  Invisibility devices based on coordinate transformation have opened up a new
field of considerable interest. Such a device is proposed to render the hidden
object undetectable under the flow of light or sound, by guiding and
controlling the wave path through an engineered space surrounding the object.
We present here the first practical realization of a low-loss and broadband
acoustic cloak for underwater ultrasound. This metamaterial cloak is
constructed with a network of acoustic circuit elements, namely serial
inductors and shunt capacitors. Our experiment clearly shows that the acoustic
cloak can effectively bend the ultrasound waves around the hidden object, with
reduced scattering and shadow. Due to the non-resonant nature of the building
elements, this low loss (~6dB/m) cylindrical cloak exhibits excellent
invisibility over a broad frequency range from 52 to 64 kHz in the
measurements. The low visibility of the cloaked object for underwater
ultrasound shed a light on the fundamental understanding of manipulation,
storage and control of acoustic waves. Furthermore, our experimental study
indicates that this design approach should be scalable to different acoustic
frequencies and offers the possibility for a variety of devices based on
coordinate transformation.
