Increased Thermal Power Handling
Ferrofluid is roughly five times more thermally
conductive than the air it displaces from the voice coil
gap. Ferrofluid added to a loudspeaker's voice
coil gap 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.
Damping
Ferrofluid in the gap provides a mechanical resistance
to the moving voice coil. The amount of damping is
proportional to the viscosity of the ferrofluid.
Voice Coil Centering
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 = 2Ms Hm
ht/r 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
Reduced Distortion
Harmonic distortion and spectral contamination caused by
radial and rocking modes of the voice coil is reduced
due to the centering force of the ferrofluid 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.
Reduced Power Compression / Improved Dynamic
Linearity
Minimizing the temperature rise of the voice coil
reduces thermal power compression effects.
Ferrofluid in the voice coil gap not only reduces
sensitivity loss but maintains the linearity of the
speaker’s output.
The Catch?
Ferrofluid, especially old,
early generation ferrofluid, tends to thicken and
the liquid component of it dissipates over the
decades. Many, if not most loudspeaker drivers
which employ ferrofluid will need servicing in 20-25
years and beyond - some more, some less.
The dissipation of ferrofluid's base-carrier
synthetic ester or synthetic hydrocarbon leaves
behind a thick layer of metallic-particles, which
either reduces the output volume of loudspeaker
drivers or seizes the voice coil / diaphragm
completely within the magnetic assembly's voice coil
gap. Hence the need for driver disassembly, cleaning and ferrofluid replenishment.
Do not add ferrofluid to the gap of an audio driver which did not ship from the manufacturer with ferrofluid already added to the gap! Two critical issues may arise: 1) the manufacturer may have used an absorbent product to construct the former that supports the voice coil winding. A soggy former will become misshapen and be the death of your driver! 2) The glue that was used to bond the voice coil windings to the former may feature a chemical composition that reacts with the ferrofluid. Again, this may precipitate a bad outcome for the driver...