The sound generating part of the headset is called the unit. The headset has four common units: 1. Moving iron unit. Moving iron unit is also called balanced electric drive unit by manufacturers. The structure is very simple. The coil is wound around the balance armature located in the center of the permanent magnetic field, the changing current magnetizes the armature, and the magnetic field generated by the permanent magnet acts on the magnetically changing armature, resulting in vibration.