Electrostatic airborne ultrasonic transducers with an air-gap structure are widely used in automatic ranging, acoustic imaging, NDE, etc., so that there is an increasing need to develop a reliable model for their optimal design. A Helmholtz resonator model and a plate-under-tension model were presented successively in recent years, but both are still not very satisfactory. In this paper, an electrostatic transducer is treated as a thin plate supported by an air spring, in other words, its diaphragm is viewed as a plate situated on an air cushion. An interesting point revealed by this model is that its fundamental frequency stems from the air-gap spring, and the higher resonant frequencies are caused by the combined contributions of the bending stiffness of the diaphragm, in-plane tensile forces applied to the diaphragm, and the air-gap stiffness. The model is applied to predict the natural frequency of a typical V-grooved transducer with a 8-μm-thick Mylar diaphragm and a 0.5-mm pitch air gap. The first resonant frequency predicted by this model is 50.8988 kHz, and its nominal measured values are 51 kHz and 52 kHz, respectively, as reported by some researchers. [Work supported by the UK Royal Society.]