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.

Figure 1

(Color online) (a) Structure of the present DNG metamaterial consisting of interspaced membranes and side holes. This structure exhibits negative phase velocity in the frequency range below 440 Hz. (b) Dispersion curves from Eq. (1); wave vector (solid line) and wavelength (dashed line) as functions of frequency.

Figure 2

(Color online) (a) Experimental setup for Doppler effect. (b) Recorded signal from the stationary detector in the metamaterial. In the case of the approaching source, the detected frequency was 342 Hz, which was 8 Hz downshifted from the source frequency. For the case of receding source, the detected frequency was 358 Hz.

Figure 3

(Color online) Theoretical curve from Eq. (3) agrees with the experimental data. Dashed line is the Doppler shifts for plain air, which is positive and nondispersive for all frequencies.