  We cloak a region from a known incident wave by surrounding the region with
three or more devices that cancel out the field in the cloaked region without
significantly radiating waves. Since very little waves reach scatterers within
the cloaked region, the scattered field is small and the scatterers are for all
practical purposes undetectable. The devices are multipolar point sources that
can be determined from Green's formula and an addition theorem for Hankel
functions. The cloaking devices are exterior to the cloaked region.
