  Among the many far-reaching consequences of the potential existence of a
magnetic monopole, it induces topological zero modes in the Dirac equation,
which were derived by Jackiw and Rebbi 46 years ago and have been elusive ever
since. Here, we show that the monopole and multi-monopole solutions can be
constructed in the band theory by coupling the three-dimensional Dirac points
in hedgehog spatial configurations through Dirac-mass engineering. We then
experimentally demonstrate such a monopole bound state in a
structurally-modulated acoustic crystal as a cavity device. These monopole
resonators not only support an arbitrary number of degenerate mid-gap modes,
but also offer the optimal single-mode behavior possible -- whose modal spacing
is inversely proportional to the cubic root of the modal volume. Our work
completes the kink-vortex-monopole trilogy of zero modes and provides the
largest free spectral range for sizable resonators.
