Fostex’s RP technology, which has undergone continuous improvement over 50 years, features a full-surface driven planar diaphragm driver. In the fourth generation, the RP driver enhances the vibrational area and uniformity of the planar diaphragm by increasing the magnet size that sandwiches the diaphragm and redesigning the pattern shape of the printed coil. Additionally, the magnetic circuit components have been revamped to optimize magnetic flux distribution, suppressing unwanted diaphragm resonances and achieving sharp response for both sound onset and decay. Leveraging various accumulated technologies, the fourth-generation driver simultaneously improves sensitivity, offers smooth frequency characteristics suitable for monitoring, and delivers excellent transient response. From deep bass to high frequencies, it reproduces sound accurately and delicately, enhancing precise localization and soundstage reproduction beyond previous versions.