On the measurement of attenuation in ultrasonic delay lines

A theoretical and experimental study has been made of the effects of coupling films on the propagation of compressional waves from a transducer to a solid medium. In practice it has been found that ‘wringing’ the transducer to the specimen with oil as a coupling medium produces a film of non-uniform thickness. Although the variations in thickness are of the order of a wavelength of light, these variations are important at ultrasonic frequencies in the region of 50 Mc/s and above.Conditions are described under which such films can lead to the propagation of a predominantly first-order mode in the specimen, resulting in an exponential decay of the amplitudes of successive reflections, with a consequent improved accuracy of attenuation measurement.The work provides a greater understanding of the problems encountered in applications of delay lines.