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Overhung vs. underhung
Vented vs. unvented
Phase plugs
Cooling
Pole piece shapes
Magnetic effects
Magnetic shielding

Driver guidelines - Motors:

1. Overhung or Underhung. This relates to the length of the VC to the height of the magnetic gap. If the coil is longer than the gap, the motor is said to be overhung. If the coil is shorter than the gap, the motor is said to be underhung. Both of these attempt to address motor linearity from different directions. In either case, the convention remains that Xmax is presumed to be 1/2 the absolute value of the coil length minus the gap height. That the motor may be designed either way is represented by the necessity to use the absolute value of the difference between coil height and gap length. In either case, when Xmax is exceeded, the coil begins to move out of the gap, resulting in a marked loss of motor force, and a concomitant increase in distortion. This is why Xmax is assumed to represent the maximum linear excursion.
   An overhung VC will always keep a portion of its length within the gap. Coil windings outside the gap are presumed to contribute little additional force to the motor and can thus be largely ignored. So long as the driver remains within its Xmax limit, the coil will never be out of the gap. As the VC excursion passes Xmax, the length of the coil in the gap begins to decrease, resulting in a loss of motor force. At this point, the fact that a large portion of the coil is still affected by the end flux of the magnet structure assures that the increase in distortion with over excursion may be controlled to some degree by shaping the end flux.

   

   In an underhung VC, the total length of the coil is always within the magnetic gap. An underhung motor is, therefore, inherently more linear within its Xmax limits. However, once Xmax is exceeded, the linearity of an underhung motor drops off more rapidly than an overhung motor. Also, because of the need for such a massive magnet structure, underhung motors are inherently more expensive than overhung motors.

   

   Both designs can perform well, but from the preceding, it's obvious that overhung designs require more careful design of the magnet structures to approach the linearity of an underhung design.
Vented or unvented magnet assemblies. If you consider the design of a high-excursion motor with an unvented magnet, it quickly become obvious that a problem exists under the dustcap. While the cone can radiate to both the front and rear, the rear wave from the dustcap has no where to go. The most common solution is to provide a central vent through the magnet's central pole piece to allow the back wave from under the dustcap to exit the assembly.  vented pole piece (Lambda) 

Other schemes for venting drivers include a vented dustcap and formers with vents between the top of the VC and the point of attachment to the cone.

2. Phase plugs. Another common solution to magnet venting in midrange and some mid/bass drivers is to eliminate the dustcap completely. One problem common to all dustcaps is that reflections from the walls of the cone can interfere at higher frequencies, causing ragged response at the top end of a driver's range. To solve this problem, many drivers use a conical-shaped device called a "phase plug" which screws into the pole piece and extends out into the central area of the cone.  with phase plug (Focal)   without phase plug (Focal) 
3. Cooling. In any driver, too much power causes the coil to heat up. Consider that, as a transducer, a nominally 90 dB/W/m driver is still less than 1% efficient at converting electrical energy to mechanical energy. What this means is that if you feed 100 Watts to such a driver, 99 Watts will have to be dissipated as heat! This is bad, because as the motor heats up, its parameters change. It becomes less efficient and a phenomenon called power compression sets in where twice the power results in something less than twice the movement. Among the various ways to get rid of this excess heat are:
   1. Our old friend the phase plug. You'll notice that many of the more expensive drivers using phase plugs use metal phase plugs. This is for cooling. As long as the designer is going to extend the magnet pole piece into the relatively cool outside air, why not use a material with good thermal conductivity?  copper phase plug (Seas)   aluminum phase plug (Lambda) 
   2. Non-magnetic metal parts. Many motor designs use shorting or Faraday rings (see below) to help control linearity in overhung designs. By making these parts of copper or aluminum, the designer has the double advantage of maximized thermal and electrical conductivity.

   3. Large diameter coils have a greater surface area than smaller diameter coils. This greater surface area facilitates the transfer of heat from the coil to the magnet's pole pieces. The downside of a large diameter coil is that it makes it more difficult to achieve really good high-end extension.
   4. Former material. The thin cylindrical bobbin on which the VC is wound is called the former. Two of the most common materials are Kapton^TM and aluminum. Black anodized aluminum is great at helping to remove excess heat. Kapton is a dimensionally stable polymer with good high temperature characteristics. Both have advantages. In addition to heat, aluminum also conducts electricity, and so a simple cylinder would form a large shorted coil. For this reason, aluminum formers are made with a split to open the electrical circuit, which degrades their mechanical integrity to a degree. Kapton, like any other plastic, has nowhere near the thermal conductivity of aluminum.
   5. Heat sinks. Some vendors, most notably Eminence and Eton, have gone to the trouble of adding special thermally conductive structures to couple coil heating to the outside air. Such devices are usually used on high power woofers and are mounted in the place of a phase plug.  heat sink (Eminence) 
   6. The radial chassis is used only by Volt on its 12" and larger woofers. The radial chassis turns the entire driver structure inside out. Instead of mounting the motor to supporting arms attached to the mounting ring behind the cone, the radial chassis design attaches the motor to the mounting ring with supporting arms cantilevered in front of the cone. This exposes much more metal to the outside air, thus helping cooling.  cross-section   view 
4. Shape of the pole piece. As noted earlier, designing a good overhung motor with controlled symmetrical performance at its excursion limits is a challenge. Originally, the design of motors was pretty simple with straight pole pieces, as illustrated below. Nowadays, the following designs are common:

   

Using a T-shaped pole piece is now common practice on many advanced drivers. It concentrates the magnetic field between the two narrow poles rather than spreading around the surface of a large straight pole piece. Also called an undercut pole piece, the downside is earlier magnetic saturation and considerably less surface area to help cool the VC.



