Reversed Doppler effect in double negative metamaterials

Doppler shifts in double negative metamaterials have never been observed. This Rapid Communication presents experimental results on Doppler effect in a double negative acoustic metamaterial. We observed that frequency was downshifted when the source was approaching and upshifted when receding. Notably, while in ordinary media wavelengths corresponding to downshifted frequencies are longer, we demonstrate that in double negative metamaterials wavelengths increase as the frequencies increase. Consequently even though the frequencies were downshifted in front of the moving source, the wavelengths became shorter.