Diffraction and Nonlinear Propagation in wide-band ultrasonic spectroscopy.

For many years there has been interest in the use of ultrasonic spectroscopy for non-destructive evaluation and in other fields such as materials characterisation and food science. In all cases, the requirement is to isolate the desired frequency-dependent effect from any others that might be present in order to maximise the accuracy and reliability of the measurement. Diffraction and nonlinear propagation are two frequency-dependent effects that cannot always be eliminated and therefore need to be minimised by a careful choice of experimental parameters. Although diffraction and nonlinear propagation have been investigated for narrow-band beams, their combined effects in wide-band ultrasonic fields are not well-documented. This paper presents both theoretical and experimental treatments to provide a clearer understanding of these important frequency-dependent effects and how they can be minimised. The results show that for a plane-piston transducer, measurements made close to the transducer face are preferable. If this is not possible then using the far-field requires a relatively simple correction for diffraction, but a much more complicated one for nonlinear propagation.