Driver magnetic question

[Tom Loizeaux]

I just purchased a pair of JBL 2426 compression drivers and want to mount them in an existing cabinets with fiberglass horns. The cabinets are built with a steel plate, drilled for the JBL 1" compression drivers and is attached to the walls of the cabinet to support both the horn throat and the driver itself. When I began to attach the drivers, I felt a strong magnetic pull between the driver and the steel plate. The question is: Will mounting these drivers to the steel plates affect or reduce the performance of these drivers? Will it weaken the magnetic strength now...or over time?
Thanks.

Tom

[Tom Loizeaux]

Come on, Giskard, Scott, Bo, etc. There is a lot of expertise here in this forum with knowledge about JBL practical application and technical theory - that there must be some opinions on the affect, if any, of mounting a compression driver against a steel support plate.
Any info would be appreciated.
Thanks,

Tom

[4313B]

I would use a non-magnetic metal.

[Zilch]

Magnetism is not a consumable. If you're concerned, re-make the plate of aluminum or non-magnetic stainless steel....

[scott fitlin]

Originally posted by Tom Loizeaux
I just purchased a pair of JBL 2426 compression drivers and want to mount them in an existing cabinets with fiberglass horns. The cabinets are built with a steel plate, drilled for the JBL 1" compression drivers and is attached to the walls of the cabinet to support both the horn throat and the driver itself. When I began to attach the drivers, I felt a strong magnetic pull between the driver and the steel plate. The question is: Will mounting these drivers to the steel plates affect or reduce the performance of these drivers? Will it weaken the magnetic strength now...or over time?
Thanks.

Tom I definitely dont think the pull of the driver towards the steel plate will weaken the magnet over time!

Would it cause some type of weird magnetic interferences? I dont know, but maybe its possible!

If I had to guess, I would say I wouldnt want anything in my cabinet that the drivers magnet is attracted to, it might cause eddy currents, and that would negatively affect sonic performance!

[speakerdave]

Originally posted by Giskard
I would use a non-magnetic metal.

I would too, but it may not be necessary. In their alnico magnet structures JBL designed to focus the field in the gap by adding a cast pot "return." The poles of a flat ferrite magnet are on the flat faces. I can't say I have a clear picture of the field geometry of a ferrite-magnet driver, except that it seems obvious that the field of the pole on the back of the magnet is just allowed to run wild, which is the reason they are not so good next to TV's. If there is steel in this field it will to some extent redirect it. Whether that will have an effect on the pole used in the voice coil gap I don't know. However, when these magnets are "shielded" I think what they do is glue a bucking magnet onto the back of the speaker magnet. If that has no deliterious effect on the function of the speaker I don't see how a piece of steel would. But this is just my reasoning based on limited experience and extremely limited technical background.

David

[Tom Loizeaux]

Thanks for the replies so far. As I expected, the opionions are mixed.
To explain further; the cabinet has a steel plate that spans the width of the cabinet. The plate is about 6 inches tall and about 1/16" thick. It has the 1" hole for the driver opening and the three mounting holes on JBL centers. The plate bolts to the sides of the cabinet and the driver (JBL 2426H) mounts on one side of the plate and the fiberglass horn on the other. The three bolts go through the fiberglass horn mounting ring, pass through the plate and then thread into the driver. When I hold the driver up into position, I can feel the magnetic pull of the driver as it nears the plate.
These horn boxes were designed about 30 years ago and these boxes had older JBL 1" compression drivers mounted in them all those years. I don't know, of course, if those drivers were affected by this steel plate design, but I'm concerned as I'm putting in these new 2426Hs.
Pulling these plates out would be difficult and would require me to make plates, probably out of brass or stainless steel to exact dimensions to fit and align these componants correctly...not an easy task!

Is there a test I can do to determine if there is signigicant magnetic interference?
Thanks to all,

Tom

[scott fitlin]

I really dont think you will have magnetic interference problems. And i dont know how to test for magnetic problems though! What I do know, is that if you have serious magnetic interference, you would be able to hear it! For instance, electro-magnetic interference shows up audibly as hashy sound!

