We study the normal modes of torsional waves in an elastic beam consisting of a set
of
cuboids of varying heights. We present experimental, theoretical, and numerical
results. We show that some analogies to the Wannier–Stark ladders resonances,
originally introduced by Wannier in 1962, are exhibited by this classical system. The
original ladders studied by Wannier consist of a series of equidistant energy levels for
the electrons in a crystal in the presence of a static external electric field with the
nearest-neighbor level spacing proportional to the intensity of the external
field. For the case of torsional waves in the beam we have observed a similar
behavior, namely, the vibrations of the beam show resonances of equidistant
frequencies with the nearest-neighbor spacing proportional to parameter
associated with the geometry of the beam analogously to the electric field. However,
this analogy is not perfect; we address the origin of the differences.