Driver design
The most common type of driver, commonly called a
dynamic loudspeaker, uses a lightweight diaphragm, or
cone, connected to a rigid
basket, or
frame, via a flexible suspension, commonly called a
spider, that constrains a voice coil to move axially through a cylindrical magnetic gap. When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current
in the voice coil, making it a variable electromagnet. The coil and the
driver's magnetic system interact, generating a mechanical force that
causes the coil (and thus, the attached cone) to move back and forth,
thereby reproducing sound under the control of the applied electrical
signal coming from the amplifier. The following is a description of the individual components of this type of loudspeaker.
The diaphragm is usually manufactured with a cone- or dome-shaped
profile. A variety of different materials may be used, but the most
common are paper, plastic, and metal. The ideal material would be 1)
rigid, to prevent uncontrolled cone motions; 2) have low mass, to
minimize starting force requirements and energy storage issues; 3) be
well damped, to reduce vibrations continuing after the signal has stopped with little or no audible ringing due to its resonance
frequency as determined by its usage. In practice, all three of these
criteria cannot be met simultaneously using existing materials; thus,
driver design involves trade-offs.
For example, paper is light and typically well damped, but is not
stiff; metal may be stiff and light, but it usually has poor damping;
plastic can be light, but typically, the stiffer it is made, the poorer
the damping. As a result, many cones are made of some sort of composite
material. For example, a cone might be made of cellulose paper, into
which some carbon fiber, Kevlar, glass, hemp or bamboo
fibers have been added; or it might use a honeycomb sandwich
construction; or a coating might be applied to it so as to provide
additional stiffening or damping.
The chassis, frame, or basket, is designed to be rigid, avoiding
deformation that could change critical alignments with the magnet gap,
perhaps causing the voice coil to rub against the sides of the gap.
Chassis are typically cast from aluminum alloy, or stamped
from thin steel sheet, although molded plastic and damped plastic
compound baskets are becoming common, especially for inexpensive,
low-mass drivers. Metallic chassis can play an important role in
conducting heat away from the voice coil; heating during operation
changes resistance, causing physical dimensional changes, and if
extreme, may even demagnetize permanent magnets.
The suspension system keeps the coil centered in the gap and provides
a restoring (centering) force that returns the cone to a neutral
position after moving. A typical suspension system consists of two
parts: the
spider, which connects the diaphragm or voice coil to the frame and provides the majority of the restoring force, and the
surround,
which helps center the coil/cone assembly and allows free pistonic
motion aligned with the magnetic gap. The spider is usually made of a
corrugated fabric disk, impregnated with a stiffening resin. The name
comes from the shape of early suspensions, which were two concentric
rings of Bakelite
material, joined by six or eight curved "legs." Variations of this
topology included the addition of a felt disc to provide a barrier to
particles that might otherwise cause the voice coil to rub. The German
firm Rulik still offers drivers with uncommon spiders made of wood.
The cone surround can be rubber or polyester foam,
or a ring of corrugated, resin coated fabric; it is attached to both
the outer diaphragm circumference and to the frame. These different
surround materials, their shape and treatment can dramatically affect
the acoustic output of a driver; each class and implementation having
advantages and disadvantages. Polyester foam, for example, is
lightweight and economical, but is degraded by exposure to ozone, UV
light, humidity and elevated temperatures, limiting its useful life to
about 15 years.
The wire in a voice coil is usually made of copper, though aluminum—and, rarely, silver—may
be used. The advantage of aluminum is its light weight, which raises
the resonant frequency of the voice coil and allows it to respond more
easily to higher frequencies. A disadvantage of aluminum is that it is
not easily soldered, and so connections are instead often crimped
together and sealed. These connections can corrode and fail in time.
Voice-coil wire cross sections can be circular, rectangular, or
hexagonal, giving varying amounts of wire volume coverage in the
magnetic gap space. The coil is oriented co-axially inside the gap; it
moves back and forth within a small circular volume (a hole, slot, or
groove) in the magnetic structure. The gap establishes a concentrated
magnetic field between the two poles of a permanent magnet; the outside
of the gap being one pole, and the center post (called the pole piece)
being the other. The pole piece and backplate are often a single piece,
called the poleplate or yoke.
Modern driver magnets are almost always permanent and made of ceramic, ferrite, Alnico, or, more recently, rare earth such as neodymium and Samarium cobalt.
A trend in design—due to increases in transportation costs and a desire
for smaller, lighter devices (as in many home theater multi-speaker
installations)—is the use of the last instead of heavier ferrite types.
Very few manufacturers still produce electrodynamic loudspeakers with electrically powered field coils,
as was common in the earliest designs (one such is French). When high
field-strength permanent magnets became available, Alnico, an alloy of
aluminum, nickel, and cobalt became popular, since it dispensed with the
power supply problems of field-coil drivers. Alnico was used for almost
exclusively until about 1980. Alnico magnets can be partially degaussed
(i.e., demagnetized) by accidental 'pops' or 'clicks' caused by loose
connections, especially if used with a high power amplifier. This damage
can be reversed by "recharging" the magnet.
Wiring connections
