Planar magnetic transducer is commonly known as planar diaphragm unit (also known as field pole type) or quasi ribbon unit. If the horn is made of this unit, it is called planar diaphragm horn. As early as 1920's, the planar diaphragm horn was developed simultaneously with the moving coil unit. However, due to technical limitations, it was not commercialized until after 1970's. The planar diaphragm unit is driven by magnetic force just like the moving coil unit, but its voice coil is not rolled into a circle like the moving coil unit, but scattered evenly on a planar diaphragm. The flat diaphragm unit can be used as a treble unit, a midrange unit, or a bass unit.
　　Some flat diaphragm horn is a whole horn, which is a plastic diaphragm (Mylar) stretched into a plane, and a conductor (usually aluminum foil, relatively light) is attached to the diaphragm, just like adding an ultra-thin gland ring to the flat diaphragm. After the current flows into the conductor, the aluminum foil attached to the plane diaphragm interacts with the permanent magnet to produce a magnetic field, which makes the whole diaphragm be pushed forward and pulled back, thus making a sound. Some flat diaphragm loudspeakers use flat diaphragm for high and middle tones and cone basin moving coil unit for low tones. Why design like this? Because if the plane diaphragm is used as the bass unit, the diaphragm area potential must be very large, and the low-frequency sense will be enough, so the cone basin moving coil unit is used instead.
　　The biggest difference between planar diaphragm horn and electrostatic horn is that the planar diaphragm horn is driven by magnetic force, while the electrostatic horn does not use magnet, and the diaphragm moves completely by the high-voltage repulsion effect (electric field effect).