In the automotive industry, acoustic insulation of the engine compartment is one of the key points for the NVH comfort of vehicles. Grommets of the firewall are an important path of noise from engine to passengers' cabin. Acoustic requirements defined by automotive manufacturers for firewall insulating materials and for grommets are more and more constraining. One of the main indicators used by OEMs to determine the acoustic performance of grommets is the Insertion Loss (IL). It is the difference of the sound pressures between two configurations: a reference and the configuration with the part to be assessed. One way to measure the IL of a part is the use of a small cabin. It consists in a dedicated bench having one emitting room, one receiving room and the tested part at the interface. Suppliers may need to test several prototypes to meet the OEMs' specifications. In this context, the numerical simulation of the small cabin can provide a helpful support in the design phase of the grommets. The finite element software Code_Aster is used in this study. Code_Aster is an open source software developed by EDF (French Electric Company). The open source software Salome is used for the pre-processing (geometry and mesh) and post-processing. A parametric model is developed in order to easily modify the geometry, the mesh or the input data (material properties, frequency range, boundary conditions, ...). The objective is to compare IL computed with different models. Correlation between experiments in small cabin and simulations is achieved up to 10 KHz. It demonstrates equivalent trends of the IL spectra and levels. Numerical results show that the damping factor and the stiffness of materials deeply modify the IL. Several enhancements could still improve the numerical model. Perspectives of the work are to use frequency dependent materials properties as inputs and to take into account acoustic leaks in the simulation.