Broadband diffuse field sound insulation of double layer metamaterial plates lined with porous material

We propose a composite acoustic metamaterial consisting of double layer metamaterial plates (DLMPs) lined with porous material (PM). Numerical results show that DLMPs lined with PM can produce a very high diffuse field sound transmission loss (STL) beyond the mass law over an ultra-wide low-frequency range. The unusual behavior is interpreted by explicit formulas. For validation, a large-scale specimen is fabricated using light surface mass density (5.72 kg/m2) and small thickness (5 cm). Measured data show that the specimen possesses an excellent diffuse field STL deeply breaking the mass law at broadband low frequencies (248–1624 Hz), and simultaneously, it has an improved diffuse field STL within a wide low-frequency band (208–850 Hz) compared with the conventional double layer homogeneous plates lined with PM. Owing to the simple construction, light weight, and thinness, the proposed composite metamaterial has promising applications in noise control engineering.