  An acoustic topological insulator (TI) is synthesized using topology
optimization, a free material inverse design method. The TI appears
spontaneously from the optimization process without imposing requirements on
the existence of pseudo spin-1/2 states at the TI interface edge, or the Chern
number of the topological phases. The resulting TI is passive; consisting of
acoustically hard members placed in an air background and has an operational
bandwidth of $\approx$12.5\% showing high transmission. Further analysis
demonstrates confinement of more than 99\% of the total field intensity in the
TI within at most six lattice constants from the TI interface. The proposed
design hereby outperforms a reference from recent literature regarding energy
transmission, field confinement and operational bandwidth.
