  Oscillations in sunspot umbrae exhibit remarkable differences between the
photosphere and chromosphere. We evaluate two competing scenarios proposed for
explaining those observations: a chromospheric resonant cavity and waves
traveling from the photosphere to upper atmospheric layers. We have employed
numerical simulations to analyze the oscillations in both models. They have
been compared with observations in the low (Na I D2) and high (He I 10830 \AA)
chromosphere. The nodes of the resonant cavity can be detected as phase jumps
or power dips, although the identification of the latter is not sufficient to
claim the existence of resonances. In contrast, phase differences between
velocity and temperature fluctuations reveal standing waves and unequivocally
prove the presence of an acoustic resonator above umbrae. Our findings offer a
new seismic method to probe active region chromospheres through the detection
of resonant nodes.
