  The acoustic response of defect-based acoustic interferometer-like designs,
known as Coupled Resonator Acoustic Waveguides (CRAWs), in two-dimensional
phononic crystals (PnCs) is reported. The PnC is composed of steel cylinders
arranged in a square lattice within a water matrix with defects induced by
selectively removing cylinders to create Mach-Zehnder-like (MZ) defect-based
interferometers. Two defect-based acoustic interferometers of MZ-type are
fabricated, one with arms oriented horizontally and another one with arms
oriented diagonally, and their transmission features are experimentally
characterized using ultrasonic spectroscopy. The experimental data are compared
with finite element method (FEM) simulations and with tight-binding (TB)
calculations in which each defect is treated as a resonator coupled to its
neighboring ones. Significantly, the results exhibit excellent agreement
indicating the reliability of the proposed approach. This comprehensive match
is of paramount importance for accurately predicting and optimizing resonant
modes supported by defect arrays, thus enabling the tailoring of phononic
structures and defect-based waveguides to meet specific requirements. This
successful implementation of FEM and TB calculations in investigating CRAWs
systems within phononic crystals paves the way for designing advanced acoustic
devices with desired functionalities for various practical applications,
demonstrating the application of solid-state electronics principles to
underwater acoustic devices description.
