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Equivalent circuit-based analytical open-circuit sensitivity modelling of a capacitive-type MEMS acoustic sensor for Internet of things (IoT) application is presented. It can not only evaluate simply the characteristic of the sensitivity on wafer level, but also improve the accuracy of the sensitivity due to including the fringing field between the diaphragm and each etching hole in the back-plate. The effective capacitor model is obtained by applying the approximately linearized electric-field method (ALEM), resulting in the equivalent circuit-based dynamic model. From the sensor with a 325 µm-radius diaphragm, the effective radius and the effective residual stress of the diaphragm were extracted to be 299 µm and +23.0 MPa, respectively. With the pull-in voltage of. 12.0 V and the pad capacitance of 0.23 pF; the open-circuit sensitivity was modelled to 11.3 mV/Pa at 1 kHz in the bias of 10 V.