Old magnets--weakened magnetism?

Schurkey

I was watching a Youtube video about making speaker drivers. At one point in the process, they shove the non-magnetized ceramic "magnet" into a machine (electromagnet) that, when energized, causes the ceramic to become magnetized.

One wonders how much magnetism has been lost from our 30+ year-old ceramic magnets. I have no idea how powerful those magnets were when new. Ceramic magnets are said to lose magnetic power very slowly...but it depends on what grade of ceramic was used, and the operating/storage conditions--heat, vibration, etc. all play a part. Which then makes me wonder what sort of process would rejuvenate them back to "full" "saturated" magnetic power--restoring efficiency and perhaps improving sound quality.

I can imagine a bigass coil of copper wire, and a "24-volt" battery charger as a high-current DC power supply. In fact, I don't see much info on the apparatus needed to "recharge" a ceramic magnet.

Anyone experimented with re-magnetizing a ceramic speaker magnet? Tips? Advice?

msg

Interesting. I found this:
https://www.canuckaudiomart.com/forum/viewtopic.php?t=43676

Looks like there are some services out there that do this. At first glance, I wonder, how does one determine much magnetism a driver magnet needs?

xschop

I ran into this when I was looking into vented pole-pieces...
I'm sure our vintage Polk's mags are all ferrite....

http://www.audioheritage.org/vbulletin/showthread.php?4031-The-Great-Alnico-Ferrite-Debate

Gardenstater

Interesting and I learned a lot. I wonder what a "copper shorting ring" would look like:

"Ferrite designs can equal or exceed an Alnico magnet’s flux stability with the addition of a copper shorting ring around the pole piece. With the use of vented cooling and heat sinking, you can manage the heat build-up on a ferrite driver to where it stays below the threshold of non-linear response. Finally with the addition of secondary magnets, you can shield the motor of a ferrite driver to the same degree as an internal ferrite equivalent."

PS: https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=5815587A&KC=A&FT=D&ND=3&date=19980929&DB=&locale=en_EP

SIHAB

my guess from the "summer of chemistry" was 10% max loss.
I found a page that states:
"Ferrite magnets can last for several years if it is properly used and cared for. Since ferrite magnets are permanent magnets, they will only lose less than 10 percent of their magnetism every 100 years."

I do remember remagnetizing stuff as a kid. I would guess one would
only need a larger donor magnet and time to revitalize a woofer.

mhardy6647

The issue is (much) more pronounced with AlNiCo magnets -- which are also pretty sensitive to mechanical shock.

Speaking of shock -- the apparatus used to recharge (AlNiCo) magnets is pretty impressive... and terrifying.

xschop

I've yet to see a Classic Polk MW that has the copper plated pole-piece that was mentioned in the old artcle/pamphlet.

Schurkey

msg wrote: »

I wonder, how does one determine much magnetism a driver magnet needs?

Assumption: They're magnetized to "saturation" 'cause there'd be no advantage to doing less.

msg

Oh, well, duh, that's a good point - the assumption that a magnet does not simply become ever more powerful with whatever the process for charging is, that its full strength be limit by its design.

xschop

msg wrote: »

Oh, well, duh, that's a good point - the assumption that a magnet does not simply become ever more powerful with whatever the process for charging is, that its full strength be limit by its design.

The exception being here on CP, is the natural laws of physics don't apply. If you can bend Ohms law to achieve your power needs, anything's possible.

SIHAB

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