Using an extended pole piece creates a trapezoidal magnetic field while still leaving clearance for the cone. Extended pole pieces may be either straight or undercut. Although the field isn't symmetrical, it does help keep more of the coil (assuming an overhung design) in the gap. Properly done, an extended pole piece can provide a BL curve comparable to an underhung motor's.



5. Magnetic effects. A driver motor is subject to all of the same considerations as a rotary DC motor. Therefore, one important thing to recall is that when a DC motor is turned, it becomes a generator. Even when running, a motor's inductance will generate a "back EMF" - a voltage opposite of that which is driving it. All motors, even linear motors, exhibit this effect. In a speaker, it means that at the same time as an amplifier is trying to drive the speaker, the coil in the speaker is simultaneously trying to drive a counter-signal back into the amplifier. How successful it will be is determined by the damping factor of the amplifier. For the speaker designer looking to achieve maximum linearity, dealing with or counteracting back EMF is a crucial part of the design process. A side effect of back EMF is that the magnetic field in the gap distorts the flux in the gap as the VC moves.

   Another problem which may be solved in the magnetic circuit is how the driver behaves between Xmag and Xsus. Ideally, there should be some mechanism which prevents the VC from destructively colliding with the motor structure. Usually, there is some mechanical protection in the form of spider stiffness, damping materials, etc., but a well designed motor can assure that by the time the VC reaches Xsus, it no longer has sufficient force do damage itself or other parts of the structure.

   The following magnetic circuit designs deal with the above issues:

   Faraday rings are essentially single-turn shorted coils (typically either aluminum or copper) which react to the back EMF generated in the VC as it moves in order to stabilize the magnetic field. Physically, this shorted turn is a fixed part of the magnet structure. In less expensive drivers, the Faraday ring may consist of a simple conductive non-magnetic plating on the pole piece as illustrated below on an extended pole piece.

   

   One popular approach is to use an aluminum Faraday ring below a T-shaped pole piece. This sort of design stabilizes the flux a lot, and is quite economical to manufacture. Also the large surface area of the aluminum ring helps considerably with cooling. Again, this is illustrated below with an extended pole piece.

   

   Another design, patented by Scan-Speak for its highly regarded SD-1 motors, is to use dual Faraday rings above and below the pole piece, as illustrated below. The interaction between the Faraday rings and the VC occur outside the main part of the magnetic gap, since. the copper effectively creates a virtual T-shaped pole piece.

   

   Shorted VC turns are part of the moving assembly rather than the fixed assembly. Useful only on overhung motors, when Xmax is exceeded, the shorted turns move into the magnetic gap, where they resist further motion. Seas uses these on its Excel and other woofers and calls the system "dynamic damping".

   

   

   Note that most of the above design features can be used in combination to good effect. For example, here is an illustration of the extreme motor design used by Lambda Acoustics. (This is the same motor used above to illustrate a vented magnet assembly, to better help you visualize the venting.) Note that the ends of the vent in the extra large extended pole piece are flared for better aerodynamics resulting in less noise. Also note that the gap is vented in addition to the dustcap. Finally, the enormous extended pole piece is fitted with a full-length copper Faraday ring. Even with the long coil shown, such a motor will remain extremely linear. The most controversial feature of this design is the Faraday ring. Using a full-length ring as Lambda does pays off in extremely flat impedance and good extended FR (assuming the cone is up to the task). The downside is that it also saps a lot of the motor's strength which means more money spent on the magnet assembly than would otherwise be required. The full-length Faraday ring combines with a gap with extremely tight tolerances which also helps to remove a lot of heat, although maintaining such tight tolerances also adds cost.

   Follow this link to see another example of an advanced motor design, this time from TC Sounds. Note that this design also uses a double-stacked magnet assembly. The extra-large extended pole piece is also flared as was the Lambda driver. The special feature of this motor design is the highly symmetrical magnetic field produced by the specially contoured pole pieces. Other, non motor-related, features of modern high-excursion woofers are also illustrated in this design - note the large half-roll surround and dual spiders.

   One final illustration… Follow this link to see an illustration of an underhung design, also from TC sounds. Note the enormous length of the motor assembly. Also illustrated is the BL product (magnetic strength) in the gap. As previously noted, with an underhung design, this is extremely linear, all the way up to its Xmax limit (+/-14 mm in this case).