Why Suspend a Spider?


When you build your own drivers like we do, over the years you become very familiar with their working parts and conventional ways of putting them together. Sometimes it is worthwhile examining those conventions more closely.

This is what it takes to assemble one of our woofers:

1. Stamped metal basket. Buy these in Chicago from an outfit called Alden Mfg which has made nothing else for 47 years. Use 14 gauge and 12 gauge steel for strength. Yes, we could buy diecast frames from Taiwan like some of those pro-sound guys. Then you find out the casting is brittle (with a disconcerting tendency to shatter in transit) because it’s pot metal, the cheapest alloy. Tolerances on the cast basket are nowhere near as precise as on our domestic stamping, and the homegrown steel is more than strong enough to support an 2.3kg magnet, thick frontplate, long pole piece etc (I’m getting ahead of myself). If you want the basket to vibrate less, like we do, apply Soundcoat sheets to the outside.

2. Backplate/pole piece. Buy these metal parts from a local outfit called Punch Press Products. Backplates and pole pieces carry the magnetic charge from the “South” pole of the ferrite doughnut, which is the actual magnet, up to where it meets the “North” pole of same, so you have opposing poles in close juxtaposition, necessary for push-pull operation. A Phase Plug attaches to the pole piece to prevent reflections inside the cone and move the acoustic center of the woofer forward to meet the planar sources.

3. Voicecoil/former, insulated copper wire (called “magnet wire” ) wound around a heat-conductive cylinder of Aluminum or Kapton which is usually split to allow expansion from voicecoil heating. Since Mr. Maxwell discovered the physical principle 150 years or so ago that alternating current (i.e. music) sent through a wire suspended in a magnet field causes proportional movement thereof, we thread the voicecoil leads through the...

4. Cone, to which the former/VC combo is glued, then suspended inside the airgap formed by the pole piece and the...

5. Front plate, another steel part which focused the magnetic field on as wide a section of the voicecoil/former as possible. The idea is the thicker the front plate, the more linear the travel of the former/VC, since the magnetic field should always cover as much former as possible. The voicecoil is “overwound” so that when it moves (pistonically, up and down), the front plate still covers it with magnetic field, even if the former moves say 16mm in either direction as it will on really loud lowbass. 

6. Magnets, made of strontium ferrite, are charged in a gaussing machine once they have been glued to the front and back plates, the basket riveted to the front plate, the VC/former suspended in the air gap (centered and held in place by Teflon shims) and the cone surround glued to the edge of the basket and the cone apex, or throat, glued to the... 

7. SPIDER, the part I want to talk about today. 

OK, not many parts so we can afford to do them at the high quality level. For example, instead of using one magnet which is about 25mm thick, we stack two and use a thicker (12.5mm) front plate and longer pole piece. This means (and this is very important) that we can overhang the voicecoil at both ends of the former and still have no danger of the former bottoming out (usually with a loud bang) against the back plate, which is impossibly far away due to the stacked magnets!

But, I was going to talk about spiders, not just the fact that you can build a woofer that will not and cannot bottom out if you’re willing to spend the dough. Regardless of how important that may be. BTW power handling is improved as well with this technique.

The spider is a round piece of corrugated cloth or plastic with a hole cut in the center a little larger than the former (itself 5cm or so diameter) and which constitutes the springy support for the cone throat. Without a spider the cone cannot restore to center when the signal is removed. In other words, the spider helps keep the cone centered while providing a spring to regulate its motion. The spider is assisted in its work by the cone surround, which has the additional job of absorbing traveling waves (sound energy that stays within the diaphragm, rather than departing it to excite the air which makes for the sound you hear). We use natural rubber surrounds on Woven Carbon Fiber cones for our best woofers, found in the RM 2 and SRE series VMPS speakers. 

But I wanted to talk about the spider. Since the spider supports the cone it also transmits traveling waves to its edge. Now spiders come in two flavors--flat and walled. A walled spider stands on a “leg” about 16mm tall which elevates the corrugated (springy) part of the spider above the back of the speaker basket, so that the cone doesn’t knock against the basket on long excursions. The wall is just an extension of the spider material--imagine the lid on a mayonnaise jar, with the spider the flat top of the lid and the wall the part that screws onto the glass. If a mayonnaise jar lid had a 5cm hole cut in the center you analogy would be perfect. So traveling waves reach the wall, or edge of the spider, and reflect their energy back into its center, muddying the sound.

It occurred to me that if the spider had a termination just like the cone’s surround, traveling waves would be reduced and the sound of the woofer would improve. So I did just that, starting about November 2000, on all spiders on all woofers and passive radiators we make. THD dropped about 20% in our measurements, and the drivers sounded notably cleaner and clearer, actually much cleaner and clearer. The cost of so doing is under a buck, and we are the only manufacturer to do so.

This is a fundamental improvement in the design of the dynamic loudspeaker and I am most pleased to tell you about it now. Hope the competition isn’t listening.

Brian Cheney