A metamaterial superacoustic absorber: the bubble raft

Abstract : The concept of bubble raft was introduced in 1947 by William Lawrence Bragg (L. Bragg and J. F. Nye, Proc. R. Soc. London Ser. A 190, 474 (1947)) as a dynamic model of a crystal structure. A bubble raft is a compact assemblage of bubbles, a millimetre or less in diameter, floating on the surface of a surfactant solution. Here we show that a bubble raft is also an acoustic metamaterial that can be used to absorb sound in a selective frequency band determined by the bubble size. To form the bubble raft we inject monodisperse small bubbles by blowing air through a 20 µm-diameter capillary at the bottom of a cylindrical container. Reflection measurements are performed by acoustic pulse emission and reception with a couple of broadband air transducers at an incidence larger than 20°, i.e., larger than the critical angle for an air/water interface. Contrary to first intuition, the reflection is not total but is strongly decreased at a frequency that scales with the inverse bubble size. Using an MST calculation (Multiple Scattering Theory) we relate that decrease to absorption in the bubble raft, which exhibits an effective viscosity 250 times higher than that of water.
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Alice Bretagne, Valentin Leroy, Arnaud Tourin. A metamaterial superacoustic absorber: the bubble raft. Acoustics 2012, Apr 2012, Nantes, France. ⟨hal-00811032⟩

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