Proper selection of a ferrofluid requires a careful balance of the properties of the ferrofluid vs. the properties of the loudspeaker. The five most important factors to consider are quantity, viscosity, magnetization, compatibility and volatility.

Quantity

The optimum quantity of ferrofluid for a loudspeaker is determined by the physical dimensions of the air gap and can be calculated with the use of the following formulae:

English V [µl] = 56500 A [E2+C2-B2-D2] (where A, B, C, D in inches) Metric V [µl] = 3.5 A [E2+C2-B2-D2] (where A, B, C, D in mm) where:  A= Top plate thickness  B= Radius of pole  C= Inner radius of voice coil  D= Outer radius of voice coil  E= Inner radius of top plate

The recommended tolerance on ferrofluid quantity is ±10% which is best maintained through the use of a positive volume displacement dispenser. Maintaining the proper ferrofluid quantity is critical as overfilling the gap is wasteful and can lead to leakage while underfilling the gap minimizes the heat transfer benefits of the ferrofluid, compromises the fluid's long term reliability and can lead to response anomalies in the loudspeaker.

Viscosity

Once the ferrofluid amount has been determined, the viscosity of the ferrofluid should then be selected with respect to the desired amount of damping.

Magnetization

The magnetization value of the ferrofluid should be balanced against the loudspeakers air gap flux density and voice coil excursion.Tweeters or compression drivers having high air gap flux and minimal coil excursion require ferrofluid magnetization values in the 100-200 gauss (10-20 mT) range. Woofers, on the other hand, typically have much lower air gap flux and much greater coil excursion and require ferrofluid magnetization values in the 300-400 gauss (30-40 mT) range. High gauss ferrofluids generally have a shorter life at high temperatures. To ensure long term reliability, it is therefore recommended that only as much magnetic material as needed is used.

Compatibility

Successful ferrofluid application engineering requires that the ferrofluid be fully compatible with all the materials and adhesives which have the potential to come into contact with the ferrofluid including:

• Voice coil bobbin/former material: The critical property here is that the material be non-absorbent which eliminates kraft papers and uncoated Nomex^® materials. Compatible materials include aluminum, and Kapton^®.
• Collar material: Again, the critical property here is that the material be non-absorbent. Coated Nomex and Kapton have all been used successfully in this application.
• Voice coil wire adhesive: All popular methods (wet winding, self bonding) and related adhesives are compatible with ferrofluids and no incompatibilities are known to have been encountered. New techniques or adhesives should be analyzed by Ferrofluidics Corporation to ensure their compatibility with ferrofluid.
• Adhesive joints: The glue joint at the bobbin/diaphragm or bobbin/cone/spider junction as well as the magnet system structural adhesive must be compatible with ferrofluid. Commonly used acrylics, epoxies and cyanacrylates are compatible with virtually all hydrocarbon based fluids and most ester based fluids. Ester based fluids should not be used in conjunction with rubber based adhesives. When introducing ferrofluid into a design, or in changing the adhesives used, compatibility should be fully tested.

Avoiding fluid migration

Care should be taken to ensure that there are no capillary paths present by which the ferrofluid could migrate from the gap. The most common are:

• Machining grooves in the magnet top plate: If these grooves terminate in the air gap, ferrofluid can travel along the grooves via capillary action.
• Cotton impregnated lead wires: The cotton can absorb the ferrofluid if they come into contact with each other. This type of material should be wax coated if used in conjunction with ferrofluid.
• Lead wire migration: Ferrofluid can travel between the collar and former along the lead wires via capillary action. This pathway can often be successfully sealed using a wicking adhesive (such as Loctite 290) or a gel type adhesive (Hernon's Quantum 134) which is draped over the lead wires before the collar is applied.
• Bobbin slit migration: If the gap between the two ends of the voice coil former is too narrow (< 0.001" / 0.025 mm) ferrofluid can travel up the slit via capillary action.

Volatility

The thermal capacity of the ferrofluid should be chosen with respect to the operating temperatures of the voice coil. Moderate to low temperature operation requires our more standard products such as the APG 800 or APG 900 series Ferrofluids (hydrocarbon based materials) while more demanding high temperature applications should utilize the ester based APG "O" or "S" series or the hydrocarbon based APG 2100 series.