First Timer with Lots-O-Questions

[husserm]

I am a first time poster on this or any forum. I have been turned on to this site by my buddy and have been reading it for a while...you guys know your sh...stuff .

But any way I used to be a car audio installer, and have recently purchased a house and am getting ready to build an entertainment center in place of an ugly fake fire place. I need advice on how to kill the magnetic field of my speakers that are screwing with my TV. I have heard of this stuff called magnetshield http://www.lessemf.com/mag-shld.html
and I am not sure if it will work. What would ya'll do?

I'm sorry but I am not at home so I don't have the model of the equipment or pictures of the project area. I will post them later. Thanks

husserm

[ThomasW]

MuMetal does work but boy it's really $pendy. It has extremely sharp edges when cut (hint think razorblade).

I've lined speaker cabinets with thick (16 gauge) sheet metal and that does the job.

Sometimes one can simply put a piece of plate steel between the TV and the speaker.

[Amphiprion]

What is the relative permaeability of MuMetal? I didn't think it got a whole lot better than good 'ol steel.

[ThomasW]

http://www.mumetal.com/mumetal_specifications.html

[Amphiprion]

Good lord, I guess it does get a lot better than steel!

[ThomasW]

Good lord, I guess it does get a lot better than steel!

Yep pretty amazing stuff with an equally amazing price... 8O

[Amphiprion]

No kidding. But I agree with Thomas, steel plate should work great for this application. Steel is about 3000-5000 times more permeable than air IIRC, so it should soak up almost all the stray flux.

[husserm]

Time for an experiment

Thanks guys. I'm curious so I'm going to invest in a small amount of "Magnetshied" and cunduct a little experiment.

The speakers that I'm trying to shield are Focal kevlar dual voice coil car speakers (I've just got them laying around).

Is there a way to change the resistance of a total of 4ohms to 8 omhs in the x-over? or will I have to come up with some creative wiring (like adding another mid to the mix)? :twisted:

[ThomasW]

Thanks guys. I'm curious so I'm going to invest in a small amount of "Magnetshied" and cunduct a little experiment.

You might consider shopping around for different sources of MuMetal. That's what the Magnetshield people are selling, but they're pretending it's something exclusive to them.

[husserm]

Ooooooh! Thanks I appreciate the heads up, I will be searching around. :B

[mmoeller]

I've had a number of sheilding discussions before. Conductive metals will all offer some sheilding. The real trick is at what frequency. Low frequency magnetic fields will be attenuated less than high frequencies. I have attached a nice explanation of sheilding from Analog Devices. It shows that below 100hz there is very little that can be done, but by 1kHz even a very thin layer of steel will help tremendously. The only thing that will really help the low frequencies is distance.

http://www.analog.com/UploadedFiles/Application_Notes/41727248AN_347.pdf

The explanation of what is going on with magnetic sheilding get a little technical. I'm not even sure I fully understand, but that's for another day.

[husserm]

Well I have not ordered my experiment material yet, but I did e-mail the head engineer at MuMetal. This guy suprised me by saying:

"Our materials would work but they may be overkill. Get a piece of galvanized steel from your local hardware store and place it between the speaker and the TV. My bet is that it will work."

The fact that he did not push his companies product and gave an honest answer was the part that suprised me. 8O

But just for the fun of it I am going to get some of the material and find out what happens and post the results for all to see. :T

[ThomasW]

Sheet metal does work. Understand that overtime it to will become magnetized. That's not a major problem because hitting it will a hammer will decrease the amount of stored up magnetism.

But just for the fun of it I am going to get some of the material and find out what happens and post the results for all to see.

One of the online suppliers has a sample kit that's reasonably priced. Sorry but I don't remember which supplier offers it since I researched this years ago...

[mmoeller]

How much of this material would you need to test Thomas? I might be able to dig some up at work. We have an sheilded anecohic room for these type of measurements just at higher speeds. The spectrum analysers and frequency generators won't generally dip to the audio frequencies.

[ThomasW]

Probably just enough to wrap around the magnets

[husserm]

I have already ordered my test quantity and it was pretty cheap but thanks for the offer. I may even have enough to do all the speakers for a true test.