I was just reading a thread on AA where someone mentioned that one should rotate bass drivers to prevent the voice coils from sagging. It sounds like a problem for geriatric speakers, but is there any truth to this?
Not urban legend at all! Anyone with enough years in speaker restoration will have seen this problem. I have a variation on the sag problem: A friend and I are restoring a pair of Fulton J Modular speakers. The 12" subwoofer drivers face downward, firing into a slot. 30+ years of continuous 1G force has pulled the cones downward by maybe 3/16 of an inch, even when the drivers are held facing "forward" or "upward", resulting in measurable changes to the Fs and Qts, compared to a new old stock driver. And this shift has also reduced the symmetrical Xmax. The cones are still centered properly and there is no voice coil rub. We plan to "exercise" the drivers for a few days on a table top with an amp driving them at high excursion at the free-air resonant frequency (to keep power in the voice coil low). Hopefully this will loosen the spider and pleated-cloth surround and recenter the cone at the mid-point of the excursion range. If not, then scary combinations of heat and amateur chemistry will be attempted. Any advice based on real experience would be appreciated.
After looking at it and reading it again, they may also be concerned that the person doesn't push the former all the way down, and bottom it out especially more so, since they use shims.
The article should be more specific. They should state something in the order that the sag could cause it to be off center. I think their main concern is to get the person doing it with no knowledge, good even clearance around the coil. If they're referring to upward firing woofers, I can't think of any companies that do this. I wasn't sure if you knew that I understand how the speakers are designed. I think we both know what is going on.
I'm just talking from my personal experience, as a speaker reconer. The article YOU cited, was referring to the spider sagging towards the magnet structure(not the frame), and states exactly that. Like I said, I don't doubt that could occur, do to age, but- my encounters have been a result of damage caused by Florida's high humidity.
Rodman, if your referring to me, the woofer cone is pulling on the spider causing the distortion in its (spider) shape. If there was no cone weight on it, I doubt the spider would sag in its lifetime. The midrange and tweeter with their light cones gives anyone an idea, the spider itself is not the problem. That's self explanatory with the woofers weight being the major cause of this problem. FWIW, there's still a problem with sagging from the weight of the cone. Some have it more than others. Larger gaps allow for more error. Also lighter cones, stiffer surrounds, and stiffer spiders along with other design factors come into play for the failure rate. Again, the problem still exists for some.
Step Six is speaking of the spider sagging toward the magnet(read it again). Something that I ran into frequently, with older woofers, when in the reconing business. Just the opposite of what one would get, in a downward firing woofer. That would occur quite often, in the humid Florida climate. The moisture would weaken the sizing(stiffening agent) in the spider, and allow the pleats to distort/warp. On occasion; I'd run into a woofer, with a pleated cloth surround, that had the same problem, and a rubbing voice coil. It's not hard to see how the same thing might happen, do to age.
Supposedly, Lowthers are famous for this. The other thing is picking up dirt and all manner of other stuff in their voice coils, and needing to both rotate and clean out the gunk.
So far, with my Hornings, I've not done either. Then again, with their circumcised drivers, maybe it's not necessary? Or, perhaps, I'm just whistling through the graveyard...
Step six in this talks about sag in older drivers. I don't think it's their imagination, or something they dreamed up. [http://www.decware.com/newsite/refoam.htm]
If you check the net, there are a lot of people that heard what they describe as buzzing and was told to rotate their woofers, and they had good results after doing it.
This isn't a theory, it's a fact. It's not like a cable type of discussion where people have different results, without any measurements to back it up. It's just like people put their classic cars on blocks to keep the springs from sagging. The spider and surrounds on the speaker are it's suspension. And gravity does pull them down, whether their forward, or downward firing. There is evidence of it around the world, not just theory. It's not something that comes out of the minds of bored audiophiles. I imagine if you ask any one that recones vintage speakers, they will probably have seen one that has damage from the voice coil, or former rubbing, do to sag. I have seen it visually, and felt it moving the cones.
You might notice; that article, and it's accompanying remedies, addresses downward facing woofers only(as in some subwoofers). Many of which have heavy cones and/or long excursion(some double) coils. The effect of that type of sag would displace the voice coil, from it's centered/at rest position.
Viridian, I beg to differ. Unless I misunderstood you, I was under the impression you made it personal with your comment; because you and others in your club never experienced woofer sag that it must be a myth. Isn't that what an urban legend is? If that is the case then you were implying my explanation was BS.
Viridian, Your use of the term ad hominem was inappropriate. I am sharing with this forum what I know to be true through experience for the benefit of interested parties. Just because you have not experienced this with your antique speakers does not mean it doesn't exist.
VMPS(still in business)used to weight their sub drivers in the old days and recommended they be rotated occasionally. I don't know of any other manufacturer that did this.
Viridian, The surround on your Klipsch speakers is a continuation of the cone material. Also, the woofer in your speaker dictates the sensitivity of the speaker, so with 102db sensitivity the woofer cone is very light weight. It's not very likely to sag like some of the heavier materials, but if it does you know what to do.
It is true that some drivers need to be rotated 180 degrees due to becoming off centered. It has to do with the weight of the driver putting pressure on the surround over a long period of time. This is less likely to happen with smaller lighter drivers than with larger heavier drivers and the surround material has a lot to do with it. For example, a 12" poly cone woofer with an inverted butal surround would most likely benefit from periodic rotation.
If your drivers need to be rotated you should be able to see a difference in the width of the surround by comparing the top to the bottom.
Some of the weighted drivers from VMPS needed to be rotated occasionally. It's the cone that sags not the voice coil. I don't know if they are still around.
Yes, the talk on the net is for real. They used that dope on midranges and maybe even tweeters. Some said it also was for dampening. It always seems to stay shiny and new looking.
The talk about the voice coil rubbing happens too. I don't recall the brands, but they must of had heavy woofers. These were all vintage that I know of, but not all brands. The spider keeps the voice coil centered, but the weight of the cones on the surround, and spider will let the gap get tighter. I've felt older speaker cone movement that was tight do to this. They would scape on the one side only, that was apparently do to sag, if you carefully moved them. I rotated them before selling or trading them.
With newer materials, there doesn't seem to be any problem anymore, but who knows when they get older.
There were and maybe still are speakers that have a coated surround that is usually made out of cloth. This was done a lot in the '60s to '70s and maybe before or after that time period. If you see some old woofers from around then, that has a liquid looking coating on the surround, it usually sags to the bottom. Those are probably what they are referring to. The owners of those usually do rotate them them a half a turn to put the heavy part at the top, and eventually the liquid will sag down again. It's sort of a super thick honey looking compound. This is done on some accordion surround edges of all paper/wood pulp cones to help keep them flexible and to help prevent cracking or tearing from all the movement they do.
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