There are three important physical properties of ferrofluid that must be understood to successfully design fluid-filled audio speakers and select the best fluid: saturation magnetization, viscosity and volatility.

Saturation Magnetization

Saturation mag-netization is determined by the nature of the suspended magnetic material and the volumetric loading of this material. The greater the quantity of magnetic material in suspension, the higher the saturation magnetization of the ferrofluid. Audio ferrofluids are manufactured with a range of saturation magnetization values from 75 to 400 gauss (7.5-40 mT). Compared with iron which has a saturation magnetization of 17,000 gauss (1.7 T) ferrofluids are weak magnetic materials.

Viscosity

Viscosity is a measure of resistance to flow. It is defined as a ratio of viscous shear stress to shear rate and is measured in centipoise (cp) or Pa s. Audio ferrofluids are manufactured with a range of viscosity values from 25-10,000 cp (25-10,000 mPa s) (measured at 27°C). The most commonly used values fall within a much narrower range, typically from 100-2,000 cp (100-2,000 mPa s).

Volatility

Several factors influence the volatility of ferrofluid in a loudspeaker:

• Evaporation rate of the carrier: Evaporation Rate: expressed as % weight loss or loss of material (in grams) from a surface area of one cm2 in one second. At 175°C, the evaporation rates of audio ferrofluids range from 1.0 to 8.5 x 10-7 gcm-2s-1.
• Average ferrofluid temperature: A temperature gradient exists across the ferrofluid in an air gap such that the fluid is warmest next to the coil and coolest next to the top plate and pole. A loudspeaker with a coil temperature of 150°C and top plate/pole temperature of 80°C suggests an average ferrofluid temperature of 115°C.
• Exposed surface and ferrofluid quantity: These factors are defined by the physical dimensions of the air gap.