Experimental demonstration of a space-time-modulated airborne acoustic circulator

Achieving strongly nonreciprocal scattering in compact linear acoustic devices is a challenging task. One possible solution is the use of time-modulated resonators; however, their implementation in the realm of audible airborne acoustics is typically hindered by the difficulty of obtaining large modulation depth and speeds while managing noise issues. Here, we propose a practical and cost-efficient route to realize simple modulated resonators and observe experimentally the strong nonreciprocal behavior of an acoustic circulator. We propose to modulate the neck cross-section areas of three coupled Helmholtz resonators using rotating circular plates actuated by an electrical motor, and control their phase difference via meshed gears, thereby implementing a modulation scheme with broken time-reversal symmetry that effectively imparts angular momentum to the system. We experimentally demonstrate tunable nonreciprocal behavior with a high nonreciprocal isolation of 34 dB and reflection as low as \ensuremath{-}9 dB, with insertion losses of 5 dB and parasitic signals below \ensuremath{-}20 dB. All the experimental results agree well with theoretical and numerical predictions.