Extraordinary absorption of sound in porous lamella
Invited paper
DTU Fotonik
Tuesday 2 june, 2015, 10:20 - 10:40
0.7 Lisbon (47)
Abstract:
Extraordinary absorption of sound in porous lamella-crystals
Johan Christensen
Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
jochri@fotonik.dtu.dk
The damping of sound waves can be understood by conversion of the mechanical
energy into heat. Mechanical energy is extracted from oscillating systems
through dissipation such that the amplitude of, e.g., free acoustic
vibrations decay exponentially with time or accordingly is maintained finite
at resonance. Acoustical material losses are governed by the intrinsic
topology in the form of porosity or fibrosity which is leading to viscous
frictions and heat exchanges and consequently to acoustic absorption.
Several efforts have been made by functional materials in which systems
containing oscillating building blocks give rise to strong resonances and
produce perfect absorption for narrow or selective range of frequencies. To
achieve a broadband response with full absorption for any direction of
incident sound remains a desirable challenge of paramount importance and is
indispensable when noise pollution is to be kept at a minimum. Here we show
that strong all-angle sound absorption with almost zero reflectance takes
place for a frequency range exceeding two octaves in artificially structured
porous materials. We demonstrate that lowering the crystal filling fraction
increases the wave interaction time and is responsible for the enhancement
of intrinsic material dissipation, in analogy with slow-light enhanced
light-matter interactions [1].
[1] J. Christensen, V. Romero-Garcia, R. Pico, A. Cebrecos, F. J. de Abajo,
N. A. Mortensen, M. Willatzen, and V. J. Sanchez-Morcillo, Sci. Rep. 4, 4674
(2014).