We propose a simple solid-fluid layered structure where narrow Gaussian acoustic beam propagates practically without spreading. It is demonstrated that the coefficient (so-called diffraction coefficient), which defines the rate of linear spreading, vanishes if a certain relation between frequency and direction of propagation is hold. Unlike nonspreading “light bullets” recently proposed in optics, acoustic monochromatic signal can propagate in diffraction-free regime without sophisticated modulation. In the experiment with steel-water layered structure we observe nonspreading propagation of a signal with frequency ~ 100 kHz at a distance ~ 1m. Such long-range collimation is accompanied by negative phase velocity refraction, anomalous dispersion, and very strong anisotropy of the effective mass density. Such long-range collimation is accompanied by negative phase velocity refraction, anomalous dispersion, and very strong anisotropy of the effective mass density.
Reciprocity is a fundamental property related to T-symmetry of wave equation. Nonreciprocal acoustic transmission becomes possible in a nonlinear or in a moving medium. Viscous losses, which break T-symmetry, are not considered as a nonreciprocal factor. We demonstrate that transmission through a finite-length dissipative phononic crystal is nonreciprocal for asymmetric scatterers. Asymmetric transmission is known even for inviscid background. However, additional nonreciprocal contribution related to the vorticity mode is usually missing. For infinite dissipative phononic crystal we prove that the decay coefficients turn out to be equal for the opposite directions but the velocity remains nonreciprocal due to broken PT-symmetry.
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