Low-frequency electroacoustic absorbers have recently been developed as a solution for the modal equalisation. Firstly investigated in waveguides, the technique consists in matching the acoustic impedance at a closed-box loudspeaker diaphragm to the characteristic acoustic impedance of air. Extending the results in a duct to rooms brings up several challenges. Some parameters, such as the position and orientation of absorbers, the total area, as well as the acoustic impedance achieved at the diaphragms may influence the performance, especially in terms of modal decay time reduction. In this paper, the optimal values of a purely resistive acoustic impedance at an absorber diaphragm, whose area varies, are first investigated under normal incidence and grazing incidence in a finite-length waveguide. The optimal acoustic resistance values are then investigated for a given position, orientation, and total area of absorbers in rooms of different size. From these results, the target acoustic impedances with multiple degrees of freedom are defined with a view to assign to the absorber diaphragms. These impedances are then optimised from a global criterion, so that these impedances approach at best the different optimal resistance values found to minimise the modal decay times. Finally, an experimental evaluation of the performance of the electroacoustic absorber in a waveguide is provided.