 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 work only against the centering forces of the spider and surround, and do not actively monitor the position of the driver element or attempt to precisely position it.