Novel Acoustic Materials for Passive Hearing Protection

A bi-stable meta-material which rejects high amplitude harmful sound but transfers sound levels needed for the situation awareness is proposed. A threshold intensity of the sound is determined by the material design and is set to 90 dBA all signals exceeding this level will be attenuated at least at 25 dB. More specifically, the material is composed by meta-structural elements having two stable states: an element transmits a low intensity pressure wave by uniaxial elastic deformation. However, under the load exceeding the critical level, uniaxial deformations are replaced by bending deformations of the meta-structure. Since bi-stable elements are prone to mechanical instability and are switchable by the sound wave, they will perform best by having linear sizes much smaller than the wave length. In addition, the desired acoustic performance can be achieved within a few millimeters thick monolayer of the meta-elements. This is a critical advantage for designing wearable protective devices. Finally, the material can be mass fabricated using lithography free low cost manufacturing approaches.