  The jamming transition is an important feature of granular materials, with
prior work showing an excess of low frequency modes in the granular analog to
the density of states, the granular density of modes. In this work, we present
an experimental method for acoustically measuring the granular density of modes
using a single impact event to excite vibrational modes in an experimental,
three dimensional, granular material. We test three different granular
materials, all of which are composed of spherical beads. The first two systems
are monodisperse collections of either 6 mm or 8 mm diameter beads. The third
system is a bidisperse mixture of the previous two bead sizes. During data
collection, the particles are confined to a box; on top of this box, and
resting on the granular material, is a light, rigid sheet onto which pressure
can be applied to the system. To excite the material, a steel impactor ball is
dropped on top of the system. The response of the granular material to the
impact pulse is recorded by piezoelectric sensors buried throughout the
material, and the density of modes is computed from the spectrum of the
velocity autocorrelation of these sensors. Our measurements of the density of
modes show more low frequency modes at low pressure, consistent with previous
experimental and numerical results, as well as several low frequency peaks in
the density of modes that shift with applied pressure. Our method represents an
experimentally simple technique for investigating the granular density of modes
and may increase the accessibility and number of such measurements.
