Conventional electrodynamic (coil and magnet) transducers are simply linear DC
motors attached to a diaphragm (typically a cone or dome). The design of the motor
is critical to how well a driver performs. A good motor will provide a linear
relationship between the input signal and the resulting motion, and will be
designed to allow heat to escape from the VC. Although the cone is important, many
engineers feel that the motor in large part defines most of a driver's sound. It's
almost certainly true that the motor is clearly responsible for 90+% of a drivers
mid-band distortion. Features to look for in a good motor design include:
- 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
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.
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.
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.
- 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.
- 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)
- 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:
- 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)
- 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.
- 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.
- Former material. The thin
cylindrical bobbin on which the VC is wound is called the former. Two of
the most common materials are KaptonTM 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.
- 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)
- 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.
- 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.
- 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
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
One final illustration… Follow this
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).