Electret microphones dedicated to consumer electronics (mobile phone) and medical applications (hearing aids) have reached the miniaturization limits. Since the release of the first microphone based on silicon micromachining, electret microphones are constantly replaced by MEMS microphones. However, reduction of the diaphragm surface for MEMS is a fundamental limit to the miniaturization of microphones. The following paper present a novel MEMS microphone architecture that is developed in the frame of the ANR MADNEMS project. It uses micro beams that deflect in the plane of the base wafer. Signal transduction is achieved by piezoresistive nanogauges integrated in the microsystem and attached to the micro beams. Acoustic pressure fluctuations lead to the deflection of the micro beams which produces a stress concentration in the nano gauges. Such architecture enables us to reduce the surface of the deflecting element and leads to a microphone with a smaller footprint that preserves at the same time high performances. Accurate simulations based on the Finite Element Method of the discussed transducer couple acoustic, mechanical and electric behavior of the system. A 3D model of the microsystem would be too expensive in terms of computation time and memory with a poor mesh quality. This type of model is not suitable for the design of the MEMS microphone where numerous geometry parameters have to be revised. That is why a major effort has been devoted to modeling the microsystem by a 2D model. In particular, as the system is not symmetrical due to the deflection of the beams, a mechanical equivalent 2D model was proposed. This paper presents the modeling approach used to taken into account mechanical and thermal boundary condition at the walls, and the phenomena of viscous and thermal diffusion involved in the air. Its implementation in a finite element code allows the numerical calculation of the pressure sensitivity of the microphone. The numerical results were validated by an analytical model. The highlighting of the main physical phenomena in the microsystem, allowed to develop simplified models (Lumped Elements and Low Reduced Frequency) more suitable to the design of MEMS.