  We present an efficient method for the design of anomalous reflectors for
acoustic waves. The approach is based on the fact that the anomalous reflector
is actually a diffraction grating in which the amplitude of all the modes is
negligible except the one traveling towards the desired direction. A supercell
of drilled holes in an acoustically rigid surface is proposed as the basic unit
cell, and analytical expressions for an inverse diffraction problem are
derived. It is found that the the number of holes required for the realization
of an anomalous reflector is equal to the number of diffracted modes to cancel,
and this number depends on the relationship between the incident and reflected
angles. Then, the "retrorreflection" effect is obtained by just one hole per
unit cell, also with only two holes it is possible to change the reflection
angle of a normally incident wave and five holes are enough to design a general
retroreflector changing the incident and reflected angles at oblique incidence.
Finally, the concept of Snell's law violation is extended not only to the
incident and reflected angles, but also to the plane in which it happens, and a
device based on a single hole in a square lattice is designed in such a way
that the reflection plane is rotated $\pi/4$ with respect to the plane of
incidence. Numerical simulations are performed to support the predictions of
the analytical expressions, and an excellent agreement is found.
