In this work, we developed a new class of active acoustic metamaterials (AMMs) with a fully controllable effective density in realtime. The density of the developed AMM can be programmed to any value ranging from -100 kg/m3 to 100 kg/m3 passing by near zero density conditions. This is achievable for any frequency between 500 and 1500 Hz. The material consists of clamped piezoelectric diaphragms with air as the background fluid. The dynamics of the diaphragms are controlled by connecting a closed feedback control loop between the piezoelectric layers of the diaphragm (using one piezoelectric layer as a sensor and the other as an actuator). The density of the material is adjustable through an outer adaptive feedback loop that is implemented by the real-time evaluation of the density using the 4-microphone technique. Applications for the new material include programmable active acoustic filters, nonsymmetric acoustic transmission, and programmable acoustic superlens.