Evanescent-wave tuning of a locally resonant sonic crystal

Locally resonant sonic crystals can support band gaps at low frequencies defined by resonances internal to the unit cell. Band gap frequencies are dictated by the choice of resonators and their interaction with the medium supporting acoustic wave propagation. We show that locally resonant band gaps can be tuned by engineering the dispersion of the evanescent waves appearing in the propagation medium at the resonator sites. Specifically, we experimentally consider a tubular waveguide filled with different levels of water and grafted with a periodic array of acoustic resonators. Water filling continuously tunes the dispersion of evanescent waves by changing the waveguide cross-section. Dispersion relations and transmission properties are obtained with a three-dimensional time-harmonic finite element model of wave propagation. Numerical and experimental results are found to be in good agreement. The present work is relevant to the practical design of tunable acoustic devices.