Another magnetic shielding question

[Century L100]

I've been reading posts having to do with non-magnetically shielded speakers used close to CRT direct view TV sets and I have a question that I haven't seen answered, at least not directly. Namely: If you have a speaker located close to your TV -- about 15" away -- and you don't see any color distortion, should you just not worry about the issue?

That happens to be the case with my oldie-but-goody JBL L40s, one of which has to go near the TV. The picture looks just fine with the speaker located immediately to one side. Because I'm into music only, the JBLs are almost never in use while the TV is turned on, though.

The tech data in the owners manual quotes the magnet in the 10" woofer at 2.8 lb and 8500 gauss, and the 1" dome tweeter has a 1.5 lb magnet and 14,000 gauss. I'm not an engineer, but that sounds like a pretty powerful magnetic field!

This is my first post on this forum, so excuse if I'm in the wrong place. Thanks in advance for any advice

[jneutron]

I've been reading posts having to do with non-magnetically shielded speakers used close to CRT direct view TV sets and I have a question that I haven't seen answered, at least not directly. Namely: If you have a speaker located close to your TV -- about 15" away -- and you don't see any color distortion, should you just not worry about the issue?

That happens to be the case with my oldie-but-goody JBL L40s, one of which has to go near the TV. The picture looks just fine with the speaker located immediately to one side. Because I'm into music only, the JBLs are almost never in use while the TV is turned on, though.

The tech data in the owners manual quotes the magnet in the 10" woofer at 2.8 lb and 8500 gauss, and the 1" dome tweeter has a 1.5 lb magnet and 14,000 gauss. I'm not an engineer, but that sounds like a pretty powerful magnetic field!

This is my first post on this forum, so excuse if I'm in the wrong place. Thanks in advance for any advice
The data provided in the manual is the flux strength within the gap of the speaker, that is, the field strength where the wire of the voice coil is to do it's work. That is not necessarily the measure of how much stray field there is outside of the structure. Sometimes you will find a speaker manu specifying BL product. B is the field strength, L is the length of wire on the voice coil, and the product tells one how much force the voice coil will exert as a result of a current. The earth's field is about half gauss, one tesla (T) is 10,000 gauss.

The .85 T and 1.4 T gap strength is not bad, normal metals start to lose their ability to channel the field at about 1 to 1.5 Tesla, meaning more starts to spash on the outside of the magnetic circuit. Law of diminishing returns. They are strong fields, but because they are confined to the voice coil gap, are not dangerous.

As for the TV. Relax, if you see no color change, don't worry about it. Eventually, with time (months, years), the shadow mask could start to magnetize, but most likely not.

If in the event it does, just buy a demag coil and use it once a year or so..Note, many tv's (that still work) have a degaussing coil built into the set. Most telling is the louder hum at startup, my tv does this every turn on.

For a science show we give in the summer, we put a neodymium magnet on the face of a 39 inch TV to explain mag fields and charged particle beams, that magnet is so strong (>2 Tesla at the face), that it creates a four inch black circle where the beam cannot get to because of how strongly it is bent. THAT is incredibly strong. And the entire face of the tv becomes magnetized beyond belief..Yet, two seconds with the degauss coil clears it up completely.

I purchased mine online, it is a GC Electronics Professional degaussing coil, catalog number 9317. Sorry, I do not have a link..

Cheers, John

[Century L100]

Wow...thanks for the quick and very informative reply!

The TV is a newer JVC, about 3 years old. And you're right, there is a very noticeable momentary hum when I turn it on. I always wondered what caused that.

[jocko_nc]

A couple of months back, I gave my kid the magnet assemblies from a couple of old woofers I had laying around and had torn apart. I told him specifically: Do not stick these on the car. Do not put anywhere near the televisions.

Next day, what do I find? The television is a rainbow mess. I was pissed. Fortunately, Sony degausses during startup. These things were deadly at two feet.

It must run in the family... A week later I was trying to ID some old tweeters, I put them on the desk and went online. D'oh! Right in front of the monitor!

jocko

[jneutron]

Wow...thanks for the quick and very informative reply!

The TV is a newer JVC, about 3 years old. And you're right, there is a very noticeable momentary hum when I turn it on. I always wondered what caused that.
Glad to help...I work with magnets as well as play with speakers. Someday I'm sure you'll return the favor.

Cheers, John

[jneutron]

A couple of months back, I gave my kid the magnet assemblies from a couple of old woofers I had laying around and had torn apart. I told him specifically: Do not stick these on the car. Do not put anywhere near the televisions.

Next day, what do I find? The television is a rainbow mess. I was pissed. Fortunately, Sony degausses during startup. These things were deadly at two feet.

