New Perspectives on the Physical Origin of Acoustic Wood Anomalies

Acoustic Wood anomaly is a pulsed ultrasound phenomenon in which sharp spectral anomalies such as tips or dips or both occur in the spectrum of a pulse reflected from or transmitted through a periodically corrugated interface for normal incidence. To date only surface acoustic waves generated on periodic interface through diffraction mechanism have been identified as the physical source of these spectral anomalies, and each anomaly has been proved being matched by specific order(s) of diffraction of related surface wave(s) with high accuracy. However, in this study, we report the observation of several anomalies that are not originated from surface waves, and an anomaly that cannot be matched by any individual order of diffraction. Using a proposed sectional analysis approach, comb filtering effect is proved to be a new source of spectral anomalies, and a multiple surface wave theory is developed not only to provide a satisfactory explanation for the unusual anomaly observed, but also to serve as a general guideline for understanding spectral anomaly formation.