  Exotic compact objects may resemble black holes very closely while remaining
horizonless. They may be distinguished from black holes because they
effectively give rise to a resonant cavity for the propagation of low frequency
gravity waves. In a Green's function approach, the resonance structure appears
in a transfer function. The transfer function in turn is modulated by an
initial-condition-dependent source integral to obtain the observed spectrum. We
find that the source integral displays universal factors that tend to enhance
low and negative frequencies, and this increases the complexity of the
waveforms in the time domain. These waveforms also display a significant
sensitivity to initial conditions. For these reasons a standard matched-filter
search strategy is difficult. In contrast, the sharp and evenly spaced
resonance spectrum presents a much more robust signal to target. It persists
even in the absence of simple echoes. We also describe an additional
two-component structure of this resonance pattern.
