When designing an acoustic device, the study of the impedance mismatch between the
device itself and its surroundings is of critical importance to achieve good performances. In
fact, a high impedance mismatch would prevent the transmission of the energy across the
interface, thus limiting the amount of energy that the device itself could treat. Generally,
this is overcome by applying acoustic impedance matching layers, for instance in the form
of gradients, similar to what done in optical coatings. The minimal form of such a gradient
can be seen as an intermediate layer with specific properties laying between the ones of
the two media to impedance match, and, in any case, requiring a minimum thickness of
the order of at least one quarter wavelength of the lowest frequency under consideration.
The selection of materials is traditionally dictated by the required combination(s) of the
(limited) available elastic properties and densities. Nature, also constrained by the use of
a limited number of materials in the design of the biological structures, shows the way
to a different approach, where the design space is swept by varying specific geometrical
and/or material parameters. The middle ear of mammals and the lateral line of fishes are
examples of such an approach, the latter already embodying an architecture of distributed
impedance matched underwater layers. Inspired by this organ, here we describe a resonant
mechanism, whose properties can be tuned to provide impedance matching at different
frequencies by choosing appropriate values for a small set of geometrical parameters.
Similar to the lateral line organ, the mechanism at hand is intended as the base unit for
the construction of an impedance matching meta-surface. A numerical investigation and
a parameter optimization demonstrate its ability to match the impedance of water and air
in a deeply sub-wavelength regime.