It must run in the family... A week later I was trying to ID some old tweeters, I put them on the desk and went online. D'oh! Right in front of the monitor!

jocko
Same here, with a Selenium D205TI...doh!

At least your kids didn't use a neomydium magnet to see if your work ID badge (with the mag stripe) would stick to the magnet..guess I won't be hittin the tunnel anytime soon..:( (luckily, my credit cards were in my wallet (well, maybe unlucky))...

Cheers, John

[wayner86]

No naturally occuring magnet, inc. neodymium will corrupt a mag swipe/credit card, they can withstand well over 20k gauss field strength.

Myth Busted!

:p

[jneutron]

No naturally occuring magnet, inc. neodymium will corrupt a mag swipe/credit card, they can withstand well over 20k gauss field strength.

Myth Busted!

:pNeat. Everything I have found does not exactly support your statement..:confused: I have to wonder about that mythbuster program, as I am not sure of the level of scientific accuracy a program that has as a target audience... the general public. Science is way too boring..

These links say don't put a refrigerator magnet in the same pocket as a credit card, lest you de-mag them..
http://van.hep.uiuc.edu/van/qa/section/Electricity_and_Magnets/Magnets/20021023203138.htm

http://money.howstuffworks.com/question503.htm

They could, of course, simply be feeding the myth, so I searched for the manufacturers of the write heads for the stripe writers..

Here's a link to a manufacturer of writing heads for magnetic stripes.

http://www.brushindustries.com/dg2_3.htm

Note that they talk of heads which can encode high coercivity material, which they say range up to 3500 oersted's. 1 oersted is one gauss in air, it is actually equal to 1/mu..mu being magnetic permeability. I could not find permeability of the mag stripe media, but that is of no significance.. In air, 3500 oersteds is .35 Tesla, if the stripe has higher permeability than air (it cannot have less by definition), that only makes the matter worse. The issue of course, is how much field is required to flip the domains of 3500 material, or 2750 material, or what is currently in use..as for the mythbuster guys, how did they verify the field strength they used?

So, From what I have found, mythbusters is'nt exactly floating my boat..


When I go to a checkout counter, the rf coil used to zap the anti-theft widget in the tags is marked (sometimes very prominently) do not put credit cards there, as they will be erased..

Actually, given the correct materials, it is indeed possible to exceed 2, 3, even 4 tesla, but it requires some rare earth alloys to prevent saturation. We are working some designs which require that kind of stuff, but it is very expensive, even for my workplace.

But fridge magnets do not come close to the level a neo magnet can achieve. Yet, they are said to be an issue for cards..

If you have any information in support of your myth busting, or rather, how they supported their trials scientifically, I would enjoy reading it..

Thanks

Cheers, John
ps..when I first wrote this response, I didn't look at the subject line, didn't realize you invoked mb guys..doh! major edit when I realized you were simply conveying what you saw.

[Florian]

Hey we can test this! I have infront of me aprox. 12ft high and 8ft wide array of pure neodynium magnets. Send me your CC cards and work ID's and ill swipe them in a listening session and send them back to you. :-)

[wayner86]

I didn't intend for that post to be taken seriously, sorry guys, i thought the mythbusters reference and the absurdly high gauss measurment would have been laughed at :p

(I believe the actual reading was 1k gauss @ 1second exposure before the card was wiped)

[jneutron]

I didn't intend for that post to be taken seriously, sorry guys, i thought the mythbusters reference and the absurdly high gauss measurment would have been laughed at :p

(I believe the actual reading was 1k gauss @ 1second exposure before the card was wiped)
Didn't you realize that you were talking to a geek????

Geeks don't understand humor.. we don't get it!!!

If you are going to inject humor that I will realize and understand, you are going to have to type much slower...:p

I did have fun though, looking up the card reader stuff, you gave me something to think about..thanks..

And, thanks for the info...1kgauss wiped em?? Wow. In my research, I have a feeling that the newer stripe materials with the higher numbers are meant to stop the sensitivity of the stripes to external wiping by accident.

Oh, and I do enjoy that program when I am lucky enough to see it..

Cheers, John

[jocko_nc]

Wiped 'em, huh?

Hmmmm... I think I need to stash those woofer mags in the lady's wallet for a while...

jocko

[wayner86]

my old man gave me some rare earth neodym discs awhile back, and these little buggers are strong, i don't know what to do with them, besides walk by my tv with them in my pocket and watch the rainbows :)

in reality i have them hiding somwhere in my house where they can't cause any trouble, i actually hid them so well i can't find them :D