  We study the generation of tones by ideally-expanded round jets impinging on
a flat plate. Data from large-eddy simulations performed for different
nozzle-to-plate distances is explored, and we consider closure of the
aeroacoustic feedback loop responsible for the tones by guided jet modes.
Allowable frequency ranges for resonance, underpinned by the existence of modes
with upstream-directed group velocities, are computed using two different
models: a cylindrical vortex-sheet model; and a locally-parallel stability
model which considers a finite-thickness velocity profile. It is shown that
inclusion of a finite-thickness velocity profile consistent with the mean flow
in the vicinity of the plate improves the agreement between observed tones and
model predictions. The frequency of the largest tones found in the data are
found to fall within, or very close to, the frequency limits of the
finite-thickness model, correcting discrepancies observed with the vortex-sheet
model. The same trend is observed in comparisons with experimental and
numerical data gathered from the literature. Pressure eigenfunctions of the
stability model are in good agreement with upstream-travelling disturbances
educed from the data at the tone frequencies. This provides further evidence
for the involvement of guided jet modes in the resonance mechanism.
