  In this work we experimentally achieve 1 kHz-wide directional band-gaps for
elastic waves spanning a frequency range from approximately 8 to 11 kHz.
One-way propagation is induced by way of a periodic waveguide consisting in an
aluminum beam partially covered by a tightly packed array of piezoelectric
patches. The latter are connected to shunt circuits and switches which allow
for a periodic modulation in time of the cell properties. A traveling stiffness
profile is obtained by opportunely phasing the temporal modulation of each
active element, mimicking the propagation of a plane wave along the material,
therefore establishing unidirectional wave propagation at bandgap frequencies.
