The HEXADYM Magnet System

A New Approach to Loudspeaker Magnet System Design

The ceramic magnet system dominates the current loudspeaker market for good reasons. It is economical to manufacture; it is easily produced in different sizes, and its magnetic characteristics are stable. The popularity of this magnet system will probably continue for many years to come, especially in the mass market.

Despite its popularity, however, the ceramic magnet system has a number of serious drawbacks:

• It is both large and heavy, and it reflects a lot of acoustic energy back to the rear of the cone, which is especially troublesome in midrange drivers. In dome tweeters, the large magnet diameter dictates a chassis diameter which is 3 to 5 times larger than the voice coil diameter, often compromising the mounting of the tweeter in relation to the woofer.
• It creates a strong magnetic stray field that distorts the picture on neighboring TV or PC screens. While compensating magnets and shielding cups can be used to reduce the stray field, the extra components are cumbersome to install, add considerably cost, and further worsen the reflection problems in cone drivers.
• It contains a cavity between the magnet and center pole that causes undesirable acoustic resonance's along with pumping effects, especially in drivers using magnetic fluid.

In the late 1970's a new magnet material using Rare Earth Cobalt (REC) was introduced, followed in the mid 1980's by Neodymium Iron Boron (NdFeB). These new materials offered more than 10 times the maximum energy product of ferrite materials. Up to now these materials have mainly been used in small dome tweeters, based on a disc shaped magnet placed in a a soft steel pot with a disc shaped pole piece mounted on the top of the magnet. This type of magnet system is very small and light and uses the magnetic flux very efficiently. Tweeters utilizing this type of magnet system have enjoyed considerable success both in automotive and A/V applications.

Unfortunately, these magnet systems also suffer from considerable drawbacks:

• The cavity between the dome diaphragm and pole piece, as well as the cavity between the magnet and steel pot severely limit the acoustical performance of such tweeters.
• The magnet size is dictated by the voice coil diameter, thus limiting the total available magnetic flux

To overcome the problems in both types of magnet systems mentioned above, SEAS has developed the new HEXADYM magnet system (patent applied for).

The HEXADYM magnet system is based on 6 rectangular-shaped, neodymium magnet blocks which have been radially magnetized. These are affixed to a hexagonal pole piece with an axial pole. Six (6) soft steel plates conduct the flux from the magnets to the top plate, which defines the air gap around the pole. (See the attached drawing.)

The HEXADYM system offers the following advantages over previous designs:

• Unwanted acoustical resonance's and pumping effects are eliminated because there are no internal cavities.
• Simply increasing or decreasing the axial length of the magnet controls the amount of magnetic flux, allowing for a wide range of applications in both cone and dome drive units.
• The stray field is sufficiently low so that A/V applications do not require further shielding.

The cost of the new HEXADYM system is higher than a standard magnet system due to its increased complexity and costlier component parts. However, the improved performance of this system, especially in drive units designed for the high end market, justifies the additional cost.

The first product to use the HEXADYM system will be the new Excel "MILLENNIUM" type T25CF002. This high-performance soft dome tweeter uses the HEXADYM magnet systems's lack of internal cavities to full advantage, resulting in a very linear frequency response and unusually low resonant frequency.