  Soft acoustic metamaterials utilizing mesoporous structures have recently
been proposed as a novel means for tuning the overall effective properties of
the metamaterial and providing better coupling to the surrounding air. In this
work, the use of silica aerogel is examined theoretically and experimentally as
part of a compact soft acoustic metamaterial structure, which enables a wide
range of exotic effective macroscopic properties to be demonstrated, including
negative density, density-near-zero and non-resonant broadband slow sound
propagation. Experimental data is obtained on the effective density and sound
speed using an air-filled acoustic impedance tube for flexural metamaterial
elements, which have previously only been investigated indirectly due to the
large contrast in acoustic impedance compared to that of air. Experimental
results are presented for silica aerogel arranged in parallel with either 1 or
2 acoustic ports, and are in very good agreement with the theoretical model.
