Acoustic Wood Anomaly: A Unique Phenomenon of Diffraction and Surface Acoustic Wave Generation on Periodically Corrugated Surface – investigation of physical origins

Abstract

International audienceWood anomaly is widely known as a fascinating optical phenomenon in which the spectrum of an incident white light reflected from a diffraction grating contains some anomalies such local intensity increases and decreases at specific frequencies. Similarly, in acoustics, sharp dips were observed in the reflection spectra of a wideband acoustic pulse perpendicularly incident onto a periodically corrugated interface, which has been called acoustic Wood anomaly. The greatest concern of this acoustic phenomenon is its physical origin. In precious studies, the sharp dips were interpreted as missing energy, which was thought to be lost in the interaction of incident waves with periodic interfaces, and correlation was found between the anomaly frequencies and the critical frequencies at which surface acoustic waves were potentially generated through diffraction. This observation suggests that the ”missing” energy ais not really missing but retarded. We also show that in pulsed acoustics the occurrence of spectral anomalies highly depends on the time-domain signal windowing, and by properly selecting the portions of signal to be analyzed we can control the occurrence of the spectral anomalies in reflection spectra. We further show that the physical processes from the incident wideband waves to the appearance of spectral anomalies are involved with generation pseudo surface acoustic waves and two-time diffraction on periodically corrugated interfaces. Moreover, we show that acoustic Wood anomalies not only exist in the reflection fields, but also in the transmission fields and diffraction fields. In this sense, these anomalies are not abnormal phenomena any more. As a fundamental study of the interaction of sound with periodic structures, this work may shed light on the further investigation on phononic crystals and acoustic metamaterials, the main structural feature of which is periodic organization of constituent components