Aperture size effects in wideband attenuation measurements [ultrasonics]

In wideband ultrasonic measurements of attenuation and dispersion of samples, transducers of different diameters are often used. A new, simple, computationally efficient expression for the diffraction loss and phase of unequal coaxial circular apertures is presented to estimate the extent of these effects or to correct for them in measurements. Calculations indicate that considerable diffraction artifacts can result from the use of small-diameter hydrophones or receivers. Conditions where large-area hydrophones or receivers reduce these effects are also discussed. The influence of diffraction can be minimized in the wideband insertion measurement method in which signals with the sample inserted are compared to those with the sample removed. Diffraction loss and phase ratio curves for the insertion method are introduced, along with guidelines for determining tradeoffs among aperture sizes and the extent of diffraction error over the measured bandwidth