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Slow sound is a frequently exploited phenomenon that metamaterials can inducein order to permit wave energy compression, redirection, imaging, soundabsorption and other special functionalities. Generally however such slow soundstructures have a poor impedance match to air, particularly at low frequencies,and consequently exhibit strong transmission only in narrow frequency ranges.This therefore strongly restricts their application in wave manipulationdevices. In this work we design a slow sound medium that halves the effectivespeed of sound in air over a wide range of low frequencies, whilstsimultaneously maintaining a near impedance match to air. This is achieved witha rectangular array of cylinders of elliptical cross section, a microstructurethat is motivated by combining transformation acoustics with homogenization.Microstructural parameters are optimised in order to provide the requiredanisotropic material properties as well as near impedance matching. We thenemploy this microstructure in order to halve the size of a quarter-wavelengthresonator (QWR), or equivalently to halve the resonant frequency of a QWR of agiven size. This provides significant space savings in the context oflow-frequency tonal noise attenuation in confined environments where theabsorbing material is adjacent to the region in which sound propagates, such asin a duct. We term the elliptical microstructure `universal' since it may beemployed in a number of diverse applications.

Deepening subwavelength acoustic resonance via metamaterials with universal broadband elliptical microstructure