Handbook for Sound Engineers

598 Chapter 17

and of themselves, and their convergence in the field of
loudspeaker design results in one of the most complex
combinations of art and science that has ever existed.

17.2 Transducer Types

There are a number of ways in which electrical energy
can be converted into acoustic energy. Of all the possi-
bilities for carrying out this function, a relative few have
become dominant in practical loudspeakers: electrody-
namic, electrostatic, and piezoelectric. In general, an
electroacoustic transducer contains three elements:
motor, diaphragm, and suspension. The motor converts
electrical energy into mechanical (motional) energy and
the diaphragm converts mechanical energy into acoustic
energy (vibration of the transmission medium, usually
air). A suspension supports the diaphragm, allows it to
move in an appropriately constrained fashion, exerts a
restoring force proportional to displacement from its
equilibrium position, and provides a damping force
proportional to the velocity of motion that serves to
prevent the diaphragm from oscillating in an undesired
manner.

17.2.1 Electrodynamic Transducers

The most common type of transducer used in loud-
speakers is the electrodynamic driver. In this type of
transducer, a time-varying current passing through a
conductive coil suspended in a time-invariant magnetic
field creates a force on the coil and the parts to which it
is attached. This force causes the parts to vibrate and to
radiate sound.
There are a number of viable implementations of
electrodynamic transducers. By far the most common is
the cone driver. In a cone driver, a cone-shaped
diaphragm is suspended at its outer periphery by a
structure called a surround and (usually) near its center
by a spider. The motor consists of a permanent magnet
assembly that concentrates the magnetic field in an
annular gap, in which is placed a voice coil that is
attached to the center of the cone via a cylindrical coil
former. An electrical signal is applied to the voice coil,
and the current in the voice coil interacts with the
magnetic field in the gap to create a time-varying force
that vibrates the diaphragm. Fig. 17-1 shows a typical
cone driver. The most commonly used magnetic mate-
rial is ferrite, or ceramic. Other magnetic materials used
in loudspeakers include aluminum/nickel/cobalt
(alnico) and neodymium/iron/boron, (neodymium or
neo). The magnet structure is typically held together
with an anaerobic thermoset adhesive. Some loud-

speakers are assembled with bolts through the magnet.

In this case, stainless steel or brass screws must be used,

so as not to magnetically short the top plate to the back

plate. A rear cover may or may not be used. A vent

through the pole piece may be provided. It serves to

prevent the addition of a spring constant due to the

small air cavity under the center cap (dust cover) and to

reduce turbulence-induced noise due to pumping effects

in the magnet gap.

17.2.2 Diaphragm Types

The most common direct-radiation device is the cylin-

drical voice coil–driven paper cone. The cheapest cone

to make is the folded cone, which is cut from a sheet of

paper, rolled, and bonded at the seam. A more expen-

sive and difficult to make cone is the molded-paper

cone. These are one piece, molded by straining a slurry

of water and paper pulp through a strainer mold in the

shape of the desired end product. The formed wet mat

of pulp is then pressed and baked to remove residual

moisture, bearing a dry, strong one-piece cone, free of

joints. Ribs and concentric rings are sometimes molded

into the cone, and the cones can be formed with straight

or curved sides of varying depth. These are all available

from suppliers of cones.

While most mathematical models of a direct radiator

assume a rigid piston, in practice this is impossible to

achieve. In some cases, diaphragm rigidity is intention-

ally reduced in order to produce specific desired

behavior. Two examples involving a controlled breakup

are shown in Figs. 17-2 and 17-3. The whizzer cone in

Fig. 17-2 is intended to radiate high frequencies as the

larger cone decouples from the motor. The Biflex prin-

ciple, as popularized by Altec Lansing in the 1950s, is

shown in Fig. 17-3. The inner cone is attached via a

compliant element at A to the large outer cone in hopes

of decoupling the outer cone at high frequencies.

Figure 17-1. Typical woofer parts identification. Courtesy

Yamaha International Corp.

Voice Coil

& Coil Form Center

Cap

Surround

Gasket

Frame

Connection

Terminal

Back Plate

Rear Cover

Shorting

Ring

Rear Cover Vent

Gasket

Pole Piece

Ferrite Magnet

Spider

Screen