An implantable device includes a first sealed flexible shell configured for implantation within a breast of a human subject, an elastic filler material contained within the first sealed flexible shell, and a second sealed flexible, inelastic shell, which is disposed within the elastic filler material inside the first sealed flexible shell and is inflated with a volume of gas. The second shell includes a material selected such that a root-mean- square (RMS) sound pressure caused by a deformation of the second shell not exceeding 50 mm does not exceed 20 micro- Pascals.
