600 Chapter 17
early method of making porous spiders was to die-cut
them from solid phenolic-impregnated linen sheet stock.
The spider is not required to seal the edge of the cone to
its enclosure as is the surround. In a typical cone driver,
the spider contributes the majority of the stiffness in the
suspension.
17.2.4 Mechanical Construction
The Peavey Black Widow bass drivers are unusual in
that they have a streamlined magnet structure, called
focused-field geometry, Fig. 17-5. It employs a
magnetic circuit that has smoothly flowing flux lines, as
might be intuitively preferred for a fluid flow channel.
Other manufacturers have adopted similar approaches
to magnet design. An added benefit of this approach is
minimization of weight.
JBL ferrite-magnet drivers have symmetric field
geometry. Fig. 17-6 shows the top plate configuration,
which makes the magnetic leakage flux at the top and
bottom of the gap symmetric, thereby, according to the
manufacturer, reducing magnetic drive asymmetry and
the resulting low-frequency distortion.
Another form of electrodynamic transducer is the
dome radiator. Most commonly used for high frequen-
cies, dome radiators have the advantages of compact-
ness and predictability of acoustic behavior. Domes can
be made from linen, impregnated phenolic fabric,
Mylar™, paper, aluminum, titanium, beryllium, and
composites such as carbon fiber/epoxy. Soft dome
tweeters have been in widespread use for a number of
years. Some of this popularity may be due to the fact
that there is no abrupt transition from piston radiation to
breakup. Instead, most of the radiation from a soft dome
comes from the region immediately adjacent to the
voice coil, making it function as a ring radiator.
Several flexible diaphragms have been used on
magnetic drivers, all sharing the same basic construc-
tion: etched aluminum conductors on Mylar™ film.
These are operated in various magnetic field configura-
tions to produce sound. One of the earliest of these is
the Magneplanar® loudspeaker, which consisted of an
entire field of magnets over which the diaphragm
conductor was mounted. Magneplanars are in the shape
of large panels. The Heil high-frequency driver, used in
systems manufactured by ESS, used direct radiators
similar to Magneplanar® in that the voice coil was
printed on Mylar™. The ESS-Heil unit, however, was
corrugated, and the sound was produced by these
vertical pleats moving open and closed, thereby
squeezing air into radiated sound. An extension of this
was used also by ESS in the Transar system, which used
hollow spheres modulated by electromagnetically
driven rods. Mitsubishi Electric (Japan) developed a
printed conductor high-frequency device called the leaf
tweeter, as shown in Fig. 17-7. The ribbon loudspeaker
is the simplest and has excellent potential for good
high-frequency response due to the fact that the
diaphragm is the conductor. No extra diaphragm struc-
ture is used on the ribbon.17.3 Compression DriversOne means of improving the performance of an electro-
dynamic transducer that will be used to drive a horn is
to create a compression driver. In a compression driver,
the diaphragm radiates into a compression chamber and
its output is typically directed through a phasing plug to
the driver’s exit, which is attached to the throat of the
horn.
The advantage of a compression driver is that rela-
tively small diaphragm velocities are converted to larger
particle velocities at the exit of the driver. The effect of
this transformation is that less diaphragm excursion is
required for a given acoustic power output. The tradeoffsFigure 17-5. Peavey focused-field geometry magnet struc-
ture with one-piece backplate/pole piece forging. Courtesy
Peavey Electronic Corp.
Figure 17-6. JBL symmetric field geometry versus asymmet-
ric design. Courtesy JBL.
B. Flux distribution with symmetrical field geometry
showing equal fringe field on both sides of the gap.A. Flux distribution in nonsymmetrical gap
showing an uneven fringe field.