Semi-anechoic rooms are used for the acoustic characterization of noise sources. They involve heavy infrastructures and thick absorbent lining. The purpose of the ongoing project "DADA" (Dôme Anti-Diffraction Acoustique), is to demonstrate a lower-cost alternative by supplementing a thin passive lining with active technology. This project targets an actual size functional demonstrator, an important milestone before tackling a fully active anechoic room at the Laboratory of Mechanics and Acoustics (LMA). Previous work achieved the active reduction of the pressure scattered by the reflective wall of a semi-anechoic room, in the 80 − 200 Hz frequency band. These results validated an innovative approach allowing to control the scattered pressure throughout the measurement volume, using a setup located over its periphery. The DADA project extends these results by dealing with the five non-reflecting walls of a semi-anechoic room. Analytical and FEM simulations are described and discussed. They allow a parametric study of many design aspects, including the structure of the active control system and transducers locations. The resulting design will be presented, together with simulations able to deal with test sources of increasing complexity.