Technology Highlight
Suspension System
Modern loudspeakers are subjected to very high input power levels and are expected to generate very high acoustic output power. The function of the suspension system is to provide both a mechanical restoring force for the loudspeaker cone and to provide mechanical centering of the voice coil assembly. The centering function of the suspension system is very important as this function prevents the voice coil assembly from rubbing on the magnetic structure steel parts and producing a loudspeaker failure.
The overall suspension system consists of two parts, the spider assembly and the cone surround. Both the spider assembly and the cone surround are required to move in a complex manner that makes controlled motion very difficult.
The image below represents a typical large format voice coil assembly and a single spider. Figure 1 shows this assembly in a "rest" position.
Figure 2 is the same assembly but the voice coil has now moved 10 mm. This is a typical displacement for a low-frequency loudspeaker. Note that this 10mm displacement has required the spider to rotate thru a 20-degree angle and that the spider no longer is touching the coil form! In order to remain bonded to the coil form, the spider must stretch!
This is a very important point and it illustrates that the spider (and cone surround as well) must not only move up and down but it must also stretch in order to remain bonded to the voice coil form.
The spider must stretch 6% more than its original length in order to allow the voice coil to move a mere 10mm in the example shown above. One of the major sources of poor reliability, reduced power handling and increased distortion in a loudspeaker system is a direct result of this suspension stretching.
A simple solution is to increase the diameter of the spider. Each time the clamp diameter (that section of the spider that is bonded to the loudspeaker chassis) of the spider is increased the amount of stretch required for a given displacement of the voice coil is reduced.
Figure 3 shows three spiders, each with a progressively larger clamp diameter. For a given motion of the voice coil assembly, the spider with the largest clamp diameter would offer the best reliability, highest power handling and lowest distortion of the three examples shown because it would be required to stretch the least amount.
Low Stress (LS)™ Suspension System
The spiders used on several of P Series are extremely large clamp diameter designs and were designed to substantially minimize the stretching found in common spider designs. Several P.Audio Series loudspeakers use these new "low stress" LS™ designs. 15 inches is shown in Figure 4. The LS spider is shown in the yellow color.
Because the LS spiders are subjected to very low percentage stress levels they provide a more linear restoring force and provide excellent voice coil assembly centering at high input power levels.
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