In this paper, the impedance and mobility compact matrix (IMCM) method for prediction of acoustic modal characteristics of two-dimensional irregular cavities with the rigid wall boundary is presented. This method consists of discretizing the whole cavity into a series of subcavities either of regular or irregular shape. Continuity of both pressure and velo- city between adjacent subcavities is ensured using a virtual membrane with zero mass and stiffness. Mathematical formulation for acoustic cavities with the irregular shape has been explained in detail. A finite element model has been developed to calculate the acoustic natural frequency and mode shape. The proposed method is validated using a regular and irregular cavity and compared with finite element modelling results and available results in the literature. © 2021 Polish Society of Theoretical and Allied Mechanics. All rights reserved.