Flexural wave modes have sinusoidal variation in their displacement pattern around the circumference, in integer values ranging from 1 to Infinity. Active focusing involves the transmission of multiple flexural wave modes, with time and amplitude corrections applied, in such a way that a circumferential node from each wave mode will arrive at the target position at the same time, the same circumferential position and with the same phase, causing constructive interference. At other circumferential positions the circumferential nodes of the flexural wave modes will arrive out of phase with each other and will interfere destructively. Adjusting the excitation conditions can rotate this focal spot around the pipes circumference. Comparing the response from different circumferential positions can allow the operator to more accurately predict the circumferential position and extent of a defect.
