Results 1 to 12 of 12

Thread: Re-magnetizing loudspeaker magnets ?

  1. #1
    Senior Member Flodstroem's Avatar
    Join Date
    May 2004
    Location
    Norway
    Posts
    401

    Post Re-magnetizing loudspeaker magnets ?

    Some time ago there was a discussion about de-magnetizing of loudspeaker magnets and why the get de-magnetized and how to.... and where to....etc..

    This is what I found out about magnets and why they become weaker:
    (this is a very oversimplified text)

    When making the magnetic materials:
    During formation of the material the atoms align themselves in the same magnetic direction over a small volume called a "domain". Each domain is a sort of a tiny small magnet by itself, then, but its field is canceled out at a distance by that of other domains, so that the net effect at a distance is zero.
    In a magnetic material (eg. Alnico), when an exterior magnetic field is applied all the domain tend to ALIGN with it. Some domains align more easily than others and so the resulting magnetic moment depends on how STRONG the applied field is, up until all possible domain are aligned (material is then saturated).

    Now, when the exterior magnetic field is removed these domains are held in aligment itselvf, but some of the most weakly domains will collapse back to randomly realign and then weakened the magnetic material in some degree.

    This weaknes is caused by high temperatures, by shock or BY TIME.
    So, that´s the oversimplified explanation to why magnets get weaker by time. Of course, they can again get re-align when applying a new, strong enough exterior magnetic field.

    This information (oversimplified) came from Joseph J Stupak Jr. at the Oersted Tech. Corp. who was the author of the document:
    "Methods of Magnetizing Permanent Magnets". EMCW Coil Winding Show
    1 october-2 november 2000, Cincinnati, Ohio. (E-mail oersted@oersted.com)

    When speaking to a re-magnetizing company i Stockholm, Sweden, they told me that when re-magnetizing loudspeaker magnets their experience was they become 10-15% stronger. These magnets was from the 50´s and 60´s
    mostly.

    Place to re-magnetize loudspeaker magnets in Sweden as far as I know is HYAB in Stockholm. Talk to Mr Lars Aakesson on phone number
    +46 8 261 080.

    Best regards
    Flodstroem
    Flodstroem

  2. #2
    pangea
    Guest

    Tack för tipset!!!

    Hej!

    Tack för tipset, har ett par 2441'or som behöver ommagnetiseras.

    /Roland

  3. #3
    Senior Member
    Join Date
    Sep 2006
    Location
    San Jose
    Posts
    847
    Some where in all these many bits of data it is addressed to the level of certain drivers whos vc's would never be able to demag them and certain models that when subjected to certain types of use are almost guaranteed to loose some magnetism. I claim no expertise but I know it is in here straight from JBL.

  4. #4
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Y'all need to make a little effort to find this stuff:

    http://audioheritage.org/vbulletin/s...522#post110522

  5. #5
    RIP 2013 Rolf's Avatar
    Join Date
    Apr 2003
    Location
    Skien, Norway
    Posts
    2,322
    Quote Originally Posted by pangea View Post
    Hej!

    Tack för tipset, har ett par 2441'or som behöver ommagnetiseras.

    /Roland
    I do not believe any mid or hi frq driver need re-magnetizing. They can not be driven that hard ... maybe in a pro setup for concerts, but not in a private home.

  6. #6
    Senior Member Flodstroem's Avatar
    Join Date
    May 2004
    Location
    Norway
    Posts
    401

    Practical measurements to older Alnico driver

    Hello

    For about a couple of years ago I bought a Gaussmeter and that gave me the opportunity to do some measurements to older Alnico-V drivers
    (what facts do you refer to when you say they probably dont need re-gaussing?)

    I have done some measurements on large compression drivers for a while now and my impression is that most of them has been weaker regardless of if they have been used as a home (theater) driver or used in professional environments. Most of them had a measurable loss by the value of 5-15% of their magnetic field strength and for a 2440 it means the value of the loss lies between 1.1 to 3.2 kG

    Measurement methods is based on a ±0.5 % accuracy professional F.W.Bell probe and instrument.

    It means the results of the measurement of for example an 2440 could differ between + or - 107.5 Gauss if the driver still holds the specified JBL data sheet value (21.5kG/2.15T).

