Analysis of bandpass spectra of phononic defect-mode waveguides based on mode coupling between point defects

Abstract

Pass bands of defect-mode waveguides fabricated in sonic/phononic crystals have desirable characteristics for their practical applications to acoustic bandpass filters. Their mechanism is considered to originate in mode-coupling characteristics of point defects, a chain of which composes a defect-mode waveguide. Especially, mode confinement in each point defect or their mode coupling is found to decide the flatness of the pass band. For example, with a relatively strong mode confinement, the transmission of the pass band swings between 0 and -16 dB with frequency, although the transmission outside the pass band remains still typically at -50 dB. On the contrary with a moderate mode confinement, the transmission remains fairly flat only with a swing of 2 dB. We have obtained these characteristics not only by an elastic FDTD method but also by acoustic experiments. These have been investigated theoretically and experimentally based on analyses of the mode coupling between point defects. Two sonic/phononic crystals are considered; one is composed of acrylic-resin cylinders in air, and the other steel cylinders in water.