AN3340

Mechanical structure of a loud speaker

Doc ID 18453 Rev 1

5/24

3

Mechanical structure of a loud speaker

Figure 1.

Cross section of a speaker

A basic loudspeaker is made from a diaphragm or cone connected to a metal basket or a

rigid structure, via a flexible suspension. This suspension constrains the voice coil, which is

connected to the diaphragm, to move axially through a cylindrical magnetic gap.

The audio signal from the power amplifier is applied to the voice coil. This generates a

variable electric current which interacts with the static magnetic field in the cylindrical gap

between the poles of a permanent magnet to move the diaphragm, which is mounted on the

voice-coil former. The diaphragm suspension must be elastic and must operate in its linear

region to ensure that the diaphragm is displaced according to the amplitude of the input

signal.

The diaphragm movement generates a variable sound pressure which is proportional to the

drive signal applied, that is, the output of the power amplifier.

The surround and the spider keep the diaphragm mechanically centered and maintain the

voice coil in the neutral position when no signal is applied.

The voice coil is usually made with a copper winding on a cylindrical former and oriented

coaxially inside the cylindrical air gap within which the magnetic field is uniform over the

range of travel of the coil.

In special designs, more cones can be combined in a single enclosure to make a multi-way

coaxial speaker, as shown in Figure 2.

Another approach is to design the speaker to enlarge the frequency bandwidth rather than

to achieve a full-range speaker. Such a speaker could have a non-unity aspect ratio as

shown in Figure 3. Although the frequency range is quite large the frequency response is

generally not flat.

Diaphragm

or Cone

Spider

Permanent

Magnet

Voice Coil

Surround

Voice Coil

Assembly

Frame

or

Basket

Dust Cup

or Dome