Damping induced by ferrofluid seals in ironless loudspeaker

• Ferrofluid seal damping in unsteady and not spatially constant magnetic field.
• A model determines the viscous damping coefficient induced by a ferrofluid seal.
• Ferrofluid seal damping coefficient as function of this geometry and local viscosity.
• The higher the magnetic saturation, the higher the damping.

Damping induced by ferrofluid seals in ironless loudspeakers is investigated in this paper. The magnetic field is steady but not spatially constant. A model to determine the viscous damping coefficient induced by the ferrofluid seal is derived. It is a function of geometrical parameters and local viscosity of the ferrofluid in which dependence from magnetic field, shear rate and frequency is accounted for. Comparison with experimental results shows a good agreement for the thinner seals. An overestimation of the damping is observed for higher volumes. This discrepancy comes from geometric irregularities of the magnet assembly made out of several tiles.