Experimental demonstration of a space-time modulated airborne acoustic circulator

TL;DR

This paper demonstrates a practical airborne acoustic circulator that uses space-time modulation of resonators to achieve strong nonreciprocal sound transmission, with high isolation and low reflection, validated by experiments.

Contribution

It introduces a cost-effective, mechanically modulated resonator design for nonreciprocal acoustics, experimentally demonstrating tunable nonreciprocal behavior in airborne sound.

Findings

Achieved 34 dB nonreciprocal isolation

Reflections as low as -9 dB

Insertion losses of 5 dB

Abstract

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 to obtain 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 -9 dB, with insertion losses of 5 dB and parasitic signals below -20 dB. All the experimental results agree well with theoretical and numerical predictions.