Audio Engineering

706 Chapter 24

(0.6 m) in diameter for use at high treble down to low bass. There are at least 15,000
different types of cone materials, textures, and weights available. Most are made of paper
pulp, but plastics, metals, composite materials, and laminated combinations are also used.
Every one sounds different and measures differently.

There are as many permutations again for the voice coil’s diameter, height, and wire
gauge; the type of dust cap, magnet, the chassis, and the fl exible jointing, called the
surround (at the front), and centering device at the rear, the suspension or spider. With all
the moving parts, ruggedness and stiffness are pitted against the need for agility, hence
levity. This is the main reason why the radiating part is cone shaped. This shape can stiffen
the most limp paper against the axial force applied to it by movements of the voice coil.

24.1.1.2 Compression Drivers

The second most common type of driver, at least in professional sound, is the
compression driver ( Figure 24.2 ). This is simply a specialized form of moving coil drive-
unit. The depth of the cone is replaced by a much shallower and usually opposite-facing
and dome-shaped radiating surface, called the diaphragm. The voice coil is attached
peripherally between the edge of the dome and the suspension. This type is made for
some midrange but mainly hf speakers, which are horn loaded. All bass bins and most
midrange horns employ specially adapted but ordinary-looking cone drivers; these
alone can handle the larger excursions required. A compression driver cannot handle
more than very small excursions. To avoid large excursions and potential ripping of the

Magnet (ceramic) Polepiece

Top plate

Basket

Cone

Suspension (‘surround’)

Suspension (‘spider’)

Voice coil Dust cap

Flexible ‘lead-outs’

Gasket

Terminals

Figure 24.1 : A moving coil drive-unit, with key parts identifi ed.

(Courtesy Funktion One Research)