Acoustic wave propagation in structurally helical media - art. no. 011703

A theoretical analysis is given of the acoustic wave propagation in periodi cally nonhomogeneous media made of a solid material whose stiffness tensor is uniformly rotating along a given axis. In the last years, such media hav e been studied theoretically as well as experimentally, in particular for w hat concerns sample preparation and possible applications. A detailed analy sis of their acoustical properties is given here, based on fully analytic a nd simple propagation equations. For axial propagation: (i) the dispersion curves of media where the transversal field components and the longitudinal ones are not coupled show only one forbidden band, that gives selective Br agg diffraction; in the opposite case they show at least a second forbidden band, that involves the quasilongitudinal and one of the quasitransversal eigenmodes; (ii) in the first case (absence of coupling), the medium gives pure acoustical rotation for p much less than lambda, where p is the helica l pitch and lambda the acoustical wavelength, a nonperfectly uniform but ve ry large rotatory power for p of the order of lambda, and a guided rotation for p much greater than lambda; (iii) in the presence of the coupling, reg ions of mode exchange between the longitudinal component and a transversal one are generally present. The cases of lossy media and of quasiaxial propa gation are also considered, and the analogies between optical and acoustica l properties discussed.