Abstract: Apparatus is disclosed for detecting separations or air pockets hidden within truck tire casings which are being inspected to determine their fitness for retreading. The operator holds a roller against the inner surface of the tire casing which is being rotated. Regularly spaced cams which project from the inner surface of the roller engage and lift one or more impact hammers. The hammer when released strikes an anvil mounted on the side frame of the roller and the ensuing shock is transmitted through the axle to the roller. A microphone mounted near to the roller picks up the sounds produced by the shocks. Any variations in the sound frequencies, such as would be produced by an air pocket, are detected electronically and an alarm signal is given. The apparatus has broader application.