Shielding a subwoofer? Monitor is not happy.

[bluevolume]

I'm installing speakers in the bezel above my monitor, and am having problems with a woofer that isn't shielded.  The left and right speakers are already shielded.

Is it difficult to add shielding (another magnet I assume)?  It is a fairly cheap 4" speaker, maybe it would make more sense to just buy one that is already shielded.

[Green Giant]

Why not save yourself the trouble and just place the sub on the bottom of the cab away from the monitor?  The sub will sound just as good pointing directly at you at the top as it will on the bottom.

[bluevolume]

There is a sealed box that is perfect for a woofer (there was one there originally) above the monitor.  I've already built the plate out of MDF, so this design decision has pretty much been made.

[Zebidee]

Buying a new shielded subwoofer speaker is probably your best bet.  Or pulling one out of something else (eg some old TVs have them) You can buy mag shielding material, but it isn't cheap.

[BobA]

You can sheild with anything that is magnetic.   This means steel or any other metal that is magnetic such as tin.  Check a cookie sheet with a magnet.  If it is the cheaper tin/steel type not aluminum it will give you raw material to sheild your subwoofer.   Some people have shielded smaller speaker with tin cans but since yours is larger a cookie sheet is probably your best bet.  Take a fridge magnet along to test it.

[Zebidee]

Some people have shielded smaller speaker with tin cans but since yours is larger a cookie sheet is probably your best bet. 

I've tested this wives tale thoroughly with cans and steel cladding, and found that it does nothing (or virtually nothing) to stop the magnetic field.  Commercial shielding material is much, much better, around 20,000 more powerful than steel plate at stopping magnetic fields.

[BobA]

Before spending approximately $20 a square ft for a nickel alloy foil I would give a try to the old wives tale method since there is probably a cookie sheet around the house that would work for testing for free.

[bluevolume]

Before spending approximately $20 a square ft for a nickel alloy foil I would give a try to the old wives tale method since there is probably a cookie sheet around the house that would work for testing for free.

Are you talking about just laying the cookie sheet down flat below the speaker, or do I need to cut it and roll it around the driver?

[Zebidee]

Bob, I really did give the old wives tale a really good go but it all came to nothing.  The speakers still affected my monitor.  Even with several layers of steel building brace rolled around it and a layer of steel behind.  It'll even affect my sony PVM, which has metal shielding around it as well!  MYTHBUSTED!  Might work on the earth's magnetic field, but not on strong speaker magnets.

You can buy rolls of magshielding stuff that is flexible, sort-of like a fridge magnet, and easy to work with.  It literally sticks itself to the magnetised back of the speaker.  Worth buying if you have a lot of speakers to do, I suppose.

However, I love being proved wrong about these things so long as someone can demonstrate to me that it actually works!

[scotthh]

Here are the options:
1. Replace the subwoofer with a shielded one (I did a little searching, and couldn't find one)
2. Shield the subwoofer:
    DIY instructions.
    Discussion that suggests this isn't cheap or easy.
3. Move the subwoofer.

Subwoofers are non-directional (it doesn't really matter where you put them, unlike your left and right stereo speakers). Why not splice the wires and place the subwoofer in the bottom of your cabinet? Although you planned on putting it behind the marquee, it looks like changing your design is the cheapest, easiest most practical solution.

[BobA]

Just place the sheet between the speaker and crt with it closer to the speaker.   Degauss your monitor and see if it makes a difference.    If it does nothing then Zebidee is right about it being basically an old wives tale.

[specfire]

It is NOT an old wives tale, the property of the material required is that it be ferromagnetic  http://en.wikipedia.org/wiki/Ferromagnetic.  The success of the shielding is dependent on the material used and its thickness. 

If a magnet will stick to it, that is a good start, however, it is not a foolproof method.  In otherwords, one man's cookie sheet may not necessarily be like another man's cookie sheet. 

Many metals can be used to form an alloy with which such things as cookie sheets can be made, so even if a magnet "sticks to it", it may not have enough ferromagnetic material to be effective for your application.  Often aluminum will be a significant component of most metal consumer goods even if there is some iron in them as well.  (aluminum is not ferromagnetic.)

Dedicated shielding material is expensive because it uses expensive metals such as cobalt, magnesium or nickel.  BTW, most speakers sold as "shielded" do not use such expensive shielding materials.  The dedicated shielding material is used in applications that must guarantee a certain level or degree of shielding such as in medical devices or in particular industrial applications where an errant magnetic field might wreak havoc to a particular manufacturing process etc. 

When it comes to consumer goods such as speakers and CRT's, really bad things don't happen even if the shielding is not very good (in other words, no one dies, at worst, the CRT has a colour haze or ring.)

I personally would go to a hardware store and get some flat sheet metal (usually found in the heating section amongst all of the ductwork.)  and then cut it to size to form a dedicated enclosure. 

Good luck.

spec

[Zebidee]

It is NOT an old wives tale, the property of the material required is that it be ferromagnetic.  The success of the shielding is dependent on the material used and its thickness. 

Hey Spectre, I agree with you 100%.  Why I say "old wives tale" is because simple iron or steel sheeting is not nearly powerful enough to counter the magnetic field of your average powerful speaker.  Commercial mag shielding materials are like 20,000 times more powerful.  But also expensive.

Telling you guys, I've done this experiment but always happy to watch someone else have a go    Like I said, love to be proven wrong and learn something new.

[jcoleman]

Most speakers are magnetically shielded (those that are, anyway) by essentially reversing the magnetic field outside of the area that drives the voice coil.  In other words, another magnet is applied that supplies an inverse field, thus cancelling out the field of the magnet supllying power to the speaker.  This is non-trivial for a DIY-er.

Coleman

[Zebidee]

... In other words, another magnet is applied that supplies an inverse field. ...
Coleman

Wow, no I've learnt something new!  I've often wondered how they did that relatively cheaply for multimedia speakers and similar.

Incidentally, I'm shocked to say that MOST of the old original arcade cabs I've worked on originally used UNSHIELDED speakers  , and in many cases closely positioned speakers would have had a significant affect on the original monitor - WTF  !%#$%@ .  I STRONGLY recommend to anybody with an original cab that they check their speakers out closely, and replace as necessary.  For example, I've had 6w unshielded original cab speakers that have caused distortions to my SHIELDED sony video monitor, which was at least 6-8 inches away.

[lanman31337]

They do that with almost all shielded speakers.  they use two small magnets, and have them with inverse poles, which makes the magnetic field very weak.  One down side in it is that power handling is significantly decreased.  You usually don't have to worry about that with television speakers because they're very low power output.

[Zebidee]

TV speakers can make a very good substitute for games cabinet speakers.