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.