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Andy Rondeau's Do-It-Yourself (DIY) Speaker Project

Magnetically Shielding the Drivers

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The Worlds First Do It Yourself (DIY) Magnetic Loudspeaker Shielding Page

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Disclaimer!

I am not responsible for any data loss and damage to electronic equipment that you may incur using the methods on this page.  Use these methods at your own risk.

Additional Tips!

Use common sense!  Do not perform experiments on your new Sony Wega; use a $30 dollar TV or old computer monitor instead.  If weird artifacts remain onscreen after removing a magnet, they will fade after a few hours.

This Page is now Outdated!

A few years after creating this page, I came across a web site that sells DIY speaker shielding supplies.  I have not yet tried their technique.

Background

If you understand the need for magnetic shielding, you may skip this section.

Why do speakers need to be shielded?

A problem with most speakers is that they contain permanent magnets and generate electromagnetic fields.  During playback, to generate sound, an electromagnet uses the current from the amplifier to pull the cone forward, and the permanent magnet pulls the cone back.  Unfortunately, because traditional CRTs use magnetic fields to operate, placing speakers close to a television or computer monitor can cause serious picture distortion.  Speakers with very strong magnets can cause data loss when placed close to computer hard drives, magnetic disks, and magnetic tape.

Magnets can mess my TV up?

Take a strong magnet and place it next to your TV.  (It won't work if you have an LCD or Plasma TV.)  See!  Picture tubes work by using electromagnets to whip a beam of electrons across every square inch of the screen 30 times a second.  (Or 25 if you're in Europe.)  By placing a magnet near the screen, you interfere with this process.  Click here for more information.

Why is this page unique?

I believe that I am the first person to publish an account of DIY loudspeaker shielding on the web.  (Well, I haven't found a page about DIY shielding yet!)  I expect that this topic will become more common as more people decide to DIY home theater, where the center channel can pose serious distortion to traditional CRT televisions.  (Then again, DIYers who can afford LCD and plasma screens need not read any further!)

In my travels through the web, I've noticed that most DIYers only build for 2-channel stereo.  If they do use home-built speakers with their television, they are usually placed far enough away that the magnets will not cause interference. Before I built my center speaker, I placed my original pipes far enough away from the speaker so that the distortion was not noticeable.

Can one avoid this problem?

The simplest way to avoid the problem of magnetic shielding is to select speaker drivers that are already shielded.  In my case, I wanted all speakers to use the same exact drivers, so that there would be no alterations in sound as it pans from a side speakers through the center.  Also, I really liked the sound of the Radio Shack 40-1044s, and was not willing to try and find another driver, even though the model has a 10-ounce magnet.

How does shielding work?

To eliminate magnetic fields from an object, one must completely cover it with a material that is attracted to the magnet, at a thickness to capture all fields.  Click here for more information .  The problem with this theory is that if one wraps an entire speaker in metal, no sound will escape!  Typically, a shielded speaker will cover the magnet and back of the driver with an additional layer of metal, such as steel.

The Quick and Dirty Approach to Shielding a Non-Shielded Driver

Ideas Considered

A fellow student at WPI suggested that I try wrapping the speaker in a wire screen.  I found some old wire mesh that was magnetically responsive in my parents shed, but it didn't work.  My friend, the DIY Guy, suggested that I try copper flashing.  Before I decided to invest money in an unproven idea, I googled up some information about materials to use.  What I found was that typical shielding applications use steel, not copper.

The Quest to Home Depot

Before I left for Home Depot, I pocketed a small refrigerator magnet.  I planned to use it to test materials to see if they would react to magnetic fields.  Also, I measured the diameter of the back of the drivers so I would know how big of an object I was trying to cover.  (They measured about 3.25")  I made an attempt to find steel foil, as it would be the easiest material to work with.  It didn't exist.

At the Depot, the first place that I went was the electric isle.  (Naturally!)  First, I saw steel junction boxes, which the magnet stuck to.  I looked closely at a few, but I found that their surface had a few undesirable holes.  Next, I went over a few isles to look at the air-duct materials.  The flexible tubes were made out of aluminum, which the magnet did not stick to, but the hard junctions were made out of steel, which practically pulled the magnet out of my pocket.  Bingo!

I walked down the aisle to find a rack of round pipe-caps for air ducts.  The magnet stuck.  Perfect!  I bought one four-inch diameter steel air-duct pipe cap, returned a DVD, and rushed home.  The cap stuck perfectly onto the back of the driver, and reduced interference on my test-CRT by about 50%.

The materials that you may use are either a fully-closed electrical steel junction box, or, preferably a steel pipe cap.  You must use a metal that a magnet will stick to, or your shield will not work.

How I Shielded My Drivers

After the above experiment, I decided that the shielding provided by the four-inch diameter steel air-duct pipe caps was sufficient for the right, left, and surround pipes.  They are not placed directly next to any sensitive electronics or data.  (Now I won't have to keep my laptop ten feet away from my speakers!)  Images below:

Back of driver and pipe cap used for shielding
Back of driver and pipe cap used for shielding

In the event that it slips, I placed a small piece of electrical tape on the cap near the electrical contacts for the driver to prevent shorts.  The magnet appears to be strong enough to hold the cap on the driver.  (I'll find out if I need adhesive if the caps fall out of the pipes.)

Shielding (pipe cap) applied to driver
Shielding (pipe cap) applied to driver

The center speaker proved more challenging because even a small amount of magnetic field would cause picture distortion.  This is because the speaker is placed directly on top of the television screen.  (Granted, if I had lots of income, I could avoid this by using an LCD or plasma television!)  I decided to double the amount of steel over the magnet by using a five-inch cap over the four-inch cap.  My experiments showed distortion to be minimal with two pipe caps.

Because the fields are not as strong for the second cap, I decided to hold it on with a bit of duct tape between the two caps.  Also, in addition to the electrical tape on the inner cap, I placed a strip of tape on the electrical contacts in case the outer cap falls onto them.

Center driver with duct tape being applied to inner cap
Center driver with duct tape being applied to inner cap

Center driver with both caps applied
Center driver with both caps applied

When I flipped the driver over to place it into the hole, the outer cap moved an unacceptable amount.  I fixed the problem with duct tape.

Finished center driver
Finished center driver

Results

Testing with a Television

After I finished assembling my speakers, I put the center channel on top of the test television and fired it up.  I'm happy with the results, see for yourself.  Images follow:

Driver on top of the television
Driver on top of the television

Close-up
Close-up

(Remind me to take another shot without any shielding for comparison)

Addendum:  1 week later

When I wanted to watch a movie with my new surround system, I replaced the 13" Commodore monitor with my 27" Magnavox.  There is no distortion, except for a purplish hue in the upper-right corner of the screen.  The DIY Guy, after reading this page, made a suggestion, "... A custom made 3/16 plate steel and wrought iron cap mounted using steel strapping and the speaker mounts should kill any signal."  Maybe when I'm able to afford it, I'll order a custom-built shield or invest in some metalworking tools.

Conclusion

The techniques discussed here should be appropriate for a hobbyist home-theater DIYer who needs or desires to use drivers that do not contain any shielding.  These techniques are also adequate for someone to shield an existing speaker with drivers that are not shielded.  It is possible to use better methods that may be more durable.  Likewise, I have not stress-tested the shielding, and the caps may fall out of placement in the future.  A professional speaker builder should research proper shielding techniques, as the methods used here are not acceptable for speakers intended to be sold.