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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Raul Ruiz on November 21, 2015, 02:02:41 am

Title: jbl 2268hpl demagnetized??
Post by: Raul Ruiz on November 21, 2015, 02:02:41 am
Hi anybody ever had a neodymium speaker demagnetize??

I sent a JBL 2268hpl 18" woofer to get reconed and they told me
it could not be reconed because the speaker was demagnetized.
And was told  the speaker was no good anymore. The speaker
had to be replaced. So I had to buy a brand new one.
Anyone ever had this happen? or knows what could of happened to the driver?
Title: Re: jbl 2268hpl demagnetized??
Post by: Bob Leonard on November 21, 2015, 07:16:54 am
Unlike ceramic or ALNICO magnets, Neo suffers from a severe loss of magnetic property when faced with extreme heat. Where other type magnets may "recover", neo will not. However, it takes extreme heat for this to happen and usually, but not always, the motor assembly will fail first. Have I seen this? Yes, but NOT with a JBL driver.
Title: Re: jbl 2268hpl demagnetized??
Post by: Dave Pluke on November 21, 2015, 10:24:48 am
Where other type magnets may "recover", neo will not.

This is interesting, and kind of disturbing, news to me.  And by "recover", do you mean it will not accept a re-magnetization?

Perhaps it's better to live with the weight of ceramic or alnico magnets?

Dave
Title: Re: jbl 2268hpl demagnetized??
Post by: Bob Leonard on November 21, 2015, 10:53:56 am
Read this.

https://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets
Title: Re: jbl 2268hpl demagnetized??
Post by: Dave Pluke on November 21, 2015, 11:47:48 am
Read this.

https://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets

Learn something new every day!

Dave
Title: Re: jbl 2268hpl demagnetized??
Post by: Art Welter on November 21, 2015, 01:41:29 pm
Hi anybody ever had a neodymium speaker demagnetize??

I sent a JBL 2268hpl 18" woofer to get reconed and they told me
it could not be reconed because the speaker was demagnetized.
And was told  the speaker was no good anymore. The speaker
had to be replaced. So I had to buy a brand new one.
Anyone ever had this happen? or knows what could of happened to the driver?
Raul,
Sounds like a your reconer is full of it. I have not heard of a reconer checking the flux density of a magnet structure prior to reconing. Ask him what it was (should be .6)
The Neo used by reputable loudspeaker manufacturers has a Curie temperature of 310 to 400C (590 to 752 F) . If you recall "Farenheit 451" is the temperature which paper burns, your speaker would likely have been in flames well before it was demagnetized.

Art
Title: Re: jbl 2268hpl demagnetized??
Post by: Jim Rutherford on November 21, 2015, 07:24:10 pm
Yes, but what is the upper temp for the speakers?  The reference paper above suggests that the temperature which they begin to weaken could be much lower than the Curie temp. 

JBL doesn't specify a max storage or run temperature.  I would presume that it is significantly high enough to not worry about?
Title: Re: jbl 2268hpl demagnetized??
Post by: Bob Leonard on November 22, 2015, 11:18:40 am
This is why Art is calling BS on the re-cone facility. In order to completely demagnetize those drivers it would take excessive heat for very, very long periods of time, or heat to the point the voice coils would burn in a pretty spectacular way. Voice coils get hot, very hot, but properly powered the should never heat to the point of combustion or total destruction. And by the way, the temp we're talking about is capable of making the magnets glow red.

I hadn't considered the re-cone facility might be pulling a fast one here, but now that I think about this I would have to wonder if the re-cone facility was authorized, and if not might have been looking to sell a driver, taking the easy way out.
Title: Re: jbl 2268hpl demagnetized??
Post by: Tim McCulloch on November 22, 2015, 11:52:10 am
This is why Art is calling BS on the re-cone facility. In order to completely demagnetize those drivers it would take excessive heat for very, very long periods of time, or heat to the point the voice coils would burn in a pretty spectacular way. Voice coils get hot, very hot, but properly powered the should never heat to the point of combustion or total destruction. And by the way, the temp we're talking about is capable of making the magnets glow red.

I hadn't considered the re-cone facility might be pulling a fast one here, but now that I think about this I would have to wonder if the re-cone facility was authorized, and if not might have been looking to sell a driver, taking the easy way out.

Raul doesn't say where he's located, but if he sends the speaker back to JBLfor reconing, they can easily remagnetize the motor.
Title: Re: jbl 2268hpl demagnetized??
Post by: Ivan Beaver on November 22, 2015, 01:11:26 pm
Raul doesn't say where he's located, but if he sends the speaker back to JBLfor reconing, they can easily remagnetize the motor.
Yeah-that is a quick simple 2 or 3 second job-IF you have the right tools.

And they could evaluate it and see IF it lost it magnetic strength and possibly why.