Operation
In operation, a signal is delivered to the voice coil by means of electrical wires, from the amplifier through speaker cable, then through flexible tinsel wire to the moving coil. The current creates a magnetic field that causes the diaphragm to be alternately attracted to, and repelled by, the fixed magnet as the electrical signal varies. The resulting back-and-forth motion drives the air in front of the diaphragm, resulting in pressure differentials that travel away as sound waves.
The spider and surround act as a tensioned spring centering mechanism, to both concentrically center the voice coil within the magnet assembly, and to center the front-to-back voice coil position within the magnet assembly when power is removed.
The voice coil and magnet essentially form a linear motor working against the centering spring tension of the spider and surround. If there were no restriction on travel distance imposed by the spider and surround, the voice coil could be ejected from the magnet assembly at high power levels, or travel inward deep enough to collide with the back of the magnet assembly. The majority of speaker drivers only work against the centering tension of the spider and surround, and do not actively monitor the position of the driver element or attempt to precisely position it, as occurs in servomechanisms.
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