    When measuring those drivers I had to take in to account the fact that the magnetic gap isnt exact symmetric . the top-plate has been milled in two places for to give space for the two connecting leads (to the V.C.) and the measurement must be done in several places for to get a right value (a sampled value)

    Another interesting thing is the fact that I have never seen a ferrite magnetic speaker measure anything else than the specified JBL data sheet value.

    Hope this helps a little

    Regards
    Flodstroem

  7. #7
    Senior Member Guido's Avatar
    Join Date
    Apr 2003
    Location
    Germany
    Posts
    2,506
    Quote Originally Posted by Flodstroem View Post
    Most of them had a measurable loss by the value of 5-15% of their magnetic field strength and for a 2440 it means the value of the loss lies between 1.1 to 3.2 kG
    Regards
    I doubt they where ever 21,5 kGauss.

    This is serial variance.

  8. #8
    RIP 2013 Rolf's Avatar
    Join Date
    Apr 2003
    Location
    Skien, Norway
    Posts
    2,322
    Quote Originally Posted by Flodstroem View Post
    Hello

    (what facts do you refer to when you say they probably dont need re-gaussing?)

    Regards
    No facts Flodstroem, personal experience with a pair of 1979 model 4333A's. Got them all checked professionally, and only the woofers needed re mag. Also several members on this forum has told me that those driver's probably never will be "stressed", not like a Alnico woofer who is much easier to stress.

  9. #9
    Senior Member Guido's Avatar
    Join Date
    Apr 2003
    Location
    Germany
    Posts
    2,506
    Quote Originally Posted by Rolf View Post
    Also several members on this forum has told me that those driver's probably never will be "stressed", not like a Alnico woofer who is much easier to stress.
    The portion of the overall power going to the Mid's and "burned" there just isn't enough to demag them.

    What could happen is that the are dropped! But this won't cause a 2kGauss loss only.

  10. #10
    Senior Member Storm's Avatar
    Join Date
    Jun 2006
    Location
    Phoenix, Arizona
    Posts
    1,266
    So, is Alnico better?

    -Storm

  11. #11
    RIP 2011 Zilch's Avatar
    Join Date
    Jun 2004
    Location
    Berkeley, CA
    Posts
    10,036
    Quote Originally Posted by Storm View Post
    So, is Alnico better?
    With respect to demagnetization, Alnico is worse, generally speaking.

    http://audioheritage.org/vbulletin/s...ead.php?t=4031

  12. #12
    Senior Member Steve Schell's Avatar
    Join Date
    Jun 2003
    Location
    Long Beach, CA
    Posts
    1,474
    I have a small handheld gaussmeter and have checked many drivers over the years. The 2440s and 2441s I have checked usually look good, even beat up road dogs. My meter just blinks over 20 kilogauss, and most of these drivers make it blink. The lowest I've seen with them is about 18kg.

    Different story with Altec 288s. Many old Cs and Ds measure only 13 to 15 kilogauss. Never measured any later big pot alnico Gs or Hs, but the ceramic Ks I have checked make the meter blink.

    Jim Dickinson wrote articles years ago on servicing and upgrading the small and large VOTT systems. He describes a failure mode where the motors slowly weaken over time, and as they do so they begin to blow diaphragms more and more often. As the driver sensitivity falls off the system is fed more power to compensate, leading to the failures.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. JBL LX155 (3-Way Loudspeaker)
    By Oli_B in forum Lansing Product Technical Help
    Replies: 2
    Last Post: 02-28-2017, 08:47 AM
  2. Group Delayed Speakers, anyone?
    By MJC in forum General Audio Discussion
    Replies: 36
    Last Post: 10-30-2010, 12:32 PM
  3. Building a better loudspeaker than the K2 s9500
    By Niklas Nord in forum Lansing Product General Information
    Replies: 20
    Last Post: 10-28-2004, 09:22 AM
  4. Fundraising for Project May Loudspeaker project
    By mikebake in forum Public Forum - Announcements
    Replies: 0
    Last Post: 02-10-2004, 07:15 AM
  5. The first real JBL 130 loudspeaker?
    By martinleewin in forum Lansing Product General Information
    Replies: 27
    Last Post: 06-24-2003, 07:16 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •