Experimental observation of resonant filtering in a two-dimensional phononic crystal waveguide

Transmission of acoustic waves through a two-dimensional composite material made of PVC cylinders surrounded by air is measured experimentally. The spectrum presents a very large absolute band gap in the audible frequency range. A waveguide created inside this phononic crystal by removing a row of cylinders can transmit very efficiently the waves falling inside the stop band. We show the existence of deaf modes in the band structure of the linear waveguide. Resonant filtering is also demonstrated experimentally by coupling the waveguide to a side branch resonator of variable length. Frequency filtering is observed in the form of narrow dips in the transmission spectrum of the waveguide. Most of these observations compare favorably with theoretical calculations of dispersion curves and transmission coefficients of model structures using the plane wave expansion and the finite difference time domain methods. Narrow dips similar to those of the guide with resonator are also observed in the transmission spectrum of a waveguide with a sharp bend.