3RD SOUND IN ONE AND 2-DIMENSIONAL MODULATED STRUCTURES

An experimental technique is developed to study acoustic transmission in one and two-dimensional modulated structure by employing third soun d of a superfluid helium film. In particular, the Penrose lattice, whi ch is a two-dimensional quasiperiodic structure, is studied. In two di mensions, the scattering of third sound is weaker than in one dimensio ns. Nevertheless, we find that the transmission spectrum in the Penros e lattice, which is a two-dimensional prototype of the quasicrystal, i s observable if the helium film thickness is chosen around 5 atomic la yers. The transmission spectra in the Penrose lattice are explained in terms of dynamical theory of diffraction.