Mixing of gravitational wave echoes

Gravitational wave (GW) echoes, if they exist, would be a probe to the near-horizon quantum structure of a black hole (BH), which has motivated the search for echo signals in GW data. We point out that the echo phenomenology related with the potential structure might be not so simple as expected. In particular, if the near-horizon regime of BH is modeled as a multiple-barrier filter, the late-time GW ringdown waveform will exhibit the mixing of echoes, even the superpositions. As a result, the amplitudes of successive echoes might not drop sequentially.