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]

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!

[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