What Is Passive Magnetic Shielding? (Mu-Metal Sheets)
Passive magnetic shielding uses a high-permeability material, often a nickel–iron alloy sold as Mu-metal, to guide magnetic flux around a protected space. It does not erase a field or guarantee a set reduction. Performance depends on the source, frequency, alloy, shape, seams, and openings, so measure the field before and after installation.
What passive magnetic shielding does
Passive magnetic shielding reduces the magnetic field in a defined space without powered coils or active cancellation. A high-permeability alloy offers magnetic flux an easier path through the shield than through the air inside it. The amount of reduction depends on the complete assembly, not the material name alone.
“Mu-metal” is a trade name used for certain nickel–iron magnetic alloys, not a single guarantee of performance. High permeability means a material can carry magnetic flux readily, but permeability varies with alloy, heat treatment, forming, and field conditions. The shield redirects flux; it does not make the field vanish.
A shield may help with static fields, such as Earth’s field or a nearby permanent magnet, and with some low-frequency fields. It is not automatically the right answer for every magnetic source. Electric-field shielding, radio-frequency shielding, and magnetic shielding are different design problems.
What the alloy name does and does not tell you
A product labeled “Mu-metal” does not, by itself, state its certified properties or the reduction you will get in an installed enclosure. Specify the alloy grade and procurement requirements, such as ASTM A753 where applicable, and request supplier-certified magnetic properties.
ASTM A753 covers nickel–iron soft-magnetic alloy material. It is a material reference, not a building design standard or a promise of finished-shield performance. Ask the supplier to confirm that the supplied grade and condition suit the field and fabrication method.
Which magnetic fields are covered by common test references?
IEEE Std 299-2006 describes shielding-effectiveness measurements for enclosures from 9 kHz to 18 GHz. That range does not establish how well an enclosure performs for static fields or quasi-static fields below 9 kHz.
For those fields, use a measurement method and instrument suited to the actual frequency and field strength. Record the conditions so results can be compared. Do not treat a test result from a different frequency or enclosure as proof of your installation’s performance.
Planning: define the field, protected volume, and required attenuation
A useful design begins with a measured field and a clearly marked protected volume. Record where the field is strongest, what equipment is operating, and how much reduction is needed. Without this baseline, it is not possible to choose a sound design or verify its result.
Survey the field before choosing sheet
Use a calibrated instrument suited to the field type and range. Measure the magnetic field at the locations that matter inside the planned protected volume. Note the probe position and orientation, source operating state, and frequency or frequency spectrum where relevant.
For sources that switch, cycle, or vary with load, take readings in representative states. Separate static and AC components when the instrument allows it. If you cannot identify the frequency content or source behavior, consult a magnetic-shielding specialist before specifying material.
Set a measurable target
For field-amplitude readings, calculate attenuation as:
Attenuation (dB) = 20 log₁₀(Bunshielded / Bshielded)
Here, B is the measured magnetic flux density at the same point and orientation, before and after shielding. A 20 dB reduction means the measured field amplitude is one-tenth of the baseline. Always report the readings as well as the dB value; the ratio alone can hide important context.
Agree on a target and measurement method before fabrication. There is no universal thickness or attenuation value that fits every field, source, or enclosure. A specialist can help assess whether the target is realistic for the available space and geometry.
Access and clearances: preserve service, cooling, and electrical requirements
A magnetic shield must fit around the protected equipment without blocking required access or cooling. The alloy does not create a general building-code clearance or airflow value. Follow the equipment maker’s listing, installation instructions, and service requirements, along with applicable local building and electrical rules.
Check thermal and service limits
A close-fitting enclosure can trap heat even though the magnetic material itself needs no airflow. Keep the equipment’s required ventilation paths and service clearances open. After installation, check operating temperature under representative use and compare it with the manufacturer’s limits.
Also plan how a technician will reach connectors, controls, and replaceable parts. A door or removable panel can be a weak point in shielding, so include it in both the design and final measurements.
Review supports, nearby metal, and wiring
Confirm that the structure can support the enclosure and that installation will not interfere with other building systems. Nearby ferromagnetic parts can change the magnetic field and affect results. Do not assume that a metal object is suitable simply because it is close at hand.
Maintain required electrical grounding and bonding arrangements for the equipment and installation. Whether the shield itself needs bonding depends on the equipment, use, and jurisdiction; confirm this with the responsible electrical professional or authority.
Execution: specify, form, join, and install the magnetic alloy
The enclosure’s corners, seams, openings, and doors can matter as much as its flat panels. Specify the alloy and fabrication limits with the supplier, then follow them during cutting and forming. Do not infer enclosure performance from a flat-sheet data point or a small coupon.
Request fabrication limits in writing
Ask the supplier to identify the grade, condition, certified magnetic properties, and limits for cutting, bending, joining, and any required annealing. There is no universal sheet thickness, bend radius, seam overlap, or post-forming heat-treatment schedule. The correct values depend on the alloy, field, frequency, and design.
