An acoustic metamaterial with unusual wave properties

Several structural variants of a new acoustic metamaterial are proposed with unusual wave properties, and normal wave dispersion is studied. One of the variants is a bandpass filter in whose passband the phase velocity of a normal wave is negative. In another variant, there are two passbands: in the first, the phase velocity is positive; in the second, negative. As a result, unusual physical phenomena can be observed in it, in particular, a special type of birefringence and the combined Doppler effect. In the long-wavelength range, where the metamaterial behaves like a continuous medium, the corresponding equations have been derived in partial derivatives. Essential differences from the common wave equation are noted.