Oppo claims the PM-3s are the world's lightest closed-back headphones to use planar magnetic drivers.
Just to recap, instead of having a magnet attracting and repelling a voice coil and moving a cone, a planar magnetic driver has a thin diaphragm, held between magnets. This diaphragm is printed with semiconductors, and when pushed and pulled by the magnets, the whole diaphragm moves with its surface evenly driven.
The idea is to offer greater accuracy and stability, and cut down on distortion. The Classics use a brand-new 55mm circular driver which is tuned "to have deep, tight bass and a touch of excitement".