FerroFluid is used in high and mid frequencey speakers for increased power handling. When Ferrofluid is added to the magnetic gap it enables the transducer to produce high peak output levels with minimal power compression. It is the brown substance found in the voice coil gap and needs to be renewed after cleaning the gap before the new diaphragm insertion. "Ferrofluid is a stable colloidal suspension of sub-domain magnetic particles in a liquid carrier. The particles, which have an average size of about 100 (10 nm), are coated with a stabilizing dispersing agent (surfactant) which prevents particle agglomeration even when a strong magnetic field gradient is applied to the ferrofluid. In a gradient field the whole fluid responds as a homogeneous magnetic liquid which moves to the region of highest flux. This means that ferrofluids can be precisely positioned and controlled by an external magnetic field. The forces holding the magnetic fluid in place are proportional to the gradient of the external field and the magnetization value of the fluid."