US Patent 7729504 Ferrofluid centered voice coil speaker

An audio speaker has a driver unit having a support frame with a central portion forming a magnetic structure defining an annular gap around a central magnetic post, a vibration system having a diaphragm and a voice coil, the voice coil is attached to one side of the diaphragm where the vibration system is fixed to the support frame and where the voice coil is movably mounted in the annular gap, and a magnetic fluid disposed in the annular gap only in a space between one side of the voice coil and a surface of the annular gap having a higher magnetic flux density.