  Based on a 21.5 ks \chandra\ observation of A2556, we identify an edge on the
surface brightness profile (SBP) at about 160$h_{71}^{-1}$ kpc northeast of the
cluster center, and it corresponds to a shock front whose Mach number
$\mathcal{M}$ is calculated to be $1.25_{-0.03}^{+0.02}$. No prominent
substructure, such as sub-cluster, is found in either optical or X-ray band
that can be associated with the edge, suggesting that the conventional
super-sonic motion mechanism may not work in this case. As an alternative
solution, we propose that the nonlinear steepening of acoustic wave, which is
induced by the turbulence of the ICM at the core of the cluster, can be used to
explain the origin of the shock front. Although nonlinear steepening weak shock
is expected to occur frequently in clusters, why it is rarely observed still
remains a question that requires further investigation, including both deeper
X-ray observation and extensive theoretical studies.