Drivers with ferrite magnets are placed in the same cabinet, in very close proximity to each other, and magnets, as well as other materials used in the drivers are attracted to each other, and they work without bad effects!

Install one 2426 and listen carefully. if it plays clear as a bell, you have no problem!

I definitely do not think the magnetic attraction of the driver to the steel will hurt the driver! Nor will it weakin it!

JBL drivers are engineered well enough, that I seriously doubt the steel plate will modulate the drivers flux field!

[scott fitlin]

So, I happen to be here at the bumper cars! My bumper cars have a formed piece of steel that is the stinger, the piece ontop of the cars trolley pole, that makes contact with the electric ceiling! I went into my shop and got a stinger and let it stick itself to the magnet of one of my 2404 tweeters. I went and put some music on, and went and listened, both with and without the 12in long piece of steel attached to the drivers magnet, through the rubber ring! I cannot hear a difference whatsoever!

Several times I stood right there, putting the steel blade on the magent, and taking it off, as well as listening with it on, and off while standing further out in the room! I cant hear a difference either way! Sounds the same with the steel stuck to the magnet as it does without!

:D

[boputnam]

Originally posted by Giskard
I would use a non-magnetic metal. Me too.

Sorry, Tom. I read your post and was perplexed. By itself it isn't a problem, I would think - that is, it will work.


I can feel the magnetic pull of the driver as it nears the plate. That is diffusing the magnetic field, certainly lessening it's focus in the gap, which is something JBL worked hard to get.

You are bleeding away magentic strength and over time it will matter. Note that JBL's baskets are aluminum alloy - they must know something. :hmm:

If possible, and it were me, I'd put some MDF in it's place.

[Robh3606]

Well JBL uses metal supports in the L200 on the Le-85 and 4350 for the 2440. They look to be steel rust and all so?????? Granted these are alnico but????? Does the aluminum horn between them break the circuit??

Rob:)

[Mr. Widget]

The L200 used a steel plate to secure the LE85. It was sandwiched between the driver and horn. I have no idea if having a steel bracket would affect the performance of a ferrite driver, but the ones that JBL used the steel plates with were the alnico models that are a magnetically shielded design. Does this make a difference? I don't know.

Widget

[Robh3606]

"It was sandwiched between the driver and horn."

Hello Widget

You sure it's not like the 2440 above?? That means it would be part of the horn throat. To be honest I thought I remember it that way too but it doesn't make sense to do it that way. Your right about the Alnico vs. Ferrite who knows??

Rob :)

[Mr. Widget]

Yeah, I'm sure. It is fairly thin, but they did put gaskets on both sides of the plate and sandwich it.

Widget

[Alex Lancaster]

The mounting brackets JBL sells for pro applications are steel, so who knows:confused:

[Robh3606]

Hey Tom

I just realized you are talking 2426's. The screw on adaptor plate is aluminum. You are not tapping the field directly but what ever stray magnetism is getting through the spacer and the airgap around the ferrite ring. You have at least a 1 inch gap there. The ferrite ring will start to "grab" the plate that close. I don't think you really have too much to worry about. If they can shield drivers with steel cans and bucking magnets I would think this is more benign than either. Is the spacer bar magnetized from a previous driver???

Rob:)

[Tom Loizeaux]

I just checked the support plates - and they are not magnetized!
The photo a few posts up is similar to my configuration, but my plates span the full height of the cabinet. The plate is sandwiched between the fiberglass horn and the 2426.
I'm looking in to having these plates made from brass, which would eliminate any possible magnetic question, but I won't get into that project if I can be assured that the steel won't affect these drivers.
Thanks for you input!

Tom

[Tom Loizeaux]

I spoke with Everett Watts at JBL Pro yesterday about this question and he assured me that the steel plates will not have an adverse affect on the performance of the compression drivers.

I think I will replace the plates with a non-ferrous material anyway.

Tom