Propagation of acoustic and electromagnetic waves in inhomogeneous media is a phenomenon of fundamental importance and increasing technological relevance. In contrast to the case of electromagnetic waves, to date only a few experiments on sound propagation in the strongly scattering regime have been performed. In this talk we report on the creation of nanosponges made by dispersing networks carbon nanotube bundles in a fluid and exposing the suspension to low levels of laser radiation. Remarkably, exposure to laser light leads to agglomeration of the dispersed bundles into a nanoporous structure whose average pore dimensions are tunable with light. The acoustic properties of the suspension are measured by inelastic light scattering and reveal dramatic softening of the sound velocity with increasing weight fraction of the stiff nanotube bundles. The resulting reduction in sound speed as calculated using an effective medium approximation is comparable to that seen in the experimentally studied suspension. We interpret the observed acoustic mode as the analog of the slow compressional mode in porous solids having a length scale of order 100 nm.

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