The heart of the MEMS microphone is the acoustic sensor. The acoustic sensor uses a fixed plate and a moveable diaphragm to detect sound waves. Sound waves cause a change in pressure which causes the diaphragm to move, which in turn causes the capacitance between the diaphragm and the fixed plate to change. This change in capacitance is then measured and converted into an electrical signal by a CMOS interface IC. MEMS is short for Micro Electro Mechanical Systems; they are devices that are constructed of silicon, using processes similar to those used for fabricating ICs. The difference in fabricating MEMS is that actual physical structures are created in the MEMS devices instead of just producing a flat piece of silicon as with ICs. In acoustic sensors, there are three main parts. One is the base, which is the silicon die that the microphone is built on; on top of that is a flexible membrane, and a fixed plate. The microphone measures the changing capacitance between the fixed plate and the movable membrane when the membrane moves in response to the sound waves.