5 research outputs found
Omnidirectional acoustic absorber with a porous core and a metamaterial matching layer
An omnidirectional sound absorber based on the acoustic analogy of the electromagnetic metamaterial
“black hole” has been developed and tested. The resulting structure is composed of a hollow
cylindrical porous absorbing core and a graded index matching layer which employs multiple rods of
varying size and spacing to gradually adjust the impedance of the air to that of the porous absorbing
core. A semi-analytical model is developed, and the practical challenges and their implications with
respect to performance are considered. A full size device is built and tested in an anechoic chamber
and the semi-analytical model used in the design process is validated. Finally, the theory is extended
to allow for losses in the metamaterial matching layer, and it is shown that improved performance may
be achieved with a dual purpose layer which acts as an absorber whilst also providing the required
impedance matching condition
Absorption of sound by porous layers with embedded periodic arrays of resonant inclusions
The aim of this work is to design a layer of porous material with a high value of the absorption
coefficient in a wide range of frequencies. It is shown that low frequency performance can be
significantly improved by embedding periodically arranged resonant inclusions (slotted cylinders)
into the porous matrix. The dissipation of the acoustic energy in a porous material due to viscous
and thermal losses inside the pores is enhanced by the low frequency resonances of the inclusions
and energy trapping between the inclusion and the rigid backing. A parametric study is performed
in order to determine the influence of the geometry and the arrangement of the inclusions embedded
in a porous layer on the absorption coefficient. The experiments confirm that low frequency
absorption coefficient of a composite material is significantly higher than that of the porous layer
without the inclusions
Study of Circumferential Waves on a Poroelastic Cylinder
The dispersion relations of acoustic modes in poroelastic cylinders with and without elastic shell coating are determined and solved. The influence of elastic frame parameters and the Biot parameters on the dispersion curves is studied in the configuration with and without coating. The dispersive Rayleigh and whispering gallery waves are highly sensitive to the density and the shear modulus of the skeleton, opening a perspective for the evaluation of the mechanical parameters of poroelastic materials confined in cylindrical tubes during the manufacturing process. The predicted dispersion curves are validated with experimental results obtained by use of different experimental set-up in the case of a porous circular cylinder.status: publishe