Recharging Magnets

[Guest palomar]

I remember seeing an advertisement a while back which offered to recharge speaker magnets back to their original strength. It sounds like a gimmick to me, as I didn't think that speaker magnets, being in what I assumed to be a closed magnetic circuit, would lose any significant strength over time, but does anyone know whether there is any merit to this? And if so, would certain types of magnets or magnet structures be more vulnerable (e.g. Alnico vs Ceramic).

Gary

[dynaco_dan]

>I remember seeing an advertisement a while back which offered

>to recharge speaker magnets back to their original strength.

>It sounds like a gimmick to me, as I didn't think that speaker

>magnets, being in what I assumed to be a closed magnetic

>circuit, would lose any significant strength over time, but

>does anyone know whether there is any merit to this? And if

>so, would certain types of magnets or magnet structures be

>more vulnerable (e.g. Alnico vs Ceramic).

>

>Gary

Hi Gary;

I am not an expert on this for sure.

I do believe, I read somewhere way back in time, heat will cause magnetism to decrease.

The heat from the voice coil, I do not believe, is high enough to have any effect on these magnets.

There is enough history of old classic speakers to have someone with more technical knowledge notice any deterioraion under noraml use.

Over the years, the only referrence to magnets in drivers, that I am aware of, was an upgrade article in, The Speaker Builder, regarding the AR-3A.

The writer said that some of the AR driver magnets were factory over magnetized and had to be reduced, he went on to say that they didn't know enough about magnetism at that time.

Maybe snake oil?

[Guest denmarkdrivers]

you could pick the magnet back up in 1000 years and it will still have polarity, that is correct isent it ?

maybe if the earth stops spining.

[Guest palomar]

I do remember reading a few years ago (whether this was correct or not I don't know) that one of the problems with trying to use neodymium magnets in woofers was that they were prone to demagntization with the high magnetic flux generated by woofer voice coils.

If this was true, it has obviously been solved because I now see quite a few woofers with neodymium magnets.

That was an interesting point you made about AR overcharging some of their magnets. I suppose even if there were a chance that magnets could lose a small amount of strength over time, I wouldn't want anyone going near them to try to 'charge' them, because the results of such a thing would be less bass due to overdamping.

[real1shepherd]

Sounds like 'snake oil' to me. I'm thinking the only way in principle to recharge a magnet would be with a stronger magnetic field, similar to magnetizing a screwdriver with a powerful magnet. In any event, even if possible, in a practical sense how would you measure or gauge a 'fully recharged driver'?

[Guest palomar]

I'm starting to think that maybe they charge by the minute, and when your wallet is empty, the magnets are considered fully charged!

Seriously, though, thank you all for your responses. It does indeed sound like a gimmick.

Gary

[rrcrain]

Magnets in general can be discharged/recharged or even have their polarity changed if done improperly. If done correctly, they use a machine that measures flux density and it will be displayed before and after the proceedure. I've done this myself on simple bar and "U" magnets long ago when I worked for a company that used several tons of magnets a year. So charging or recharging magnets in general is possible.

Concerning speaker magnets, I've absolutely no experience of any sort, but do know it's possible. A local shop has sent out a few JBL drivers to have the magnets 'remagnetized". what I don't know is how they sounded before and after the proceedure.

[real1shepherd]

"Magnets in general can be discharged/recharged or even have their polarity changed if done improperly. If done correctly, they use a machine that measures flux density and it will be displayed before and after the proceedure. I've done this myself on simple bar and "U" magnets long ago when I worked for a company that used several tons of magnets a year. So charging or recharging magnets in general is possible.

Concerning speaker magnets, I've absolutely no experience of any sort, but do know it's possible. A local shop has sent out a few JBL drivers to have the magnets 'remagnetized". what I don't know is how they sounded before and after the proceedure."

In never said it wasn't possible, I questioned how you would measure the results. Short of having the "machine" you mention I don't see how it can be done. If you're dealing with a reputable company, then great. Since I really am from Missouri, I say 'show me' the listening results for the $ spent.;-).

[rrcrain]

I'm a bit (ok a lot) skeptical that recharging a speaker magnet will magically improve its performance as you are. I do know that certain types of magnets do loose their strength over the years, but to what degree this will have on sound reproduction I've no clue. SPL levels should drop as the magnet discharges.

