Air inlets coupled with mechanically controlled ventilation systems are widely used to renew polluted indoor air. They are generally positionned at the top of windows inducing a reduction of the sound insulation. As the harmful consequences of noisy environments on human health are more and more highlighted, it becomes necessary to maximize the sound insulation while ensuring a sufficient fresh air flow. To this end, efficient numerical simulation appears to be a promising way for studying air inlet acoustic behavior: many parameters can be considered without the need to carry out costly and time-consuming laboratory tests. This work aims at developing a reliable numerical model reproducing the acoustic laboratories used by manufacturers for the measurements of air inlet sound reduction index. The low frequency range is studied in the present paper as the measurement uncertainties arising from experimental conditions are the greatest at these frequencies. In order to make the calculations computationally efficient, the proposed model uses a sub-structuring approach called the Patch Transfer Function (PTF) method, and combines analytical or numerical solutions, depending on the subdomain considered. Each subsystem of complex geometries is discretized using finite elements. Conversely, the PTF of simple geometries are analytically computed from a modal decomposition, enriched with a quasi-static correction numerically computed. As empirical improvements pointed out the benefits of the addition of melamine foams, porous material modeling is introduced in the study based on an equivalent fluid model.