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Sound beyond the speed of light: Measurement of negative group velocity in an acoustic loop filter

The authors describe the experimental observation of negative group velocity propagation of sound waves through an asymmetric loop filter. The characteristics of the filter are established using impulse response and direct tunneling of narrow bandwidth Gaussian pulses. The results confirm recent theoretical predictions that faster-than-light group velocity propagation of sound is possible. Further, the results show that the spectral rephasing achieved in a loop filter is sufficient to produce negative group velocities independent of the phase velocity of the spectral components themselves. Thus, superluminal propagation is realized despite almost six orders of magnitude difference between the speeds of sound and light.