Ultrasonic testing of materials

An ultrasonic apparatus for testing a material comprises an oscillator (10) which generates a selected frequency in the ultrasonic range. A transducer (1) is connected to the oscillator (10) for applying an ultrasonic signal to the material and for receiving an echo signal back from the material. A phase detector (5) receives the echo signal and an in-phase oscillator signal to generate a first display signal, and a phase detector (6) receives a quadrature signal (90° out of phase from the oscillator signal) and the echo signal to generate a second display signal. The first and second display signals are utilised in a visual display, such as a cathode ray tube (8), to generate an image. The image changes according to the phase shift between the ultrasonic signal transmitted into the material and the echo signal, which, in turn, can be utilised to determine the presence and depth of a flaw or boundary in the material.