Extended acoustic modes in random systems with n-mer short range correlations

In this paper we study the propagation of acoustic waves in a one-dimensional medium with a short range correlated elasticity distribution. In order to generate local correlations we consider a disordered binary distribution in which the effective elastic constants can take on only two values, ηA and ηB. We add an additional constraint that the ηA values appear only in finite segments of length n. This is a generalization of the well-known random-dimer model. By using an analytical procedure we demonstrate that the system displays n-1 resonances with frequencies ωr. Furthermore, we apply a numerical transfer matrix formalism and a second-order finite-difference method to study in detail the waves that propagate in the chain. Our results indicate that all the modes with ω≠ωr decay and the medium transmits only the frequencies ωr. (paper)