Broad band membrane-type metamaterial panels for sound attenuation in the 50-1000 Hz regime

Difficulty in attenuating low frequency sound is attributable to the "mass-density law" that states that for the same sound transmission loss (STL), the required mass per unit area of the panel/wall is inversely proportional to the sound frequency. Acoustic metamaterials can be treated as homogenous materials with effective dynamic mass density and dynamic modulus, which can be negative in certain frequency ranges that leads to exponential decay of elastic waves propagation with finite decay length, therefore breaking the mass-density law. Such phenomenon is experimentally realized in membrane-type metamaterials. Metamaterial panels with thickness ≤15 mm and weight ≤3 kg/m² demonstrating 19.5 dB of internal STL at around 200 Hz, and panels with thickness ≤60 mm and weight ≤15 kg/m² demonstrating an average STL of > 40 dB over a broad frequency range of 50 to 1000 Hz have been realized. In a modified version of the panels, near total absorption of incident low frequency sound waves can be achieved. The panels can also be tailor-made to block narrow band noise, such as the engine noise of jet airplane around 300 Hz, with desired STL.