Nowadays, a lot of consumer products include microelectromechanical systems (MEMS) in order to answer the constant need to reduce the size of products. For example, cellphones include many MEMS like microphones, accelerometers or gyroscopes. The market is principally focused on pressure sensors and microphones and so, the challenge is to improve the quality while reducing the size of these sensors. When it comes to microphones, the classical way to deal with acoustics, based on the wave propagation equation, is no more accurate because the thermal and viscous effects that appear on the boundaries are not taken into account. This presentation will demonstrate why it is primordial to use the Navier-Stokes equations to accurately describe the phenomena that are appearing when the thermoviscous boundary layers size is of the same magnitude order than the characteristic lengths of such sensors (millimeters). A simple analytical solution will be confronted to the simulation results obtained with COMSOL, with and without taking into account these diffusive effects and, finally, the study of a MEMS microphone will be presented.