Waves, normal modes and frequencies in periodic and near-periodic rods. Part I

Longitudinal wave motions and localized normal modes in a rod system with periodically-alternating material properties are investigated in this paper. The energy injected into the rod system is shown either to be transported through the whole rod system in pass-bands or to be trapped near the excitation source in stop-bands. For this one-dimensional continuous model, the full power of linear system theory is utilized and a new transfer matrix method is proposed to get closed-form normal mode solutions. Localized normal modes in stop-bands in perfectly-periodic rods with asymmetric bays are identified. It is shown that for this strongly-coupled elastic system, a single small disorder may produce one or two additional modes in each stop-band, these modes are localized around the disordered bay. By understanding basic behavior of such a system, it is hoped ultimately that some insights can be achieved where closed-form results are not possible.