Analyses and mitigation of spurious scattered signals in acoustic wave reflection measurements

Abstract

Reflection techniques are widely used in extracting the acoustic wave (AW) 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, the non-zero bulk acoustic wave (BAW) power flow angles (PFA) and related BAW multimode conversion and scattering at sample boundaries will generate spurious reflections. These spurious signals may interfere with the desired signals, significantly compromising the measurements. This paper reports on an original analysis that combines the effects of PFA, wave scattering, mode conversion, sample dimensions and transducer positioning in pulse echo measurements and on the importance of considering these effects in the design of the sample to be measured. Graphical and numerical analyses are used to fully explain the experimental data obtained using a 25 mm Y-cut quartz sample. Along this propagation direction, the longitudinal wave (5996.0 m/s) reflected from the back wall of the sample is detected at the transducer after approximately 8.3 and 16.7 μs. However, clearly identifiable spurious signals occur around 11.6 and 19.9 μs. These are direct consequences of the position of the transducer/buffer rod unit and thus the PFA. The correlation between the measured spurious pulse's response in time, the sample dimensions and the PFA is detailed in this paper. Based on the analyses performed, a sample dimension design method was developed and is uncovered in this paper, which aims at the mitigation of spurious signal interferences in pulse reflection measurements.

Keywords:

• Power flow angle (PFA)
• Wave scattering
• Acoustic wave reflection
• Pulse-echo measurements

Acknowledgements

This research was conducted with support from the Army Research Office ARO, Grant # DAAD19-03-1-0117, and the National Science Foundation, grants ECS-0134335 and DGE0504494. The authors want to thank the faculty and other students from the Department of Electrical and Computer Engineering and the Laboratory for Surface Science and Technology at the University of Maine for their helpful discussions.