• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

A question for you re-foaming gurus

powerdoc

powerdoc

Active Member
Have Infinity Kappa 6 woofers half-way re-foamed, the cone side is in place. What am I supposed to hear when I hook up the speaker to an AF generator through a small amp if the cone isn't centered? Should I hear noticeable scratching? I've tried it and all I notice is that around 20 Hz the outside of the surround makes noise against the basket edge. At higher frequencies, e.g. 60 Hz, I don't hear the edge noise. Does this mean I'm centered? Usually, I just use the "push on it" method to see if I'm centered and so far it's worked flawlessly but since I really want to actually use this set of speakers I want this one to be as perfect as I can get it. Any info? Thanks!
 
Register to hide this ad
The point of the tone running through your speaker is that it sets up an oscillating magnetic field in the voice coil windings. This field interacts with the static field in the motor magnet. Because of these fields attracting and repelling equally in all directions, the voice coil is natually centered in the gap, hence, no scraping sound. Crude but effective explanation - got it?
 
If you use a whole bunch of clothes pins to temporarily clamp the foam around the edge to the speaker frame...at least one per inch or closer...spaced evenly around the perimeter, the buzzing of the loose foam will stop and you'll be able to hear if all is well as you run the test tones through it. If all is well, I usually lift a few pins at a time, maybe 4 inches worth, and glue that section. Replace clothespins to clamp and work your way around until it's all glued. Let dry at least overnight. Works for me...I'm batting 1000 so far.
 
Since you have a frequency generator hooked thru an amp to the speakers you can do what I did last time around. Glue the surround(s) and wait about 5 minutes till the glue gets tacky and starts to take hold. Then run the frequency thru the speaker for a few minutes (do one at a time). This will center the cone before the glue completely takes hold. Don't over do it meaning don't make the cone pulse too much so that the cone is jumping out and becoming un-glued, if you don't over do it this method works great for centering the cone. You should have no rub at all. I use a sine wave at between 10-20Hz and adjust the amplitude and period so that you can visually see the cone moving but not JUMPING out of the basket. You should be able to hear the signal you are sending to the speaker too since you have it going thru an amp. Just dont over do it and you will be fine with your setup.

I went one step further the last time I did it and hooked an Oscilloscope to the setup so I could see the wave form on the screen.That did impress my girl whom has an advanced degree in Physics. I don't get much flack now either when I'm looking at new (old) speakers, which is a very good thing.

Cheers,

Bob
 
I've been using clothes pins in the way Ed described and it's worked great for me. I'm 6-0 on my speakers re-foam jobs, had just one that I had to re-center while the glue was still tacky, that was it. But if I could suggest, and I know this is a low tech solution, why not just shim the coil? If you're real careful, you can cut the dust cap with a sharp x-acto knife, shim the coil up, do your thing with the surrounds then glue the cap back on so that you can't even tell it was cut. Takes a little extra time, but I haven't had a voice coil rub yet, and the cap looks fine when you're done.
 
Actually tried to shim speakers once; the problem is that your shims should be the air gap and the paper was too thin. I know this is supposed to be the the "gold standard" of re-foaming but it didn't work too well for me and I ended up doing it manually. If it's going to scratch, does the testing frequency matter?
 
Clothes pins. :thmbsp: Now that sounds MUCH easier than bar and pipe clamps.

I think I used those little neodymium magnets around the edge once. I don't recall what I had against them. Maybe that they jump at each other when given half a chance.
 
If the cone isn't centered at the VC end by the spider, you have other issues.

The real goal here is to ensure that when you glue the basket edge of the surround in place, the VC is not only centered in the VC gap, but also PARALLEL with the VC gap. Rocking the cone to one side or the other is what causes the rubbing.
 
powerdoc said:
Actually tried to shim speakers once; the problem is that your shims should be the air gap and the paper was too thin. I know this is supposed to be the the "gold standard" of re-foaming but it didn't work too well for me and I ended up doing it manually. If it's going to scratch, does the testing frequency matter?
Click to expand...

Card stock works well for those larger gaps.
 
I've refoamed scores of drivers and I don't shim them. With shims you still have the opportunity to get the surround on off center and pull the coil former over when the shim is removed. I especially caution you to avoid shimming the VC and clamping the edge of the surround. In that case there's no way to test centering and it's likely to get the surround off center.
Note that cones are not necessarily perfectly round, nor are surrounds. You can't align it visually and assume it's going to center the VC.
I glue the surround to the cone first, then use slow setting glue to attach the surround to the frame. I move the cone while the glue is setting and make sure it's not rubbing. The surround can be shifted while the glue is still soft. The viscosity of the glue will keep the surround in place while it sets.
There's no substitute for careful manual work in refoaming unless you have the factory jig on hand. Most important is prep. If the surfaces aren't free of old material and smooth you'll have headaches once you apply glue. Prep everything, then test-fit the surround and fool with cone alignment to see how it's going to go.
Sometimes you have to bend the tinsel leads to prevent them from pulling the cone off center while refoaming. I ran into this on Bose 901s especially. Gods help me if I ever have to refoam 36 tiny drivers in one weekend.
Prep carefully, test everything, go slow and stop if you're not sure it's right. Refoaming is simple in theory but requires patience and care to do consistently.
 
tensleep said:
The point of the tone running through your speaker is that it sets up an oscillating magnetic field in the voice coil windings. This field interacts with the static field in the motor magnet. Because of these fields attracting and repelling equally in all directions, the voice coil is natually centered in the gap, hence, no scraping sound. Crude but effective explanation - got it?
Click to expand...

