Porous labyrinthine acoustic metamaterials with high transmission loss property

This study systemically investigates a porous labyrinthine type of acoustic metamaterials (LAMs), a sort of acoustic metasurface, analytically, numerically, and in laboratory tests. The LAMs are composed of a series of porous elements, where stainless steel plates with various lengths are inserted into the melamine foam. At the frequency of interest 2000 Hz, porous elements with a thickness smaller than one-eighth of the target wavelength are designed to generate a linearly varied phase gradient on the refracting surface and slightly varied phase responses on the reflecting surface; the elements play key roles in refracted and reflected wave manipulations, respectively. Two porous LAMs with different periodical lengths are designed based on the generalized Snell’s law to study the effect of the periodical length on refraction and reflection phenomena in the scattered sound pressure fields. By reducing the length to smaller than one-half of the target wavelength, the high-order wave modes will disappear in the refracted and reflected sound pressure fields at omnidirectional incidence, resulting in enhancements of transmission loss and also sound absorption coefficient in a wide range of incidence angles compared with the uniform melamine foam with the same thickness. The thin porous LAMs provide a method to improve sound transmission loss and sound absorption properties of an uniform porous material and show potentials to be used in cabins of high-speed trains and aircraft.