Validation of a diffraction correction model for through‐transmission substitution measurements of ultrasonic absorption and phase velocity

A systematic study has been undertaken of the influence of diffraction on ultrasonic absorption coefficient and phase velocity measurements made using the through-transmission substitution technique. A diffraction correction model dealing explicitly with the interaction of the sample with the plane- and edge-wave components of the acoustic field of a piston transducer is introduced. The National Physical Laboratory ultrasonic materials characterization facility has been used to validate the model using a pulsed technique covering the frequency range 1–15 MHz. In the validation, absorption coefficient and phase velocity measurements are presented for test specimens of cross-linked polystyrene of various thickness. Strong diffraction effects have been demonstrated in the measurements, particularly for the absorption coefficient where measured values can be typically a factor of 2–3 higher than plane-wave values. Excellent agreement is demonstrated between theory and measurement for both absorption and phase velocity under a wide range of measurement conditions.