Audible sound reduction and control is a major issue, particularly in the low frequency range. Noise pollution, inherent to human activities, has strong repercussion on health through annoyance, premature fatigue and sleep disturbance. The European Environment Agency underlines that more than 30% of EU population may be exposed. Imposed by life and industrial standards, lighter and thinner structures are needed for the absorption of lower and lower frequency sound.
The usual way to solve this problem is the use of optimized multilayered porous materials packages. The dissipation properties of porous materials yield on viscous and thermal losses. Thus, although they are quite efficient at high frequencies, they suffer from a lack of absorption at low frequencies, particularly below the so-called Biot frequency (typically around 1000 Hz). This is due to fact that below this frequency a porous material stands in the viscous regime, where the acoustic waves are diffusive and then no longer propagating.
In the last years new porous-based structures, such as the metaporous materials, the double porosity materials or the dead end porosity materials, have been proposed. These structures make use of additional absorption/dissipation mechanisms by the presence of volume and/or surface heterogeneities that lead to energy localization inside the structure, and therefore to an enhancement of the absorption. However, this kind of enhancement is efficient mainly in the inertial regime, i.e.,the regime where the acoustic waves propagate, namely above the Biot frequency.
The Metaudible project is both a theoretical and an experimental project. Its objective is to overcome the problem of low absorption of sound at very low frequencies by designing and manufacturing the thinnest possible absorbing material. To achieve this, additional absorption and dissipation mechanisms will be used, based on subwavelength resonance phenomena, nonlinear and contact dissipation.
Existing structures (metaporous materials) will be improved by embedding optimized subwavelength resonators in correctly chosen porous materials, coupled with surface irregularities. In addition, novel metamaterials (metaudible materials) will be designed and manufactured. They will be obtained by coupling new host materials, made of a complex arrangement of resonators or porogranular materials, with larger size optimized subwavelength resonators and surface irregularities. These materials will be prototyped by a 3D wax printer or laser sintering. Dedicated experiments will be developed and performed.