  We present a novel, robust sound source localization algorithm considering
back-propagation signals. Sound propagation paths are estimated by generating
direct and reflection acoustic rays based on ray tracing in a backward manner.
We then compute the back-propagation signals by designing and using the impulse
response of the backward sound propagation based on the acoustic ray paths. For
identifying the 3D source position, we suggest a localization method based on
the Monte Carlo localization algorithm. Candidates for a source position is
determined by identifying the convergence regions of acoustic ray paths. This
candidate is validated by measuring similarities between back-propagation
signals, under the assumption that the back-propagation signals of different
acoustic ray paths should be similar near the sound source position. Thanks to
considering similarities of back-propagation signals, our approach can localize
a source position with an averaged error of 0.51 m in a room of 7 m by 7 m area
with 3 m height in tested environments. We also observe 65 % to 220 %
improvement in accuracy over the stateof-the-art method. This improvement is
achieved in environments containing a moving source, an obstacle, and noises.
