  In a recent Letter, Cummer et al. give a description of material parameters
for acoustic wave propagation giving rise to a 3D spherical cloak, and verify
the cloaking phenomenon on the level of scattering coefficients. A similar
configuration has been given by Chen and Chan. In this Comment, we show that
these theoretical constructions follow directly from our earlier work
http://arxiv.org/abs/math/0611185 on full wave analysis of cloaking for the
Helmholtz equation with respect to Riemannian metrics. Furthermore, the
analysis there covers the case of acoustically radiating objects being enclosed
in the cloaked region.