If done properly, they measure the flux density of the magnet before and after recharging it and can control the EM pulse used to recharge the magnet fairly precisely. The device I used a couple of times back in the late 80's was a tool a company engineer built specifically for the various magnets the company used at the time. I'm quite sure the process has come a long way since then as have magnet materials.

[Guest palomar]

I've been trying to learn what I can by sifting through a bunch of websites. The following sums up what I have seen so far. Keep in mind, however, that these are not published scientific reports. I've just tried to find some common ground among all of the different things I've read, so please consider it with that in mind.

It appears that Alnico magnets can possibly be somewhat demagnetized if the speaker is heavily overdriven. It seems as though it can be temporarily demagnetized a bit at high power levels - apparently resulting in a softer power compression than a comparable ceramic magnet - but that this is only temporary while the voice coil is being driven hard. But really hard overdriving may cause some permanent demagnetization. Ceramic magnets appear to be much harder to demagnetize, and from what I can gather so far, don't seem to be (as?) prone to this.

I'm guessing that most hi-fi speakers of the Alnico era would likely have had their voice coils destroyed before they could generate enough of a magnetic field to have any permanent effects. But perhaps some very high powered speakers (particularly those designed for professional use) had voice coils which could survive being heavily overdriven for a short time, to the point of causing some demagnetizing.

Gary

[soundminded]

Here's what I remember about magnets in general. Permanent magnets are made from magnetically "hard" material. That means that they are not easily magnetized but once magnetized are not easily demagnitized. This is the opposite of transformer iron which is called magnetically very soft. The properties are summarized in the B-H curve for the material which compares the coercive induction force in oersteds with the induced force I think in webers per meter. The curve looks like a double S curve with flat tops and bottoms indicating saturation and the space between the two lengths of the S indicating the hysterisis loss where the induction force is resisted by the induced force. Magnetic alloys are often described as having magnetic domains which can be rotated. That's what the induction force essentially does, rotate them and line them up. magnetically hard material like permanent magnets resist this force strongly whereas magnetically soft material like permalloy and supermalloy have little resistance to it and therefore make excellent transformer cores. Magnets cannot be destroyed by heat until you reach their "eutectic" temperature which for most alloys is I think about 600 to 700 degrees. Magnetism can also be reduced or destroyed by a sharp blow. Under normal circumstances magnets do not grow weaker over time. When I was in junior high school, I made a device for magnetizing hand tools like screwdrivers and pliers. You wound 200 turns of bell wire neatly around a cardboard tube which was used as a form. one end was connected to a screw terminal on a wooden board, the other soldered to one side of an appliance cord and insulated. The other side of the appliance cord was connected to another screw about an inch away from the first one also mounted on the board. Operation was simple. You put your tool inside the cardboard tube, attached a piece of aluminum foil across the screws, placed an inverted glass over it and plugged it in. The aluminum would melt almost instantly and you unplugged the cord and voila, the tool was magnetized. The reason is that the large inrush of current created a coercive magnetic force sufficient to rotate the domains and of course the large number of turns of wire multiplies it. When the current is interrupted, the magnetic polarity of the tool is whatever it was at that instant. And it worked perfectly. For awhile, every tool I had was magnetic. When I pulled one out of a drawer, lots of screws, washers, nuts, and things I couldn't describe came with it. It is undoubtedly the same principle used for all magnets on a much larger scale even today.

[dynaco_dan]

Great summary, well done.

[rrcrain]

What I have found mildly interesting is the storage of magnets. The experts recommend a "keeper" be placed on a magnet to prevent the loss of magnetic flux over time when storing magnets. horse shoe magnets have a bar placed across their poles, bar magnets are mated with a similar size bar magnet. Nothing is ever said about long term storage of magnets built into meter movements, small DC motors, speakers, Etc. The only sage advice that I can find regarding them is don't excessively heat them (as you mentioned) and don't subject them to mechanical shock.

You can actually magnetize an iron bar by whacking one end of it with a hammer and demagnetize a magnet the same way.

To me, magnetism borders on magic in spite of all of the theory I've had to digest and puke back up for exams. For better or for worse, I've forgotten a lot of the theory I once knew.

