We discuss the new concept of a sensitive and wide-band detector, consisting of a solid sphere nested inside a hollow one: the dual sphere. The advantage is that it would be possible to keep both the omni-directionality and high sensitivity of the spherical geometry without giving up the wide band. In the few kHz range the dual sphere would be complementary to advanced interferometers. We also discuss the main technological and scientific challenges that the construction of such a system poses, particularly regarding material choice, fabrication, cooling, suspension and readout.