Selecting the Correct Audio Fluid

Several factors must be considered when selecting a ferrofluid for a particular design. The following are some basic guidelines to follow during the selection process:

Magnetization (Ms)

The magnetization value of the ferrofluid should be balanced against the loudspeaker’s air gap flux density and voice coil excursion. This is generally dependent on the application. The following table offers general guidelines.

Ms value (mT)

Typical applications

11 - 22

Tweeter, headphone, high frequency compression driver, multimedia

22 - 33

Midrange, full range, midrange compression driver, siren driver

33 - 44

Woofer, subwoofer, siren driver

Viscosity

The viscosity value of the ferrofluid should be selected with respect to the desired amount of damping at the speaker’s resonant frequency. Typically, values of less than 200 mPa·s provide light damping, values in the 200 – 1000 mPa·s range provide moderate damping and values over 1000 mPa·s provide heavy damping.

Carrier type

Synthetic ester oils typically provide higher thermal stability than synthetic hydrocarbon oils. For this reason, APG O and APG S series fluids should be considered for extreme high temperature applications. Ester oils are also attractive due to their low viscosity. Where temperature requirements are not demanding, the low viscosity APG J series can enhance the performance of headphones and multimedia loudspeakers. If exposure to strong magnetic fields or magnetic field gradients is causing a leakage or separation of ferrofluid or one of its components from the air gap, APG L series fluids should be chosen as the high colloid stability should minimize or eliminate such problems.

There are times when ester oil based ferrofluids cannot be used due to incompatibilities with adhesives and other materials. In such cases, synthetic hydrocarbon based ferrofluids such as the APG E and APG 300 series are effective alternatives.

Special Design Considerations

Material compatibility - adhesives

As previously mentioned, synthetic ester based ferrofluids may not be compatible with certain types of adhesives, particularly inexpensive, rubber-based materials. Ferrotec offers an adhesive compatibility testing service to test the compatibility of your candidate adhesive/ferrofluid system under a variety of conditions. Contact Ferrotec or your local agent for more details.

Material compatibility - voice coil

The voice coil bobbin material selected for use with ferrofluid should be a non-absorbent material such as aluminum, Kapton or certain coated papers. If a reinforcing collar is to be used in the coil design, it must also be made of a non-absorbent material or care should be taken to ensure that the collar material does not come into direct contact with the ferrofluid.

Spider removal - voice coil centering with ferrofluid

The presence of ferrofluid exerts a uniform radial centering force upon the voice coil in the air gap. The magnitude of this force is dependent upon the strength of the permanent magnetic field and saturation magnetization of the ferrofluid. The greater these values, the stronger the centering force. Reduced scrap rates on the production line, reduced field returns and reduced distortion (due to the suppression of radial and rocking modes of the voice coil) are some of the well-known benefits of this centering force.

In recent years, the availability of high saturation magnetization ferrofluids have allowed designers to completely remove the spider, relying on the ferrofluid to center the voice coil in the air gap. This technique not only yields lower cost and a simplified production process; it also removes the well-documented nonlinearities present in all spiders, resulting in lower distortion. In general terms, the saturation magnetization values of ferrofluids used in spiderless designs should not be lower than 33 mT.

Micro speakers used in computers and personal audio devices have also taken advantage of ferrofluid’s centering force. Several manufacturers of micro speakers, who had experienced difficulty properly seating the spider to the basket on the production line, have removed the spider from the design and depend on ferrofluid to center the voice coil. This approach has resulted in dramatic scrap reduction, reduced warranty returns and improved performance.

Cavity venting – woofers and subwoofers

As the loudspeaker's voice coil excursion increases, there are more forces present which can cause the ferrofluid to “splash” from the magnet system’s air gap. Increasing the saturation magnetization of the ferrofluid can often solve this problem. However, if high saturation magnetization fluids are used and splashing still persists, physical modifications to the magnet system and other components should be implemented to create pressure relief under the speaker’s dust cap/diaphragm and inside the magnet itself. It is the presence of pressure build-up in these cavities, which can force the ferrofluid from the air gap so vent holes may be added to provide pressure relief. The size, number and location of vent holes depends on the individual design and some trial and error may be required to arrive at the optimal venting configuration. See the panel to the right for a selection of common venting configurations.

Pressure relief under the dust cap/diaphragm.

Pole piece vents have been commonly used as an effective means of cooling. They rely on the coil and diaphragm’s piston motion to dissipate heat away from the voice coil and magnet steel. The addition of a pole vent is also the most common method for relieving pressure buildup under a loudspeaker’s dust cap or diaphragm.

Alternatives to venting the pole piece include vent holes in the voice coil, vent holes in the cone body or a vented dust cap. Depending on the air permeability of the spider, venting the voice coil may require additional venting of the spider or inclusion of vent holes in the basket below the spider plateau.

Pressure relief in the magnet system.

If a pole vent is present, a radially oriented vent hole through the pole piece can offer adequate pressure relief. Alternatives to radial pole vents include vent holes in the magnet’s back plate or vent holes in the magnet’s top plate.