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Woofer spider vs surround compliance contribution


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Vance Dickason has written in his Loudspeaker Cookbooks that the spider contributes "about 80%" of the total compliance of a woofer.

I decided to test that during a recent 4-woofer refoam job. I was refoaming 12 inch graphite cone woofers taken from a Kappa 9 Reference loudspeaker. After cleaning up all the old foam I tested them with a WT3 and again after refoaming prio to any break-in of the new foams.

What I found was that on average, the spider contributed 74.2% of the resonant frequency (Fs). Some other significant changes also occured.

Qes was 73.4% with just the spider. Z max dropped 24% and Cms (mech. compliance meas. using added mass) 45% with the addition of new foam.

So, at least with AS type woofers VD was pretty close. However, if vented type woofers are tested with their typically stiffer suspensions, the ratio would climb and thus 80% may indeed be a good overall average.

Worth noting for DIY refoamers of AS type speakers (i.e. most classic NE brands), keep in mind that the foam surround contributes only about 1/4 of the total compliance. If you're reconing, then be particularly fussy about the stiffness of the spider you use due to its more significant contribution to total driver compliance.

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Vance Dickason has written in his Loudspeaker Cookbooks that the spider contributes "about 80%" of the total compliance of a woofer.

Hi Carl,

Interesting numbers. It sounds like you were measuring the woofers with no surround at first, right?

I remember that there was always a lot more choice in available spiders and their stiffness. Also, spiders were available with a range of linearity characteristics. I was always looking for spiders with a decent travel but a gradually increasing stiffness. That would improve DC offset effects and overall woofer control.

A semantic quibble: you mean "80% of the total stiffness" as compliance is the inverse of stiffness.

Actually, your numbers show that the stiffnes contribution from spider and surround were about equal for those particular woofers. Resonance is proportional to 1 over the square root of mass times compliance. Mass doesn't change (much) so resonance shifts up by the square root of stiffness increase or 1 over the square root of the compliance decrease. Doubling stiffness (halving compliance) gives a square root of 2 resonance increase (times 1.414 or 41% increase). This would be for the case where spider and surround were exactly equal.

Your Cms numbers tell the story. a 45% drop means the total is now 55% of what it was. In stiffness terms the new stiffness is 1 over 55%, 1.8182, so stiffness has increased 81.8%. The square root of the new stiffness (square root of 1.8182 equals 1.3484) indicates resonance should rise 34.8%. Inverse of that means that the orriginal resonance is 74.2% of the final resonance, exactly as you found.

If the spider was a stiffness of 1 and the total becomes 1.8182, then the spiders portion was 1/1.8182=55% (again)

In this case the spider was a little over half the total stiffness.

Hope that all makes sense.

Regards,

David

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Thanks David for the primer on stiffness vs compliance. I was just quoting VD's exact words; even though he was thorough enough to explain that compliance is the reciprical of stifness. I simply interpreted his words to relate to the change in the resonant frequency impacted by the spider alone vs the addition of the surround.

I have attached the original test data in a word pasted doc from my original spreadsheet - FYI for those who may be interested.

My parts distributor lists their spiders and surrounds in terms of "texture" identified with the first 5 letters of the alphabet with A being the 'softest' and E the 'stiffest'. Not much science here.

I'm sure speaker manufacturers and OEM parts suppliers of spiders and surrounds share more quantitative compliance specs. There is even a test method and instrument to measure it. See attachments. However, these same folks seem reluctant to share their specs with us mere mortals. It's been a pet peeve for some time. I've even had private communications with VD regarding this but didn't make any headway. From my perspective, suspension parts compliance specs would be as useful to speaker builders and repairers as T/S parameters. Of course, once we got used to using them.

BTW, how did you get from 34.8% to 72.4%? I couldn't.

Tonwel_compliance_testing.pdf

Infinity_woofer_compliance_testing.doc

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From my perspective, suspension parts compliance specs would be as useful to speaker builders and repairers as T/S parameters. Of course, once we got used to using them.

BTW, how did you get from 34.8% to 72.4%? I couldn't.

Hi Carl,

Spider compliance or stiffness numbers would be useful, but the parts suppliers rarely gave them because they rarely measured them (I think some didn't know how). Some of the better suppliers would give 50 gram deflection. Ask them about linearity curves and you still get a blank stare. I think most driver designers would just try a few different samples, and if something was close ask for more resin (stiffer) or less resin (softer).

The percentage change matter is always a little confusing because a given difference is one percentage "going up" and another "going down". The stiffness factor you achieved by adding the new surround was 1.8182, not quite double the stiffness. From the formula we know the resonance will increase not by that amount but by the square root of it. 1.3484. These numbers are a multiplier to be multiplied times whatever the original resonance was, so as an example, say you started with 20 Hz free air resonance with no surround. Adding the surround gives 20 x 1.3484 or about 27 Hz. When you said "the spider contributed 74.2% of the resonant frequency" you were saying that resonance with spider only was 74.2% of the higher figure it would be with spider and surround. 20Hz divided by 27 Hz is 74.1%, confirming your number. So going upwards from 20 to 27 means going from 100% to 134.8%. Going the other way 27 has to be considered the 100%. 27 to 20Hz equates to going from 100% to 74.2%. A 34.8% increase becomes 25.8% decrease (100%-74.2%)

Fun with numbers,

David

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Hi Carl,

Spider compliance or stiffness numbers would be useful, but the parts suppliers rarely gave them because they rarely measured them (I think some didn't know how). Some of the better suppliers would give 50 gram deflection. Ask them about linearity curves and you still get a blank stare. I think most driver designers would just try a few different samples, and if something was close ask for more resin (stiffer) or less resin (softer).

...........

David

I suspect the driver suppliers rarely measure finished drivers as well once the first few production samples roll off the line and are approved.

If true, the last sentence above is really dissapointing. The science of driver design (expecially woofers), I suspect, has advanced to the stage where pretty much all of a woofer can be modeled quite accurately on a computer before a single piece of hardware is assembled. A vital component of that process is the compliance of the spider and surround and associated performance specifications. So, I find it hard to believe a designer these days tries a few samples and goes with what's 'close enough'. Maybe 10-40 years ago, but not now. Both Nu-Way and Loudspeaker components, LLC web sites list a myriad of combinations of foam, fabric spiders and treatments but don't get into any discussion or mention of compliance/stiffness. Must be highly secretive stuff :unsure: But WHY any more so than T/S parameters?

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