Membrane absorbers, which are commonly used types of resonance absorbers, convert sound energy into thermal energy by exploiting vibrations that occur on oscillating panels. Those types of absorbers are generally tuned to be efficient in a very narrow frequency range. However, if plates are used as the front panel, it is possible to broaden the effective absorbing frequency bandwidth by exciting multiple modes. This study is devoted to investigating the multimodal behaviour of plate absorbers with closed back volumes in low-frequency range. In the numerical part of the study, the box-shaped plate absorbers are modelled using a combined BEM/FEM approach. The results are validated with measurements conducted in an anechoic chamber. In addition, the sound absorption characteristics of the plate absorbers are obtained from an experimental study conducted in a reverberation chamber. The obtained results reveal that the plate absorbers can be effective in wider frequency ranges.