Do you know the answers to these frequently asked questions?
- How does magnetic shielding work?
- What causes a magnetic field?
- What is "permeability"?
- What is Mumetal®?
- What is EMI?
- What is the difference between AC and DC fields?
- Do I need to heat treat my finished shields?
- My monitor jitters, what can I do about it?
- Why is Mumetal® so expensive?
- Can I use mild steel instead?
- Can I shield one magnet from another?
- When do I need to consider high frequency shielding instead of magnetic shielding?
- My speakers are interfering with my television, what should I do?
- What is the best way to handle high permeability foils and materials?
- What is the best shape for a magnetic shield?
- Why should I purchase my magnetic shielding from MuShield?
- Who should I contact at MuShield if I have additional questions?
How does magnetic shielding work?
When magnetic lines of flux encounter high permeability material, the magnetic forces are both absorbed by the material and redirected away from its target, much as a beavers dam absorbs and reroutes water from its den.
The most effective shields are constructed as enclosures such as boxes or better yet, cylinders with end caps. Because the field follows the line of the enclosure, an enclosed shape keeps stray fields from finding gaps which could cause unintended interference.
What causes a magnetic field?
Most magnetic fields are man made. They can be found in solenoids, bar magnets, and some motors and transformers. Magnetic fields are used in creative ways to create sound (speakers), microscopic images (microscopy), and record resonance images through MRI technology. In some cases, however, the fields interfere with sensitive electronic equipment, and shielding is necessary.
What is "permeability"?
Imagine a sponge. Some sponges have big holes, some are more tightly structured, each absorbs liquid to a point of saturation. Magnetic shielding material is similar. Depending upon its structure, magnetic shielding material absorbs magnetic interference to a point of saturation. As magnetic interference increases, so too should the permeability of the material.
What is Mumetal®?
Pronounced "mew metal", Mumetal® is a trade name for one of several high permeability magnetic shielding materials. It is made up of about 80% nickel. Often times Mumetal is used as a generic word to designate a high permeability alloy. Depending upon the mix, the remainder of the materials include iron, molybdenum or chromium, and copper. Other trade names include Permalloy®, HyMu-80®, Hipernom®, Magnifer 7904®, Magnifer 75®, and Mumetal 2®. MuShield works with all of these products and will help you to determine which is the most appropriate material for your application.
What is EMI?
EMI is the abbreviation for Electro Magnetic Interference. It can mean different things to different engineers. Essentially, EMI is an electrical or magnetic disturbance that causes unwanted interference.
What is the difference between AC and DC fields?
Alternating Current is an electrical current that reverses its direction at short intervals, while Direct Current is an electrical current that flows in one direction only. DC fields include the Earth's magnetic force, magnets and some motors. AC fields are commonly generated by electrical power equipment. Both fields respond the same way to magnetic shielding.
Do I need to heat treat my finished shields?
Heat treatment maximizes the permeability of your part(s). During the fabrication process, each part is subjected to stresses that contribute to the degradation of the shielding performance of high permeability materials. In a few cases, heat treatment is not necessary, but MuShield will always recommend it if it is not otherwise specified.
My monitor jitters, what can I do about it?
There are several things you can do:
- move your monitor;
- move the source of interference (assuming you know what it is);
- replace your CRT with an LCD monitor (LCD monitors are not susceptible to EMI);
- purchase a monitor enclosure fabricated with high permeability material.
Why is Mumetal® so expensive?
There are two reasons why high permeability magnetic shielding materials are expensive relative to other alloys: 1) It is an alloy made up primarily of nickel (80%), and 2) it is made in relatively small batches by a handful of specialty steel mills around the world. When the application permits, we will recommend other lower cost alloys for shielding. High permeability materials are the best materials for redirecting and absorbing magnetic flux.
Can I use mild steel instead?
Mild steel can shield magnetic fields and is appropriate for applications requiring a small attenuation of a high flux density field. A quick phone call to MuShield will answer your question about mild steel for shielding. If it is likely to work, why spend your money on high permeability materials.
Can I shield one magnet from another?
No. There is no such thing as a magnetic insulating material. High permeability material works to shield from magnetic interference by attracting and diverting the field through itself. The only way in which to halt the attractive or repulsive forces between two magnets is to separate them by the distance proportional to their respective magnetic field strengths.
When do I need to consider high frequency shielding instead of magnetic shielding?
High frequency or RF (radio frequency) shielding is necessary when the frequency range exceeds 100 kilohertz. Materials such as copper, galvanized steel, aluminum, and some specially treated rubber and plastics shield from high frequency through their high conductivity characteristics. High permeability materials work by absorbing and redirecting magnetic flux, but its characteristics are such that it can act as a "broadband shield" by shielding both frequency ranges - if constructed appropriately.
My speakers are interfering with my television, what should I do?
Unless your speaker is of Carnegie Hall dimensions, it contains small magnets which are creating a weak magnetic field. In all likelihood, you have a large screen TV that is affected by these magnets.
All you need is a sheet of high permeability material. Call us and we'll walk you through your options.
- move the speakers away from the television;
- move the television away from the speakers;
- buy a shield to place between the two.
What is the best way to handle high permeability foils and materials?
High permeability materials increase in fragility as they decrease in thickness. Unlike other alloys, you must take care to minimize the stress introduced during cold working of the alloy (bending, punching, drilling, etc.). Foils can be cut and bent as necessary without excessive harm, but treating the material with care will increase its effectiveness as a shield.
We recommend a final heat treating to maximize permeability once a shield has been constructed. This decreases the material's permeability.
What is the best shape for a magnetic shield?
The most efficient shield shape is a spherical sheet metal shell constructed of high permeability material. In most cases, spherical shells are highly impractical. The next best shape is a cylindrical shape, where the ratio of the length to the diameter is 4:1. The rounded surface of the cylinder is conducive to the absorption of magnetic flux. End caps on your cylindrical shield significantly increases its shielding effectiveness.
When cylinders will not fit and flat sided boxes are more practical for mechanical reasons, fabricated sheet metal boxes are the next best choice. Bend radius should be as large as possible because the magnetic flux permeating through the shielding material does not turn sharp corners easily and flux leakage can occur.
The least favorable shield shape is a large flat sheet. Since a magnetic shield functions by providing a low reluctance flux path, and magnetic flux flows from its north pole to the south pole, a flat sheet will only cover a portion of the flux path. Additionally, as the angle of the magnetic field vector becomes perpendicular to the surface of the flat sheet, the shield effect nullifies.
Why should I purchase my magnetic shielding from MuShield?
We are selected by our customers to be their magnetic shielding vendor because they count on us for a consistent product at a reasonable price.
Our goal is "shielding that works"; our end result is a satisfied customer.
Who should I contact at MuShield if I have additional questions?
David Grilli, Ext. 11,
David is our Sr. Engineer. He is prepared to assist you with any shielding application or question you may have. David will provide you with custom shielding price quotations and will address all of your delivery requirements.