A novel flexible planar loudspeaker with coils on polyimide diaphragm

This paper presented a low cost flexible electro-dynamic planar loudspeaker (FEPL) with coils on polyimide diaphragm. The structure was very simple to put a flexible thin film diaphragm (electroplated traces of copper coil) above a flexible magnetic (placed in the bottom of cavity), thus forming an integration of electromagnetic actuation and planar flexible structure. The advantages are as follows: (1) the new loudspeaker became a flat flexible one, which could not only directly drive the vibration film to increase the power efficiency, but reduce the total harmonic distortion (THD). (2) It could be used as flexible electronics and easily deployed on the surface of any object with magnetic property or using adhesive transfer tapes. To optimize the performance of sound pressure level (SPLs), this study made detailed analyses on the thickness of cavity, turns of coil, magnet polarization, and diaphragm dimension. Noted as the cavity height increasing, the magnetic flux densities of both vertically and horizontally polarized magnet were decreased. But they were nearly constant over the surface with the same height for the vertically polarized magnet. On the other hand, as the cavity height of the horizontally polarized magnet increased, the magnetic flux densities would be decreased rapidly at the locations moving farther away from the center point. Several kinds of planar-type loudspeakers for the coils wounded in spiral-in, spiral-in and spiral-out, and zigzag shapes and plated with copper and aluminum were applied for performance tests, and the comparisons with the traditional cone-type loudspeaker were also made. Finally, the sound pressure levels (SPLs) were compared for three rectangular coils with different dimensions and number of turns. These effects were not discussed in previous literature of study.