[soundminded]

>What I have found mildly interesting is the storage of

>magnets. The experts recommend a "keeper" be placed

>on a magnet to prevent the loss of magnetic flux over time

>when storing magnets. horse shoe magnets have a bar placed

>across their poles, bar magnets are mated with a similar size

>bar magnet. Nothing is ever said about long term storage of

>magnets built into meter movements, small DC motors, speakers,

>Etc. The only sage advice that I can find regarding them is

>don't excessively heat them (as you mentioned) and don't

>subject them to mechanical shock.

>

>You can actually magnetize an iron bar by whacking one end of

>it with a hammer and demagnetize a magnet the same way.

>

>To me, magnetism borders on magic in spite of all of the

>theory I've had to digest and puke back up for exams. For

>better or for worse, I've forgotten a lot of the theory I once

>knew.

It's back to Maxwell's equations for you. Yes, magnetism does sometimes seem like a mystery. I have never been satisfied that physicists have explained it convincingly. Of course they haven't explained much else convincingly to me either, especially gravity which I am convinced they don't understand much about at all.

As for putting a "keeper" across a magnet, this serves as a low reluctance path for the flux to pass through to complete the magnetic circuit. It's the equivalent of storing a battery with a short across the output although the energy levels are so low there is no perceptable heating.

The only way to measure magnetic flux that I am aware of is with a gauss meter. You'd have to compare measurements over time to see if a magnet truely lost its strength. It's not something you can just conjecture about which makes many of us easy prey for people with remagnetizing services to sell. A satruated iron core is about 15,000 gauss. I once built a gigantic electromagnet for a project at work. I had enormous coils used as field coils for big solenoids. I was regularly exposed to this strength of field with no ill effects. That was 35 years ago. I was therefore rather amused when industrial hygienists in one company I worked in became alarmed when some offices measured a few dozen milligauss. It occurred to me that if magnetic fields were truely dangerous, eletricians who work around them all the time would be dropping like flies.

[rrcrain]

There is a corrolation between strong alternating EM fields and Leukemia. How strong the corrolation is, I don't know. Exposure to intense DC EM fields can cause headaches and blurred vision until the body adjusts to it. I've first hand experience with the headaches and blurred vision. The other? Nothing to date and I've been around some rather large power generating equipment over 20 years ago.

Change of topic and way off topic for this board. Supposedly, the U.S. Navy's ships degaussing systems are derived from the failed Philadelphia experiment.

Anyway, concerning speaker magnets, I suspect if the magnet really did demagnetize, it was either cheap or grossly abused.

Maxwell's theory... I forgot it existed...

Richard Crain

[soundminded]

>There is a corrolation between strong alternating EM fields

>and Leukemia. How strong the corrolation is, I don't know.

You really have to be careful about some of these studies. They get a lot of publicity when they first appear and they it's very quiet when someone finds a flaw in it. For example, utilities may use defoliants and other chemicals to surpress vegitation around the bases of high tension towers. This and not the E/M fields may be the cause of illness.

>Exposure to intense DC EM fields can cause headaches and

>blurred vision until the body adjusts to it.

Whew, I was only exposed to strong AC fields.

As I said, electricians who work around this kind of equipment every day would be dropping like flies and the IBEW would be demanding studies and special precautions but having worked in the industry with a lot of trades people, I haven't heard or known anyone who has ever heard anything out of the ordinary.

On a theoretical basis, I can understand how strong electrical fields could cause ionization. I don't see how magnetic fields could cause any physiological effect. The only ferromagnetic atoms in the human body I can think of are iron in hemoglobin and cobalt in vitamin B12. At most, these fields would align the atoms or cause them to spin but I don't see that changing their chemical reactivity and therefore creating any biological consequences. Other magnetic effects like paramagnetism are much weaker, probably much too weak to worry about. The threshold of concern I've read about is 5000 Gauss.

[soundminded]

>>There is a corrolation between strong alternating EM

>fields

>>and Leukemia. How strong the corrolation is, I don't

>know.

>

I now recall that this study was done in Sweden and later discredited. At least that is what I think I recall :-)

[rrcrain]

It was back in the 80's that I originally heard of the corrolation and it was during a time of my life that I worked with and around high current switchgear. 450 VAC isn't low voltage but its not ultra high voltage either.

