Audio Engineering

Loudspeaker Enclosures

Ben Duncan

24.1 Loudspeakers ........................................................................................................

Knowing a little about loudspeakers, the load that is audio amplifi ers ’ raison d’etre is a
prerequisite to understanding amplifi ers. In the following sections—indeed most of the
rest of this chapter—those features of loudspeakers that most defi ne or affect the design
and specifi cation of power amplifi ers are introduced.

24.1.1 Loudspeaker Drive-Unit Basics

There are six main types of speaker drive-units or drivers used for quality audio
reproduction. Another name for a driver is a transducer , a reminder that they transduce
electric energy into acoustic energy via mechanical energy.

24.1.1.1 Cone Drivers

The most universal, everyday form of the “ moving coil ” or “ electrodynamic ” type of
drive-unit ( Figure 24.1 ) has a moving cone, with a neatly wound coil of wire (the “ voice
coil ” ) attached to its rear. The coil has to be connected to and driven by an amplifi er.
The coil sits in a powerful magnetic fi eld and can move back and forth without rubbing
against anything. When driven, a signal-varying, counteractive magnetic fi eld is set up,
causing the coil and the attached cone to vibrate in sympathy with (as an analogue of) the
driving signal. The principles are akin to an electric motor, except that the vibration is
linear ( “ in and out ” ) rather than rotational.

Moving coil drive-units can be made in many ways. Most have to be mounted in some
kind of enclosure before they can be used. Drive-unit size (strictly, the piston diameter)
broadly defi nes frequency range. Most cone drivers range from 1 (25 mm) up to 24 

CHAPTER 24