Ultra-thin low frequency perfect sound absorber with high ratio of active area

A concept of an ultra-thin low frequency perfect sound absorber is proposed and demonstrated experimentally. To minimize non-linear effects, a high ratio of the active area to the total area is used to avoid large localized amplitudes. The absorber consists of three elements: a mass supported by a very flexible membrane, a cavity, and a resistive layer. The resonance frequency of the sound absorber can be easily adjusted just by changing the mass or thickness of the cavity. A very large ratio between the wavelength and the material thickness is measured for a manufactured perfect absorber (ratio = 201). It is shown that this high sub-wavelength ratio is associated with narrowband effects and that the increase in the sub-wavelength ratio is limited by the damping in the system.