  The use of cylindrical elastic shells as elements in acoustic metamaterial
devices is demonstrated through simulations and underwater measurements of a
cylindrical-to-plane wave lens. Transformation acoustics (TA) of a circular
region to a square dictates that the effective density in the lens remain
constant and equal to that of water. Piecewise approximation to the desired
effective compressibility is achieved using a square array with elements based
on the elastic shell metamaterial concept developed in [30]. The size of the
elements are chosen based on availability of shells, minimizing fabrication
difficulties. The tested device is neutrally buoyant comprising 48 elements of
nine different types of commercial shells made from aluminum, brass, copper,
and polymers. Simulations indicate a broadband range in which the device acts
as a cylindrical to plane wave lens. The experimental findings confirm the
broadband quadropolar response from approximately 20 to 40 kHz, with positive
gain of the radiation pattern in the four plane wave directions.
