SPECIFIC FEATURES OF THE ACOUSTIC DIFFRACTION FROM A PERIODIC SYSTEM OF PLANAR SLIDING-CONTACT INTERFACES

Propagation of sagittally polarized plane acoustic waves is considered in an orthorhombic medium with a periodic system of N+1 infinite planar cuts maintaining sliding contact (ideal cracks). The reflection and transmission rates are derived by the propagator-matrix method. Two essentially different types of stop bands exist, in which the imaginary part of the Bloch vector either remains finite or reaches infinity. The latter corresponds to the transmission cut-off, which may come about specifically at the sliding-contact interface. Coupling of the Bragg phenomenon with the cutting-off effect produces quite specific resonant features of reflection and transmission. Especially sharp filtering properties of the spectra come about at a small deviation Δθ from such angles of incidence, which provide total transmission (anti-reflection) independent of frequency, namely, at nearly normal incidence of the fast mode, and at angles of incidence of the slow mode close to a certain critical value. At ∣Δθ∣⪡1, the spectrum of transmission (without mode conversion) represents a nearly periodic group of abrupt dips to zero and a modulated group of secondary drops increasing with growing (Δθ)2N2, whereas the general spectral background is close to unity. In turn, the reflection spectrum at ∣Δθ∣⪡1 contains sharp principal peaks with modulated heights, reaching nearly unit height, and the secondary peaks against almost zero background. Changes in the spectra shape on varying the angle of incidence θ become drastic near the specific threshold value of θ, which corresponds to the mutual transformation of the ordinary stop bands and cutting-off bands. After the cross-over, the transmission spectrum contains significantly wide step-wise dips, within which the rate stays very close to zero if N2⪢1.