Composite Acoustic Medium with Simultaneously Negative Density and Modulus

We fabricated an acoustic composite structure consisting of a periodic array of interspaced membranes and side holes. Experimental data on the transmission, effective density, and phase velocity are presented. The system exhibits two critical frequencies, ₒ₇ and ₂. Our metamaterial is double negative and transparent for frequencies lower than ₒ₇. For the frequencies ₒ₇<<₂, the medium is opaque and only the density is negative. For the frequencies above ₂, the system is double positive and transparent. The present medium exhibits a very wide double negative spectral range that opens the possibility of the application of metamaterials for ``white lights. ''