A fast search of the internet using google for leukemia and EMF returns a plethora of hits, some I wouldn't put an ounce of faith into, others try to answer the question with scientific data. I've not read even one to any great extent but the one I flash read seemed to suggest the answer is.... they aren't sure yet. We could conjecture about this until the cows came home and for a few decades longer. My personal opinion is simply that I'm not going to worry about it but neither am I going to camp out under high tension lines.

Regarding DC EMF. Not everyone can feel the effect let alone be bothered by it. I was suffering from massive headaches, migraines and the ilk at that time of my life and I think this amplified my sensitivity to the fields. We're talking a DC EMF field strong enough to alter a ships magnetic signature and render it almost null in the earths magnetic field.

[soundminded]

I think it's important to draw a distinction between electrical and magnetic fields. As I see it, strong electrical fields are extremely dangerous and can result in severe injury, illness, even death. That's because they can change molecular behavior, even the way atoms interact. If you recall from your study of chemistry, at an atomic level chemical activity is strictly an electrical phenomenon as it's been seen since the early 20th century. Electrophoresis is an example of how electrical fields separate protiens. Fortunately, electrical fields are relatively easy to shield. A faraday cage does the trick. It's a copper mesh or foil enclosure which is grounded. Many steel buildings are something like faraday cages which is why radio signals don't penetrate them well. I've built several shielded rooms for making precise measurements of electrical fields emitted from equipment. Magnetic fields are a different story. As I said above, I'm not convinced that at low levels they are at all dangerous. And a good thing too, they are very difficult to shield against. I've seen other people's naive attempts that failed. I worked on some rooms for shielding early electron microscopes and it's a very expensive project. Fortunately, today the manufacturer builds the shielding into the equipment.

BTW, the NEC classifies anything below 600 volts as low voltage. From 600 volts to 15,000 volts is medium voltage, and anything above 15,000 volts is high voltage.

[kkantor]

Could you give me a citation on that NEC information?

It was my understanding that the NEC refers to anything over 600V as "High Voltage." That is similar to what my quick Google results indicate, too. There's a really bizarre IEEE definition table:

http://www.iaei.org/subscriber/magazine/01...ello_table1.pdf

but I believe it only refers to overhead power transmission towers. It's a bit above what most of us audiophiles are used to... even the tube heads!

Thanks.

[soundminded]

>Could you give me a citation on that NEC information?

>

>It was my understanding that the NEC refers to anything over

>600V as "High Voltage." That is similar to what my

>quick Google results indicate, too. There's a really bizarre

>IEEE definition table:

>

>http://www.iaei.org/subscriber/magazine/01...ello_table1.pdf

>

>but I believe it only refers to overhead power transmission

>towers. It's a bit above what most of us audiophiles are used

>to... even the tube heads!

>

>Thanks.

Sorry, can't find an NEC handbook at the moment.

Your link has something strange about it

It lists as follows;

Low voltage; 0 to 1000 volts

Medium voltage; 2,400 volts to 72,500 volts

High voltage; 121,000 to 242,000 volts

Extra high voltage; 362,000 volts to 800,000 volts

Ultra high voltage; 1,000,000 volts and above

So in this classification, what happens to the in-between voltages although I know they are not all that common. Of course, the classifications are somewhat arbitrary but codes use them to define installation requirements and restrictions.

Consumer audio equipment signal circuits (excluding of course circuts to the power supplies) come under the NEC classification of low voltage power limited circuits of 50 volts or less which have special provisions giving greater flexibility and less severe installation requirements than other classifications.

[real1shepherd]

"We could conjecture about this until the cows came home and for a few decades longer. My personal opinion is simply that I'm not going to worry about it but neither am I going to camp out under high tension lines."

Amend to that one....the correlation between cancer and people living under mega power lines is pretty phenomenal-enough so that I can't image anyone making a conscious choice to do so or remain there. I once camped under some mega lines in CO because it was late and I couldn't find a better place in the dark to pitch tent. The damn things hummed and popped all night-broken glass insulators riddled the landscape.....didn't sleep well either.

[rrcrain]

YOu couldn't pay me enough to camp under high tension lines. There's enough radiation coming off the lines to cause a fourescent tube to glow and small animals refuse to live under them as well.