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, ${\ensuremath{\omega}}_{\mathrm{SH}}$ and ${\ensuremath{\omega}}_{c}$. Our metamaterial is double negative and transparent for frequencies lower than ${\ensuremath{\omega}}_{\mathrm{SH}}$. For the frequencies ${\ensuremath{\omega}}_{\mathrm{SH}}l\ensuremath{\omega}l{\ensuremath{\omega}}_{c}$, the medium is opaque and only the density is negative. For the frequencies above ${\ensuremath{\omega}}_{c}$, 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.''