A typical condenser acoustic sensor includes a stretched metal diaphragm that forms one plate of a capacitor. A metal disk placed close to the diaphragm acts as a back-plate. When a sound field excites the diaphragm, the capacitance between the two plates varies according to the variation in the sound pressure. A stable DC voltage is applied to the plates through a high resistance to keep electrical charges on the plate. The change in the capacitance generates an AC output proportional to the sound pressure. In order to convert ultra-low-frequency pressure variations, a high-frequency voltage (carrier) is applied across the plates. Finally The output signal is the modulated carrier.