Significance of power flow angle interference due to finite sample dimension in reflection measurements

Acoustic wave reflection techniques are widely used in extracting the acoustic constants and temperature coefficients of anisotropic crystals. To obtain these constants with the desired precision, care should be taken in the sample preparation, including flatness, parallelism, and surface finishing or polishing. Not so obvious is that along some orientations required for the extraction of elastic constants in anisotropic materials, non-zero bulk acoustic wave (BAW) power flow angles (PFA) occur. Depending on the sample dimensions, spurious reflections due to the PFA can interfere with the desired signal. In this paper we report on the different positions of a transducer/buffer rod excitation unit with respect to a 25 mm Y cut quartz cube and show the influence of the PFA using pulse echo techniques. In this orientation, selected as an example, a longitudinal wave travels with a velocity of 5996.0 m/s. Signals reflected from the back wall of the sample are expected after about 8.32 mus and 16.7 mus. However, additional clear spurious signals occur around 11.6 mus and 19.9 mus. This is a direct consequence of the position of the transducer/buffer rod unit and thus the PFA. The time response and the correlation between the spurious pulses and the PFA in the referred orientation are detailed in the paper. The results obtained indicate that attention should be given to PFA in determining the test sample dimensions when employing pulse reflection techniques for the extraction of elastic constants in anisotropic crystals