Triangular reentrant honeycomb metamaterial structure for broadband sound attenuation using shape memory polymers.

Passive noise cancellation (PNC) earmuffs are an effective solution for mitigating workplace noise exposure, ensuring compliance with the Health and Safety Executive (HSE) and Occupational Safety and Health Administration (OSHA) guidelines. However, conventional earmuffs require removal to allow for peer communication, exposing users to hazardous noise. This study addresses this issue by proposing a triangular reentrant honeycomb metamaterial design that facilitates the passage of low-to-mid-frequency waves (e.g. speech up to 3000 Hz) while blocking high frequency noise. Finite element modelling was used to evaluate the bandgaps created by the unit cell design and the effects of lattice layers on the resultant frequency response. Experiments performed using an impedance tube showed that the optimized metamaterial design achieved a peak sound absorption coefficient of 1 at 2392 Hz and a transmission loss of 10-11 dB within the 3000-6400 Hz frequency range. This design was then tested as an insert of an earmuff to characterize speech intelligibility and hearing protection using pink noise up to 95 dB. The results demonstrate the potential of such design for PNC earmuff applications, enabling sufficient peer-to-peer communication while maintaining noise protection.