Audio Engineering

692 Chapter 23

23.14 Suspensions

The purpose of suspensions is to provide a known restoring force to the diaphragm/voice
coil assembly and at the same time to have suffi cient lateral rigidity to prevent any
side-to-side movement of the system. This latter requisite is most important when it is
remembered that the clearance between the voice coil and the magnet pole pieces is of the
order of 0.15 mm for tweeters and 0.4 mm for 150-W woofers. The average domestic 200-
mm (8-inch) loudspeaker is about 0.25 mm.

The combined stiffness of the front and rear suspensions is formulated to resonate with
the total moving mass of the diaphragm/voice coil assembly and air load to the designed
LF resonance. The front suspension radial width is usually about half that of the rear
(in order to maximize cone diameter for a given cradle size) and it is this factor that
limits the peak-to-peak displacement. Figure 23.11 shows displacement/force for a roll
surround. It will be seen that the maximum linear movement is limited: it follows the
familiar hysteresis curve of nonlinear dissipative systems.

The annulus of the diaphragm can either be an extension of the cone material itself
or, as is more usual with high-fi delity loudspeakers, be a highly compliant surround
produced from cotton or man-made fi bers, neoprene, or plasticized PVC. In the case of

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8 6 4 2 0 2 4 6 8

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In

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Figure 23.11 : Displacement relative applied force.