Metal Shielding Alloys (Attenuation Rates)

Metal shielding does not have one fixed attenuation rate. Shielding effectiveness, measured in decibels (dB), depends on frequency, alloy, thickness, joints, openings, and test method. Copper and aluminum have high electrical conductivity, while steel offers different strength and durability choices. Select a complete enclosure to meet stated frequency bands and tested acceptance limits, not a material-only rating.

Regional rules, building types, and equipment needs can change what a shielded room requires. A design in an existing building may need to work around local fire rules, electrical clearances, ventilation, and existing utilities. I start by separating the material question from the enclosure question: an alloy can help, but it cannot promise how a finished room will perform.

What attenuation means for a metal enclosure

Shielding effectiveness describes how much a shield reduces an electromagnetic field at a stated frequency and test setup. It is usually reported in decibels. A material result describes a sample; an enclosure result includes real doors, joints, vents, and other paths through the shield.

For electric-field or magnetic-field amplitude, the comparison is commonly expressed as 20 log₁₀ of the field before shielding divided by the field after shielding. A larger positive value means greater measured reduction in that test. Power ratios use 10 log₁₀, so reports must be read with their units and test method in mind.

Attenuation changes with frequency. It also depends on the type of field and how the source and receiver are positioned. A figure measured on a flat coupon cannot be treated as a whole-room rating.

For a design or purchase decision, request the frequency range, test setup, sample thickness, and measurement method behind any quoted dB value. If those details are missing, the number is not enough to compare systems.

How common shielding alloys compare

Conductivity is a measure of how readily electric current moves through a material. It can inform shielding design, but it is not a complete measure of enclosure performance. The approximate values below are bulk references near 20°C, not guaranteed specifications for a particular alloy lot or a promised shielding rating.

Material	Indicative conductivity near 20°C	Practical design considerations
Copper	5.8 × 10⁷ S/m	High conductivity; assess weight, cost, joints, and corrosion conditions.
Aluminum	3.5 × 10⁷ S/m	Lower density than copper; plan carefully for seams, bonding, and dissimilar-metal contact.
Type 304 stainless steel	1.4 × 10⁶ S/m	Lower conductivity than copper or aluminum; strength and finish may suit some assemblies.
High-permeability alloys	Varies by alloy and condition	May be considered for magnetic-field problems; verify grade, processing, and frequency-specific performance.

These values help frame a materials discussion, not rank finished rooms. Thickness, frequency, magnetic properties, fabrication, and electrical continuity all affect performance. A design for low-frequency magnetic fields may call for different material properties than a design for higher-frequency electric fields.

I would compare supplier data only when the test conditions match the intended use. Also check the exact alloy grade, temper or condition where relevant, thickness tolerance, and any coating or finish that could affect electrical contact at joints.

Planning: define frequency bands, limits, and codes

Planning sets the performance target before material or hardware is chosen. The team identifies the frequencies of concern, the required acceptance limits, the heat load, and the rules that apply in the building’s location. A qualified EMC engineer should turn those needs into a test plan.

Start with the intended frequency bands and the equipment or source driving the requirement. Define the acceptance limit for each band, then select a measurement method that covers it. IEEE 299 and IEEE 299.1 provide methods for measuring shielding effectiveness of enclosures; the test engineer should select the applicable method and range.

ASTM D4935 is a coaxial method for comparing planar materials. It can help assess sheet or panel samples under its test conditions. It does not rate a complete room, and it does not account for doors, seams, ducts, or service penetrations.

Record the locally adopted editions of building, fire, and electrical codes. Requirements can differ by jurisdiction, building use, and equipment. Confirm them with the authority having jurisdiction and qualified design professionals before installation.

Access and clearances: protect egress and service space

A shield must not block safe access to equipment or conflict with required life-safety features. Before fixing panel locations, map doors, exits, electrical equipment, ventilation paths, fire protection, and service routes. Check the adopted codes and equipment instructions for the actual installation.

Where NEC 110.26 applies to electrical equipment, working space is commonly at least 30 inches wide or the equipment width, whichever is greater. For the applicable 0–150 V-to-ground condition, the working-space depth is commonly 36 inches. Verify voltage, installation conditions, equipment scope, and the locally adopted NEC edition.

A shield is not a reason to reduce required clearance or obstruct egress. Likewise, do not assume that a conductive wall is a compliant protective grounding path. A qualified electrical designer must determine bonding and fault-current paths for the equipment and the building.

Execution: install joints, bonds, penetrations, and ventilation

The shield works as an assembly, so construction details matter as much as the sheet alloy. Follow an approved design for panels, seams, doors, filters, and service entries. Keep conductive continuity where the design requires it, and have qualified trades handle electrical bonding and fire-rated work.

