  Breaking reciprocity has recently gained significant attention due to its
broad range of applications in engineering systems. Here, we introduce the
first experimental demonstration of a broadband mechanical beam waveguide,
which can be reconfigured to represent wave nonreciprocity. This is achieved by
using spatiotemporal stiffness modulation with piezoelectric patches in a
closed-loop controller. Using a combination of analytical methods, numerical
simulations, and experimental measurements, we show that contrary to the
conventional shunted piezoelectrics or nonlinearity based methods, our setup is
stable, less complicated, reconfigurable, and precise over a broad range of
frequencies. Our reconfigurable nonreciprocal system has potential applications
in phononic logic, wave diodes, energy trapping, and localization.
