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New Ways to Design Controlled-Directivity Loudspeakers Using Electrothermomechanical Film

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Loudspeakers with a large radiating area have traditionally been constructed as dipole loudspeakers in which the radiating surface has been a membrane possibly divided into segments corresponding to different frequency ranges. Using a recently introduced transducer material, electro-thermo mechanical film (ETMF), that can be used as an electrostatic transducer that changes its volume with the applied electrical field (the film contains small flat air or gas bubbles), radiating surfaces that do not behave as gradient sources can be constructed. Since practically no transverse waves propagate in the film, the film behaves as an ideal piston source and due to the flexibility of the film curved radiator surfaces that are very difficult to construct using conventional techniques can be made. This allows controlling the radiation pattern of a loudspeaker very simply by varying the geometrical shape of the speaker according to the application. Prototype loudspeakers have been constructed both as flat panels and as surfaces with varying radii of curvature demonstrating that the loudspeakers behave as excepted. These construction principles can be applied to wide-band loudspeakers as well as to midrange and tweeter units. The paper also describes computational methods of predicting the behavior of the loudspeakers (frequency response and radiation pattern) and compares the measured data against computed results.

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