Forming can materially reduce magnetic properties. Do not field-anneal formed panels or apply a guessed heat schedule. If the supplier requires annealing, use its documented process and qualified facilities.
Treat the enclosure as a complete system
Plan seams, corners, penetrations, and access panels before fabrication. A flat panel may perform well while a completed enclosure leaks field through a seam or opening. Use the supplier’s recommended joining details, and ask a specialist to review the geometry when the target is demanding.
| Design item | What to specify or check | Why it matters |
|---|---|---|
| Alloy | Grade, procurement specification, certified properties | “Mu-metal” alone is not a performance specification |
| Sheet and forming | Supplier-approved thickness and forming limits | Forming may change magnetic properties |
| Seams and doors | Supplier-reviewed joint and access details | Discontinuities can reduce installed performance |
| Openings | Location and size of penetrations | Openings can create leakage paths |
| Test plan | Baseline points, probe direction, source state | Makes before-and-after readings comparable |
Avoid substituting unverified steel or foil as an equivalent shield. A material may be magnetic yet lack the required permeability or condition. Confirm suitability with its supplier rather than relying on appearance or a magnet test.
Verification: measure installed attenuation and check hot spots
Verification means repeating the baseline measurements after installation, at the same points and orientations, under representative source conditions. Map the protected volume, record readings, and look for local areas where the field remains high. A single reading at the center is not enough to describe the whole enclosure.
Compare like with like
Use the same instrument, measurement range, probe orientation, source state, and locations used for the initial survey whenever possible. Record the date, equipment settings, and any changes to nearby sources or metalwork. Calculate the field-amplitude ratio and dB result from matched readings.
Check corners, seams, doors, and penetrations as well as the center. If readings differ sharply by location, record the pattern rather than reporting only an average. The goal is to understand performance across the protected volume.
Recheck temperature and operation
After the magnetic survey, check that the protected equipment still has the required airflow and service access. Measure operating temperature under normal use and compare it with the equipment maker’s limits. If the enclosure changes temperature or field readings in an unexpected way, stop and review the design with the supplier or a qualified specialist.
Installation scenarios: what the measurement process reveals
These examples are illustrative scenarios, not claims of measured results. They show how the same material can lead to different decisions once the source, space, and verification plan are defined.
Sensitive instrument near a changing source
A homeowner finds that a sensitive instrument’s readings shift when nearby equipment changes operating state. The first step is to measure the field in each state and identify its frequency content. If the disturbance is not a static or low-frequency magnetic field, a Mu-metal enclosure may not address the cause.
If measurements support a magnetic-shielding design, the installer can define the protected volume and set a target with a specialist. After installation, matched readings show whether the enclosure reduces the field at the instrument and whether seams or access panels need review.
Equipment inside a tight cabinet
A tradesperson considers lining a cabinet with high-permeability sheet to protect equipment from a nearby source. The survey indicates the field and its strongest locations, but the cabinet’s limited space raises a separate concern: cooling and service access.
The installer checks the equipment instructions before finalizing the enclosure. The installed shield is then measured at multiple points, and equipment temperature is checked during representative operation. This process addresses both magnetic performance and the risk of trapping heat.
Frequently asked questions
Is Mu-metal the same as magnetic shielding?
No. Mu-metal is a name used for certain high-permeability nickel–iron alloys. Magnetic shielding is the complete design and installation that uses suitable material and geometry to reduce a field in a defined space.
Does Mu-metal block magnetic fields?
It does not erase the field. It provides a path that can guide magnetic flux around a protected space. The resulting reduction depends on the field, alloy, enclosure geometry, and construction details.
What thickness of Mu-metal sheet should I use?
There is no universal thickness. Ask the supplier or a shielding specialist to select it for the alloy, field strength, frequency, enclosure, and required attenuation. Do not choose thickness from the material name alone.
Can I bend Mu-metal sheet myself?
Only within limits confirmed for the specific material. Forming can degrade magnetic properties, so obtain written bending and handling guidance from the supplier before fabrication.
Can I anneal a finished shield at home?
Do not field-anneal it using a guessed process. If annealing is required, follow the alloy supplier’s documented procedure and use an appropriate qualified facility.
Does IEEE Std 299-2006 verify shielding for static fields?
No. Its stated measurement range is 9 kHz to 18 GHz. That range does not establish performance for static or quasi-static magnetic fields below 9 kHz.
How do I report magnetic shielding effectiveness?
Measure field amplitude before and after installation at the same points and orientations, with the source in the same state. Report the readings and calculate 20 log₁₀ of the unshielded-to-shielded field ratio.
Does magnetic shielding require a set airflow rate?
No shielding-specific airflow value applies in general. Preserve the protected equipment’s manufacturer-required cooling and service clearances, and check its operating temperature after installation.
Conclusion
Passive magnetic shielding is a measured, geometry-dependent solution, not a sheet-metal product with a guaranteed result. Define the field and protected volume, specify certified alloy properties, preserve equipment requirements, and follow supplier fabrication limits. Then verify the finished enclosure through matched measurements, including seams, openings, and thermal checks.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
