This shows that the Helmholtz resonator model is adapted to the behaviour of the polished backplates, whereas the membrane model explains the vibration over the grooves. The equation of the vibration of a membrane submitted to a mechanical stress, but neglecting the effects of its elasticity, was also compared with experimental results. These results are compared with theoretical predictions using the Helmholtz resonator model. Experimental investigations have been performed in order to study the influence of the groove widths, their layout and the value of DC bias on the transducer behaviour. Some backplates have been subjected to coarse polishing, whereas others were first polished and then rectangular grooves were machined. Several electrostatic transducers with metallic backplates have been manufactured.
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