The one‐dimensional model of the concentric‐tube resonator [J. W. Sullivan and M. J. Crocker, J. Acoust Soc. Am.64, 207–215 (1978)] assumes that (1) the impedance of the perforation is constant along the length of the flow tube and (2) spatial variations of acoustic pressure in the transverse direction of either tube or cavity are small. Some recent measurements on long resonators have indicated that when mean flow is present, net flow in the cavity, although small relative to that in the tube, affects the impedance in a bidirectional manner resulting in violation of assumption 1. Experiments using microphone probes have demonstrated the validity of assumption 2 provided the tube is perforated axisymmetrically and uniformly. When this is not the case, a circumferential cross mode can be induced which invalidates the one‐dimensional model.