Handbook for Sound Engineers

Loudspeakers 599

Damping dope is applied to the coupling connection in

an attempt to smooth the decoupling transition

frequency response. While whizzer cones are still in use

in some inexpensive ceiling speakers, there are at this

time no devices similar to the Biflex on the market.

In addition to felted paper, a number of newer mate-

rials have found use in cone-type low- and medium-

frequency loudspeakers. A variety of plastics have been

used, the most popular being polypropylene and

bextrene. The KEF Company introduced a composite

aluminum-skinned foam-core sandwich cone. Commu-

nity Professional Loudspeaker’s M4 compression

midrange similarly uses a carbon fiber/epoxy composite

diaphragm. Adamson Acoustics in Canada uses a

Kevlar fabric resin-bonded diaphragm for the midrange

driver. Mitsubishi Electric (Japan) introduced a studio

monitor, which used cone woofers fabricated from a

honeycomb core/carbon fiber skin composite.

In a loudspeaker with an alnico magnet, the magnet

is directly under the pole piece (as opposed to being

between the top and backplates), and the outside of the

magnet structure is a cast iron return from the bottom of

the magnet to the top plate. Venting may be accom-

plished via a hole covered with open wire mesh in the

center dome. Other methods include a uniformly porous

dome with no magnet vent, Fig. 17-4.

17.2.3 Suspension Methods

The suspension of a cone driver comprises two distinct

components: the surround and the spider. The surround

is attached to the periphery of the diaphragm or cone

and is itself attached to the support structure (the basket

in the case of a cone driver). The spider is attached to

the voice coil former (or to the cone in the vicinity of

the former) and is also attached to the basket on its

periphery. Because they affect cabinet sealing,

surrounds are designed to be nonporous. Surrounds and

spiders both contribute to the damping of the motion of

the diaphragm. The most popular surround construction

is heat-formed, open-weave, resin-impregnated linen

with formed-in convolutions and sealed with damping

dope. Other surrounds are made of foam or butyl rubber

formed in a half-roll. On some loudspeakers, a

viscoelastic (never-drying) dope is applied to the

surround.

Spiders are usually made of a heat-formed, open-

weave, resin-impregnated cloth that is formed into

convolutions. They are usually not treated with a

sealing material (dope). The unsealed fabric is needed

for venting, since the air beneath the spiders can other-

wise be trapped. This also tends to damp the spider. An

Figure 17-2. Loudspeaker incorporating a whizzer cone.

Figure 17-3. Loudspeaker illustrating decoupling center
cone. (From U.S. Patent 4,146,756.)

Figure 17-4. Alnico magnet woofer—Altec 515-8LF. Cour-

tesy of Altec Lansing Corp.