Acoustic band gaps in periodically and quasiperiodically modulated waveguides

We report experimental observation of the formation of phononic band structure in one-dimensional periodically and quasiperiodically (based on the Fibonacci and Thue-Morse number sequences) area modulated waveguide structures. The experimental results are compared to model calculations considering the interference of multiply reflected waves using a transfer matrix method formulation. It was found that both the scattering due to the changes in area (causing an impedance discontinuity) and also the radiation impedance acting at each area discontinuity must be considered to accurately model the experimental results. For the quasiperiodic structures, complicated transmission spectra are seen to result, characterized by wide acoustic pseudo-band-gaps interrupted by narrow defect modes around the center of the gap