I don't think it's accurate though. The magnetic field is radial (the flux lines all point inward to a central point), there is no force exerted by the magnetic field along the lines of flux. Force is always perpendicular to the flux.

What does happen is the vibration will tend to allow all radial forces exerted by the spider and surround to be equalised. Any slight unbalanced force caused by a skew surround will be stressed further, and IF there is a weak spot such as still-liquid glue, the structure will physically move to minimise that force.
 
Well I guess I need some clothes pins! Do they still sell those in grocery stores? The last ones we had were for my kids' art projects (the youngest is now 36).
 
powerdoc said:
Well I guess I need some clothes pins! Do they still sell those in grocery stores? The last ones we had were for my kids' art projects (the youngest is now 36).
Click to expand...

small binder clamps from an office supply store work fine as well. goodluck. I'm doing JBL LE10A foams tonight & it's a hassle...
 
Great advice here!

I am about to re-surround my Sound Dynamics 15's. That'd be my 1st re-surround attempt. So far the plan is:
I have to go with the shims and cant imagine any other way. The cones are too floppy without the surround in place and that voice coil tolerance is so very tight.

Based upon some advice here I'll shim, (TDK cassette case liner) glue the surround to the cone, let dry, glue it to the basket, let sit a few mins and run a low powered 20-40 Hz tone though it and listen carefully. Once satisfied with how it looks and sounds after several minutes of that, it'll sit for 24 hrs upside down with the gaskets in place to "clamp" the surround to the basket. Then it's glue the gaskets on, mount them in the cabs and play Five Days in July and I Can't Break Away at 0 - 120 Nikko watts in 60 seconds as the final test. If they pass that, it's been done right and I'm a happy speaker guy.

And of course before the final test the voice coil will be vacuumed and blown out. Once the speakers have passed all tests and are clean, the dust cap will be glued on.

Good fantasy but I hope it works out and the 15s' are back pumping out those sounds.
 
steerpike2 said:
I don't think it's accurate though. The magnetic field is radial (the flux lines all point inward to a central point), there is no force exerted by the magnetic field along the lines of flux. Force is always perpendicular to the flux.

What does happen is the vibration will tend to allow all radial forces exerted by the spider and surround to be equalised. Any slight unbalanced force caused by a skew surround will be stressed further, and IF there is a weak spot such as still-liquid glue, the structure will physically move to minimise that force.
Click to expand...

You are correct regarding the flux field produced by the magnet; it is static,with the lines encircling the voice coil gap as illustrated in this Klipsch low frequency driver flux diagram:
1221508.ashx


However, the field produced around the voice coil is dynamic in response to the current it carries. This current varies, changing the voice coil field, which then interacts with the static field in the voice coil gap. Since we are injecting an arbitrary frequency of, say 100hz, then the field will grow as the signal amplitude rises to peak. At peak, the voice coil field will be repelled out of the gap, one direction or the other, depending on polarity, by the static gap field.

As the signal passes peak and heads towards zero, the voice coil field collapses, allowing the voice coil to relax back to it's neutral position within the gap. The process repeats itself as the sine wave of the signal heads toward peak at the opposite polarity, deflecting the voice coil in the other direction.

The voice coil field is actually oscillating (maybe pulsing might provide a better illustration) in response to the 100hz sine wave we are injecting.

I hope this helps and doesn't sidetrack the op's original intent for this thread. This stuff is fascinating to me and I appreciate all the insight and sharing.
 
Sam Cogley said:
Card stock works well for those larger gaps.
Click to expand...
I've not an expert by any means, 2 sets of speakers refoamed, but the shim method worked good for me. Perhaps a good method for the novice or the odd Luddite. And don't worry about cutting your VC dust covers, no one is going to see them that often. Unless you're an exhibitionist and just like showing off your "cones".
 
I finished the speakers and they sound great! Most of the time it took was spent cleaning and preparing them. I did use shims and replaced the dust caps with new ones. I am very pleased with my 1st attempt. So pleased I did the 10" out of one of my Pioneer HPM-60's using the same method and it worked out great as well.

Since I've run out of space for attachments, a few pictures are in my profile album, here: http://www.audiokarma.org/forums/album.php?albumid=406
 
powerdoc said:
Have Infinity Kappa 6 woofers half-way re-foamed, the cone side is in place. What am I supposed to hear when I hook up the speaker to an AF generator through a small amp if the cone isn't centered? Should I hear noticeable scratching? I've tried it and all I notice is that around 20 Hz the outside of the surround makes noise against the basket edge. At higher frequencies, e.g. 60 Hz, I don't hear the edge noise. Does this mean I'm centered? Usually, I just use the "push on it" method to see if I'm centered and so far it's worked flawlessly but since I really want to actually use this set of speakers I want this one to be as perfect as I can get it. Any info? Thanks!
Click to expand...

What is used to generate the tone? (I've heard of "tone generators" used for troubleshooting phone lines--is it something similar?)

Thanks.

SubLover
 
Best thread on refoaming I have seen, thanks!

Bauhausler what is the most problematic in refoaming the LE-10s? I am thinking about refoaming my LE-14as in my JBL S99s and any advice you could offer would be much appreciated. I have refoamed smaller Avents and AR-5s and that was by feel and they came out fine, but I understand that the JBL with its narrower voice coil gap is quite a bit different and touchier than those speakers. If I do try it I'll get the kit with the 50 hz tone cd , anything else I should know?
 
You must log in or register to reply here.
Back
Top Bottom