carbon fiber-reinforced plastic (FRP) was performed. This method can be easily applied to commercially available focusing systems and allows optimization of the setup for the microscopic study of various objects.