Why do Speakers use Ferro-fluids?

Speakers reproduce sound by moving a diaphragm to displace air. The mechanism resembles a permanent magnet electric motor. The major difference is the voice coil in a speaker moves linearly instead of in a circular motion. As the coil moves back and forth in step with the electrical signals fed to it, it moves the attached diaphragm. To prevent spurious movements and unwanted oscillations of the diaphragm, conventional speakers generally use a damper. To produce sound from such speakers, extra energy is necessary to overcome the resistance of the damper.

Additionally, the damper has its own natural frequency of vibration that restricts the speaker from reproducing sound accurately at all frequencies. A new technique using a magnetic fluid to replace the damper claims to correct this anomaly by reducing energy consumption and allowing louder and clearer sound across the entire range of frequencies the speaker is capable of reproducing. To quantify the advantages, the new speaker reduces energy consumption by 35% for reproducing the same loudness of sound as from conventional speakers and the improvement in sound quality is nearly 3dB.

NASA originally developed the magnetic fluid in the 1960’s, using it for space exploration and called it Ferro-fluid. It responds to applied magnetic fields because the fluid is infused with Nano-sized magnetic particles. They do not agglomerate or cluster together because of a coating of suitable surfactants. The unique characteristic of ferro-fluids makes them useful in a range of applications. Using applied magnetic fields to control flow or movement, ferro-fluids can replace mechanical parts such as vehicle suspensions, flow of fuel in a reactor and more.

In a conventional speaker, the damper holds several components such as the diaphragm and spring in place, even when the speaker is vibrating. However, the damper causes friction while moving, thereby distorting the original sound waves with secondary vibrations, which are manifest as noise. To overcome the friction requires additional energy while driving and that reduces the speaker’s total volume output by a few decibels.

When replacing the damper in a speaker, the ferro-fluid used has a thickness of only a few microns. The magnets of the speaker create a permanent magnetic field to which the ferro-fluid responds by holding the diaphragm and the coil in place while allowing them to move linearly without any friction. As there are no secondary vibrations from the ferro-fluid, the sound is clearer. The lack of friction allows the speaker to save about 35% of the energy as compared to conventional speakers with dampers.

Ferro-fluids used for the audio field are usually based on two classes of carrier liquids – synthetic enters and hydrocarbons. Both oils are low in volatility and high on thermal stability. The environmental considerations dictate the choice of the fluid used, along with the best balance of viscosity values and magnetization for optimizing the acoustical performance.

Using different carrier liquids and by varying the quantity of magnetic material in the ferro-fluid, it can be tailored to meet different needs. The saturation magnetization depends on the nature of the suspended magnetic material and its volumetric loading. Care is taken to use material whose density and viscosity correspond closely to that of the carrier fluid.