Practical design methods for electrostatic loudspeakers using dc bias are well known. The frequency characteristics and the efficiency of a horn-loaded electret tweeter are presented. This tweeter has a very thin diaphragm (palladium vacuum evaporated on 4 µm thick polyester) between two perforated back plates on which perforated electret films (125 µm thick) are laminated. The tweeter's frequency response depends mainly upon the ratio fo the total cross-sectional area of the holes in the perforated electret to the area of the diaphragm (perforation ratio), and also upon the ratio of the throat area of the horn to the area of the diaphragm (transformation ratio of the horn). The thickness of the electret film that maximizes the driving force can be given by E*g, where E* is the dielectic constant of the electret and g is the the length of the air gap. Regarding this characteristic of dependence, the results of analysis and experiments are given to clarify the various characteristics of a horn-loaded electret tweeter.
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