Abstract: Methods of and apparatus for simulating ultrasonic pulse echoes are used to test and calibrate ultrasonic flaw detectors by simulating a train of radio frequency pulses, having a pulse repetition frequency which is indicative of both the speed of sound in a material under test and the thickness of that material. This is accomplished by generating a train of d.c. pulses having a pulse repetition frequency, which is directly proportional to and, which is a multiple of the speed of sound of the material The pulse repetition frequency is also inversely proportional to the thickness of the material. The d.c. pulses are used to drive a radio frequency pulse generator and a pulse modulator. The outputs of the radio frequency generator and pulse modulator are then combined to produce a signal having an amplitude envelope, which simulates a series of ultrasonic pulse echoes.