  Slow sound is a frequently exploited phenomenon that metamaterials can induce
in order to permit wave energy compression, redirection, imaging, sound
absorption and other special functionalities. Generally however such slow sound
structures 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 manipulation
devices. In this work we design a slow sound medium that halves the effective
speed of sound in air over a wide range of low frequencies, whilst
simultaneously maintaining a near impedance match to air. This is achieved with
a rectangular array of cylinders of elliptical cross section, a microstructure
that is motivated by combining transformation acoustics with homogenization.
Microstructural parameters are optimised in order to provide the required
anisotropic material properties as well as near impedance matching. We then
employ this microstructure in order to halve the size of a quarter-wavelength
resonator (QWR), or equivalently to halve the resonant frequency of a QWR of a
given size. This provides significant space savings in the context of
low-frequency tonal noise attenuation in confined environments where the
absorbing material is adjacent to the region in which sound propagates, such as
in a duct. We term the elliptical microstructure `universal' since it may be
employed in a number of diverse applications.
