  We experimentally study a one-dimensional uncompressed granular chain
composed of a finite number of identical spherical beads with Hertzian
interactions. The chain is harmonically excited by an amplitude- and
frequency-dependent boundary drive at its left end and has a fixed boundary at
its right end. Such ordered granular media represent an interesting new class
of nonlinear acoustic metamaterials, since they exhibit essentially nonlinear
acoustics and have been designated as 'sonic vacua' due to the fact that their
corresponding speed of sound (as defined in classical acoustics) is zero.This
paves the way for essentially nonlinear and energy-dependent acoustics with no
counterparts in linear theory. We experimentally detect time-periodic, strongly
nonlinear resonances whereby the particles (beads) of the granular chain
respond at integer multiples of the excitation period, and which correspond to
local peaks of the maximum transmitted force at the chain's right, fixed end.In
between these resonances we detect a local minimum of the maximum transmitted
forces corresponding to an anti-resonance in the stationary-state dynamics. The
experimental results of this work confirm previous theoretical predictions, and
verify the existence of strongly nonlinear resonance responses in a system with
a complete absence of any linear spectrum; as such, the experimentally detected
nonlinear resonance spectrum is passively tunable with energy and sensitive to
dissipative effects such as internal structural damping in the beads, and
friction or plasticity effects. The experimental results are verified by direct
numerical simulations and by numerical stability analysis.
