Flow-acoustic interactions due to fully turbulent inflow past a coaxial side branch resonator mounted in a duct are investigated using a technique of digital particle image velocimetry in conjunction with unsteady pressure measurements. The configuration under investigation involves two deep cavities (side branches), which are located in-line with each other and on the opposite sides of the main duct. The quantitative imaging leads to patterns of velocity, vorticity, and streamline topology. Global instantaneous images, as well as time-averaged images, are evaluated to provide insight into the flow physics during tone generation. Emphasis is on the manner in which the ratio of the width of the main duct to the streamwise length of the side branch alters the major features of the flow structure. When the main duct is relatively wide, each of the shear layers along the mouths of each of the side branches forms an independent acoustic source. However, as the main duct becomes progressively narrower, the oscillating shear layers exhibit increasingly complex interaction patterns. Depending on which of these flow regimes occur, the acoustic response of the system is substantially altered.