Treat every opening as a design detail. A door, duct, cable route, or pipe can affect enclosure performance if its treatment does not match the design. Coordinate penetrations before construction; late changes can be difficult to seal or test.

Bond metal parts where required by the electrical design and applicable code. Do not improvise a bond and assume it provides a safe fault-current path. An RF shield does not automatically replace an equipment grounding conductor.

Ventilation must remove the actual equipment heat load. For sensible heat, a useful relationship is:

BTU/h = 1.08 × CFM × allowable air-temperature rise (°F)

This is a sizing relationship, not a universal airflow target. Include equipment-maker requirements and account for duct and filter pressure losses. Shielded ventilation may need designed components, so coordinate airflow and shielding requirements rather than choosing a fan by CFM alone.

Verification: inspect, measure, and document results

Verification checks both the built enclosure and its measured performance. Inspect the assembly before testing, then measure it using the agreed method and frequency bands. A clear record makes it easier to locate and correct weak points.

Before testing, inspect seams, access hardware, bonds, seals, penetrations, ventilation components, and firestopping. Confirm that the configuration matches the approved design. Record any changes made during construction.

Use the specified IEEE enclosure method where applicable, with acceptance limits and bands set before work begins. ASTM D4935 can support planar-material comparison, but it cannot replace an enclosure test. Keep test conditions and results with the project record.

If a test finds a shortfall, investigate the location and cause rather than adding material at random. A door interface, seam, or penetration may be the issue. Correct the detail, then repeat the relevant test and document the result.

Practical case studies: why the complete assembly matters

These are illustrative design cases, not reports of measured results from named sites. They show common decisions that follow from the stated test methods and engineering principles. In each case, the project team should set acceptance limits and verify the finished enclosure.

Case 1: A room requiring a stated RF frequency range. The team compares copper and aluminum panel options using matching material data, then designs joints, a door, and service penetrations as part of the enclosure. A planar test can inform material comparison, but only an enclosure-level measurement can establish whether the built room meets its target.

Case 2: A compact equipment room with a high heat load. The team calculates ventilation from equipment heat output and allowable air-temperature rise, then checks filter and duct losses. Shielding-compatible ventilation details are included in the design. Guessing a standard airflow value could leave cooling inadequate or fail to address the shield’s openings.

Case 3: A retrofit near electrical equipment. The design team checks working space and service access before placing shield panels. An electrical designer sets bonding and grounding details. The RF shield is treated as a separate function, not presumed to be the protective grounding path.

The shared lesson: specify, build, and test the whole system. Alloy selection is one input, not the final performance proof.

Frequently asked questions

Which metal alloy provides the highest attenuation?
There is no universal winner. Results depend on frequency, field type, thickness, joints, and test setup. Compare tested data that matches the intended application.

Can I use conductivity to predict shielding effectiveness?
Conductivity is useful background information, but it cannot predict whole-enclosure performance by itself. Construction details and frequency also matter.

Does thicker metal always provide more shielding?
Thickness can affect performance, but the result depends on frequency, material properties, and the full design. Do not infer a room rating from thickness alone.

What does a shielding value in dB mean?
It is a logarithmic comparison between fields measured before and after shielding under stated conditions. The frequency range and measurement method are essential to interpreting it.

Does ASTM D4935 certify a shielded room?
No. It compares planar materials under the method’s test conditions. It does not measure a complete room with doors, seams, ducts, and penetrations.

Which standard measures enclosure shielding?
IEEE 299 and IEEE 299.1 provide enclosure measurement methods. A test engineer should select the applicable method and frequency range for the project.

How much ventilation does a shielded room need?
There is no universal CFM value. Base airflow on actual sensible heat load and allowable temperature rise, then account for equipment needs and pressure losses.

Is the metal shield a grounding conductor?
Not automatically. A qualified electrical designer must specify bonding and fault-current paths that meet applicable codes.

What should I request from an alloy supplier?
Ask for the exact grade, thickness, relevant material properties, and test conditions behind any shielding data. Confirm that the information fits the intended frequency range and assembly.

What proves the finished enclosure meets its target?
An enclosure-level test using the agreed method, frequency bands, and acceptance limits provides the relevant performance evidence. Retain the test report and document corrections.

Conclusion: specify the target, then verify the enclosure

A useful shielding specification names the frequency bands, acceptance limits, assembly details, applicable codes, and test method. Choose alloys for the design conditions, but do not treat conductivity or a sample dB figure as a room guarantee. Inspect the work, measure the enclosure, and keep the results.

(This article was written by one of our staff writers, Steven Langford. Visit our Meet the Team page.)