Gradient metamaterial layers as impedance matching devices for efficient sound absorption
Invited paper
Loughborough University
Tuesday 2 june, 2015, 09:40 - 10:00
0.7 Lisbon (47)
Abstract:
Efficiency of traditional absorbers of audible sound is often reduced due to
the impedance mismatch between air and traditional porous materials used for
sound absorption. This results in a substantial part of the incident acoustic
energy being reflected. The performance of acoustic absorbers can be improved
by providing a smooth transition from the impedance of air to the impedance of
the absorbing material in question. In the present work, such a smooth
transition is materialised via application of gradient index metamaterial
layers formed by quasi-periodic arrays of solid cylinders (tubes) with their
external diameters gradually increasing from the external row of tubes facing
the open air towards the internal row facing an absorbing porous layer. If
acoustic wavelengths are much larger than the periodicity of the array, such a
structure provides a gradual increase in the acoustic impedance towards the
internal row of cylinders. This allows the developer to achieve an almost
perfect impedance matching between the air and porous absorbing materials,
such as foams, sponges, etc. Measurements of sound reflection coefficients
from different absorbing materials combined with matching metamaterial layers
formed by the arrays of brass tubes have been carried out in an anechoic
chamber at the frequency range of 500-3000 Hz. Measurements have been
undertaken for different types of absorbing materials in combination with the
relevant numbers of rows of brass cylinders to achieve the required impedance
matching. The results show that the presence of matching metamaterial layers
brings substantial reduction in the sound reflection coefficients, thus
increasing the efficiency of sound absorption.