An acoustic metasurface by applying planar periodic arrays of resonators with a multiple folded long neck for broadband sound absorption

Abstract

An acoustic metasurface gives a broadband sound absorption by a planar periodic array combining small resonant modules tuned at different frequencies. The resonant module is often applied by a Helmholtz resonator, a membrane backed by an airtight cavity and a quarter wavelength closed tube, which have equivalent small area to a fraction of the wavelength. The author has studied the sound absorption of resonators by applying a multiple folded long neck and an airtight cavity for a thin sound-absorbing structure with high sound absorption at low frequencies below 100 Hz. In this paper, the author prototypes an acoustic metasurface by planar periodic array of Helmholtz resonators at various resonant frequency tuned by this neck and cavity of the resonator as a unit cell, and discusses the sound absorption characteristics. The prototyped acoustic metasurface gives a 40 % - 70 % sound absorption from 125 Hz to 250 Hz when it is combined with a 6.4 mm thickness of top plate including a 24 mm - 96 mm length of the multiple folded long neck and a 43.2 mm depth of the airtight cavity. And, more broadband sound absorption will be discussed by a theoretical analysis.