shielding questions

[eyedoc]

Good Morning everyone. Its time for school again (and yes, I am the student). I was not sure if I should post this in the technical help forum or the DIY forum, so moderators, feel free to put me in my place (or so to speak...).
I am interested in building/rebuilding a speaker for a center channel. Yes I am aware of all of the sonic and timbre issues, but my questions surround (no pun intended) the shielding needed to set close to the TV. So my questions are;

1) What material is used for the shielding
2) Is it a specific material or is the actual drivers different
3) If the drivers are not different, can it be added to an existing speaker
4) If not, what do I use in the construction of a new cabinet.

Thank you in advance, and as always, your responses are appreciated.

Eyedoc

[Robh3606]

Too make a long story short, try to get shielded drivers. The real material to use is Mu Metal and it's expensive, needs to be annealed and degaussed after working it for it to work properly. We use a lot's of it where I work to shield certain areas of the "physics package" in a Rubidium Frequency Standard from the earths magnetic field. Even with us purchasing for thousands of units it's not cheap. Most speakers use bucking magnets and steel covers to shield them. The results you get can on your own can be real hit or miss. Some TV's are not as sensative as others. Do you have the TV now to see how close you can get unsheilded??? I have an 801C driver parked right on top of my 27" and get no inteference at all from this unsheilded driver. The only thing I can think of is the woofer and compression driver magnets are bucking each other. Good luck.

Rob:)

[briang]

Robh3606 is right. Mu metal is a good chioce, but tough to get for a consumer and very expensive.

If you get some and don't cut or bend it, you can get away with not annealing.

Plus it is easier to sheild the TV itself than a speaker (IMHO). Place the Mu metal foil sheets inside the TV case in the areas likely to be subject to the speakers magnetic field.

Otherwise shielding the speaker cabinet is another choice (tougher to do IMHO). Or a bucking magnet and steel can over the magnet can sheild the magnetic field.

It is not easy, but not impossible.

Good luck.

[eyedoc]

Well, that makes the project less fun...
Next questions
Will I have a problem using the 35TI tweeter as well? Is there a shielded driver that is similar in sound to a 116H-1 from an L60t? My fronts are L100t3, and while I like the match of an L60t, I am running into the obvious shielding problems.

[Robh3606]

Take a look at the Studio Series Centers. I used one with L80t3 and it sounded fine. See if you can try one of them from say Best Buy. They have money back no questions asked if I remember right. Save all the packaging materials and give one a whirl.

Rob:)

[eyedoc]

Hi Rob.

Thanks for the idea, but I am looking for something a little bolder. I have an SC305, that has been replaced by an L20t3 which gave a much more full center. The 305 was very thin and nasal. Next came the L60t that was an additional improvement, but that is when I ran into the shielding problems.

[Zilch]

Use this as an excuse to upgrade your TV.

The newer flat panels don't have this problem.... :p

[Robh3606]

Well try these from PE. What do you have to loose at this point

http://www.partsexpress.com/webpage.cfm?&WebPage_ID=3&ObjectGroup_ID=571&SO=2


Rob:)

[JuniorJBL]

Yeh I went this route when trying to find a solution to this problem then looked at the bucking magnets then.....

I bought a projector instead!:applaud:

[speakerdave]

This has been discussed a number of times. Mu metal comes up every time, but I don't think anyone we know has ever used it this way.

This problem is actually why I got into JBL. The older Alnico motors with cast-iron pots are self-shielding. Nowadays some neodymium driver motors or also self-shielding, but not all.

You can try glueing a bucking magnet to the back of any ferrite driver with an exposed magnet. I don't know why this does not screw up the magnetic field in the voice coil gap, but apparently it does not.

One member posted a while back that he put a soft iron or mild steel plate on top of his TV and then his center channel speaker on that and there was no problem. Ordinarily a magnetizable material in a steady magnetic field would gradually become magnetized and then no longer serve as a shield. Mild steel has a low coercivity index and is therefore easily magnetized. But that means it would also be easily demagnetized. When a TV is turned on it degausses its own screen. It could be that it would degauss a nearby mildly magnetized iron or steel plate also. Who knows?

Mr. Z. might have the best solution.

David

[Harryup]

The older Alnico motors with cast-iron pots are self-shielding. Nowadays some neodymium driver motors or also self-shielding, but not all.
David

I have tried 2441 and 2450 on top of my Loewe crt TV and that does not work. Even a shielded Synthesis center affect the TV a bit.