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What is Magnetic Shielding?

By J. Dellaporta
Updated May 17, 2024
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Magnetic shielding is a process that limits the coupling of a magnetic field between two locations. This can be done with a number of materials, including sheet metal, metal mesh, ionized gas, or plasma. The purpose is most often to prevent magnetic fields from interfering with electrical devices.

Unlike electricity, magnetic fields cannot be blocked or insulated, which makes shielding necessary. This is explained in one of Maxwell’s Equations, del dot B = 0, which means that there are no magnetic monopoles. Therefore, magnetic field lines must terminate on the opposite pole. There is no way to block these field lines; nature will find a path to return the magnetic field lines back to an opposite pole. This means that even if a nonmagnetic object — for example, glass — is placed between the poles of a horseshoe magnet, the magnetic field will not change.

Instead of attempting to stop these magnetic field lines, magnetic shielding re-routes them around an object. This is done by surrounding the device to be shielded with a magnetic material. Magnetic permeability describes the ability of a material to be magnetized. If the material used has a greater permeability than the object inside, the magnetic field will tend to flow along this material, avoiding the objects inside. Thus, the magnetic field lines are allowed to terminate on opposite poles, but are merely redirected.

While the materials used in magnetic shielding must have a high permeability, it is important that they themselves do not develop permanent magnetization. The most effective shielding material available is mu-metal — an alloy of 77% nickel, 16% iron, 5% copper, and 2% chromium — which is then annealed in a hydrogen atmosphere to increase its permeability. As mu-metal is extremely expensive, other alloys with similar compositions are sold for shielding purposes, usually in rolls of foil.

Magnetic shielding is often employed in hospitals, where devices such as magnetic resonance imaging (MRI) equipment generate powerful magnetic flux. Shielded rooms are constructed to prevent this equipment from interfering with surrounding instruments or meters. Similar rooms are used in electron beam exposure rooms where semiconductors are made, or in research facilities using magnetic flux.

Smaller applications of magnetic shielding are common in home theater systems. Speaker magnets can distort a cathode ray tube (CRT) television picture when placed close to the set, so speakers intended for that purpose are shielded. It is also used to counter similar distortion on computer monitors.

A number of companies will custom build magnetic shields from a diagram for home or commercial applications. Shielding using superconducting magnets is being researched as a means of shielding spacecraft from cosmic radiation.

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Discussion Comments
By anon330824 — On Apr 18, 2013

I have issues now in my bandroom with four giant speaker cabs making my macbook pro video jitter. It only happens when the speakers are running. I know it's magnetic interference, but how do I solve it?

By anon309083 — On Dec 14, 2012

Guitar player here. There is a product called pedalsnake, which basically moves all your power supplies (for pedalboards with lots of power supplies) away from the pedals and placed behind the amp. This made a huge difference in noise reduction associated with having the power so close to the pedals! I have a two tiered board where the power supplies are underneath the second tier. Is there any material I can purchase to cover or wrap the supplies to manage the electromagnetic interference?

By anon304209 — On Nov 19, 2012

If Mu Metal is used, does it have to touch the face(s) of the pole(s) of the magnet to work? If a 'gap' is allowed between the face of the Mu Metal and the face(s) of the magnet, will that reduce its effectiveness? If so, by approximately what percentage?

By anon44052 — On Sep 04, 2009

To anon24772: I would suggest the best solution would be to loose the CRT - LCD or Plasma will not be affected and you could end up spending a lot of time and money getting the shielding right.

By anon24783 — On Jan 18, 2009

Thank you so much!!!!!!! I have been doing this project at school and was at a loss the whole of today searching for sites, where the explanations were not post-graduate science!!!! This site has helped sooo much, I will be sure to use it again in the future.

By anon24772 — On Jan 18, 2009

Hi, What if a person wants to figure out a DIY approach. Let's say a person has a small, narrow room in which a CRT TV is being used, and the only location for two big speakers is on either side of the TV. Could a person "homebrew" some type of magnetic shield for the speakers without having to buy mu-metal alloy foil rolls??? Or what if a person has a big subwoofer cabinet that they want to place underneath a CRT TV (for space reasons). Thanks wiseGEEK readers.

By adm4unix — On Jan 06, 2008

On the Magnetic Shielding page you state that "ionized gas, or plasma" can be used as a magnetic shield. Where can I get more information on this strategy? Are there any working demonstrations or hints on how to use this?

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