  In this paper, a novel gradient index (GRIN) acoustic metamaterial is
proposed based on the concept of optical modified generalized Luneburg lens
(MGLL). With the MGLL, double-foci and high energy density between the two foci
can be achieved, which enables the realization of an ultra-long acoustic jet
between the two foci. Compared to the generalized Luneburg lens with a single
focus, which can only achieve short acoustic jet in the near field or
ultra-long acoustic jet in the far field, the proposed MGLL can achieve an
acoustic jet extending from the near field to the far field. This capability of
the MGLL is theoretically and numerically demonstrated. Furthermore, acoustic
metamaterial based MGLL is designed with lattice unit cells having a variable
filling ratio. Numerical simulation results show that ultra-long acoustic jets
can be achieved with a jet length of up to 30{\lambda} , covering both the near
field and the far field.
