A systematic investigation of the effect of collimators on the characteristics of ultrasonic NDT transducers

The work presented in this paper addresses the use of polytetrafluoroethylene collimators commonly applied during the industrial C-scanning of components. In particular, it investigates the way collimators modify the acoustic field generated by unfocused transducers. A series of systematic studies has been carried out applying membrane hydrophones to probe the resultant spatial distribution of acoustic pressure in the field both along and perpendicular to the acoustic axis. The results are related to the characteristics of the transducer (frequency) and collimator details (aperture diameter thickness and mounting configuration), the objective being to predict and control the effects of collimation. To a reasonable approximation, the position of the last axial maximum of the collimated pressure distribution may be taken as being equal to a a(c)(2)/lambda, where a(c) is the collimator aperture radius and lambda the acoustic wavelength, the conventional expression for a plane-piston transducer. The pressure distribution along the acoustic axis of the transducer-collimator combination exhibits beam-narrowing to the new axial position, the measured -3 dB beamwidth determined at Y-0+ being given by roughly one-quarter of the applied collimator diameter.