Methods to investigate the possibilities of using a three element periodic structure to suppress the transmission of energy in an elastic tube Ole Holst-Jensen a and Sergey Sorokin b a Minus10dB, Stokrosevej 29, 8330 Beder, Denmark b Univ. of Aalborg, Institute for mechanical engineering, Pontoppidanstraede 101, 9220 Aalborg East, Denmark ole.holst@minus10db.dk Acoustics 08 Paris 1531