Optimized structures for vibration attenuation and sound control in nature: A review

Nature has engineered complex designs to achieve advanced properties and functionalities through millions of years of evolution. Many organisms have adapted to their living environments by producing extremely efficient materials and structures exhibiting optimized mechanical, thermal, and optical properties, which current technology is often unable to reproduce. These properties are often achieved using hierarchical structures spanning macro-, meso-, micro-, and nanoscales, widely observed in many natural materials like wood, bone, spider silk, and sponges. Thus far, bioinspired approaches have been successful in identifying optimized structures in terms of quasistatic mechanical properties, such as strength, toughness, and adhesion, but comparatively little work has been done as far as dynamic ones are concerned (e.g., vibration damping, noise insulation, sound amplification). In particular, relatively limited knowledge currently exists on how hierarchical structure can play a role in the optimization of natural structures, although concurrent length scales no doubt allow multiple frequency ranges to be addressed. Here, we review the main work that has been done to analyze structural optimization for dynamic mechanical properties, highlighting some common traits and strategies in different biological systems. We also discuss the relevance to bioinspired materials, in particular in the field of phononic crystals and metamaterials, and the potential of exploiting natural designs for technological applications.