Experimental demonstration of scalable active metamaterials with interacting cells of tunable bulk modulus

Abstract

Active metamaterials can be designed to exhibit acoustic properties that are challenging to obtain using passive structures without relying on narrowband resonant behavior. Arrangements of unit cells that independently sense the local acoustic conditions and produce a coherent response, which is determined by the gain of electronics between the sensor and driver, act as a material with programmable properties. Such an active metamaterial has been previously demonstrated, but only for a single or a few non-interacting cells due to a limited understanding of how the programmed gains relate to the effective properties. Recently we developed a polarized source model to address this challenge, and based on that work we now demonstrate experimentally an active metamaterial composed of interacting cells with programmable bulk modulus. Each unit cell consists of a speaker and microphone mounted in the bottom plate of a 2D waveguide, with electronics of adjustable gain stored underneath. The field generated by a 2 × 3 arrangement of cells in response to an incident pulse was compared to the simulated scattered field of the equivalent continuous material. By tuning the gain, we accurately reproduced the behavior of a medium with negative, fractional, and large relative bulk modulus.