Extraordinary absorption of sound in porous lamella

Invited paper

Johan Christensen

DTU Fotonik

Tuesday 2 june, 2015, 10:20 - 10:40

0.7 Lisbon (47)

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).

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