Electroacoustic metamaterials: achieving negative acoustic properties with shunt loudspeakers

Acoustic metamaterials constitute a new class of structures that exhibit acoustic properties not readily available in nature. These properties can be a negative mass density, expressing the opposition of the acceleration of a particle to the application of pressure, or a negative bulk modulus, signifying the rarefaction of the particle in reaction to a compression (resp. a condensation in reaction to a depression). However, these artificial behaviors result from a periodic arrangement of passive unit-cells (such as membranes and side holes), and not from individual "meta-properties" of each unit-cell. It is however possible to observe such intrinsic metamaterial properties out of a passive electroacoustic resonator. This concept encompasses a loudspeaker, connected to a specific electric load, thus altering the acoustic dynamics of the loudspeaker diaphragm when subject to an exogenous sound source. It is especially possible to achieve negative acoustic impedance at its diaphragm thanks to the connection of passive electric shunt circuits, such as simple RLC series resonators. This paper aims at highlighting the metamaterial nature of such electroacoustic resonators through computational and experimental results, followed by discussions on ongoing developments.