  It is commonly believed that multipath hurts various audio processing
algorithms. At odds with this belief, we show that multipath in fact helps
sound source separation, even with very simple propagation models. Unlike most
existing methods, we neither ignore the room impulse responses, nor we attempt
to estimate them fully. We rather assume that we know the positions of a few
virtual microphones generated by echoes and we show how this gives us enough
spatial diversity to get a performance boost over the anechoic case. We show
improvements for two standard algorithms---one that uses only magnitudes of the
transfer functions, and one that also uses the phases. Concretely, we show that
multichannel non-negative matrix factorization aided with a small number of
echoes beats the vanilla variant of the same algorithm, and that with magnitude
information only, echoes enable separation where it was previously impossible.
