Abstract: An ultrasonic pulse-echo apparatus tests railroad wheels passing through a test zone by sending ultrasonic energy into the rim of the wheels. Sensing means are disposed to discern the type of wheel e.g. locomotive wheel or freight car wheel to be tested and provide a corresponding control signal which adjusts the sensitivity of the test apparatus. This arrangement causes the apparatus to be conditioned for registering relatively small defects in locomotive wheels, but passing over small defects in other wheels which are stressed to a lesser extent, e.g. freight car wheels. Hence, the sensitivity of the test apparatus is continuously responsive to the wheel category undergoing test.