Ferrofluid is roughly 5 times more thermally conductive than the air it displaces from the gap.The fluid provides a much lower thermal resistance between the coil and pole/top plate, lowering the voice coil operating temperature under both transient and steady state conditions. This increases power handling capabilities.
Ferrofluid in the gap provides a mechanical resistance to the moving coil. The amount of damping is proportional to the viscosity of the ferrofluid.
When the voice coil is displaced in the radial direction in the gap, a restoring force is obtained which is proportional to the displacement. Although this force is a fraction of that provided by the suspension, it is still enough to influence the centering of the moving coil. This force constant is given by:
k = π Ms Hm h t r-1 [N/m]
Ms = saturation magnetization in tesla
Hm = maximum field strength in the gap in ampere meter
h = height of fluid in the gap in meter
t = width of the gap in meter
r = radius of the gap in meter
Harmonic distortion and spectral contamination caused by radial and rocking modes of the voice coil is reduced due to the centering force of the fluid upon the voice coil. Ferrofluid in the gap also creates a seal, or liquid "O" ring around the coil which eliminates air modulation noise in the gap, particularly within the piston band.
Minimizing the temperature rise of the voice coil reduces thermal power compression effects. Ferrofluid in the gap not only reduces sensitivity loss but maintains the linearity of the speaker's output.
Due to the centering and lubricating properties of the ferrofluid, manufacturers have reported improvements in production yields ranging from 30%-60% when introducing ferrofluids into existing products. This reduction in scrap can often offset the cost of the ferrofluid itself.
Ferrofluid's ability to control a driver's behavior at resonance and, to some extent, break-up modes at the top end of the pass band, eliminate the need to address these problems in the crossover network, eliminating the need for additional expensive resistors, caps and inductors.
A 1 in (25.4 mm) voice coil driver with ferrofluid can achieve the same power handling as equivalent sized drivers which utilize 38.1 mm (1.5 in) or 50.8 mm (2 in) diameter coils. The cost savings from the smaller magnet/coil more than offset the ferrofluid cost. The weight reduction may also be attractive in many applications.