Select material, thickness and frequency to instantly compute skin depth, absorption loss, reflection loss and total shielding effectiveness in dB, and compare materials on the full frequency-spectrum chart.
The foundation of shielding is the skin depth (δ). It defines the distance over which the electric field of an electromagnetic wave decays to about 37% (1/e) of its surface value inside a conductive material. It depends on the material's resistivity (ρ) and magnetic permeability (μ), and the angular frequency (ω) of the wave.
$$\delta = \sqrt{\dfrac{2\rho}{\omega\mu}}$$Where:
δ = Skin depth (m)
ρ = Electrical resistivity (Ω·m)
ω = Angular frequency = $2\pi f$ (rad/s)
μ = Magnetic permeability = μrμ0 (H/m)
A smaller δ means the wave is absorbed in a thinner layer, leading to higher absorption loss for a given material thickness.
The total Shielding Effectiveness (SE) is the sum of Absorption Loss (A) and Reflection Loss (R). Absorption is directly proportional to thickness (t) and inversely proportional to skin depth. Reflection is a surface phenomenon based on the impedance mismatch between free space and the shield material.
$$A = 8.686\,\dfrac{t}{\delta}\quad \text{(dB)}$$ $$R = 168 - 10\log\!\left(\dfrac{f\mu_r}{\sigma_r}\right)\quad \text{(dB)}$$ $$SE = A + R \quad \text{(dB)}$$Where:
t = Shield thickness (m)
f = Frequency (Hz)
μr = Relative permeability
σr = Relative conductivity (relative to copper)
The constant 8.686 comes from converting Nepers to decibels (20 log(e) ≈ 8.686). The constant 168 in the reflection formula is derived from the properties of free space and copper.
Medical Device Shielding: MRI machines generate intense magnetic fields at specific radio frequencies. The shielded room, or "Faraday cage," uses layers of copper or aluminum to contain these signals, preventing interference with other hospital equipment and protecting patient data. The thickness and material are chosen based on the precise frequency used.
Consumer Electronics: The inside of your smartphone or laptop is a maze of small metal shields (often thin nickel-plated steel cans). These protect sensitive components like the processor and radio chips from interfering with each other, ensuring your Bluetooth, Wi-Fi, and cellular signals remain clear and your device passes electromagnetic compatibility (EMC) regulations.
Aerospace & Defense: Aircraft and military vehicles use shielding to protect onboard digital systems from both internal noise and external threats like electromagnetic pulses (EMP). Here, materials like aluminum are favored for their good shielding-to-weight ratio, which is critical for flight.
Industrial Automation: In factories, variable-frequency drives (VFDs) that control motors emit strong electromagnetic interference (EMI). Metal enclosures with proper gasketing and thickness are calculated to shield this noise, preventing malfunctions in nearby robotic arms or sensor systems.
First, there is the common assumption that "a higher Shielding Effectiveness (SE) is always better." While high SE is ideal, it involves trade-offs with cost, weight, and manufacturability. For instance, if a device requires 40dB of SE but you design for 80dB, costs can skyrocket due to the need for materials like Mu-metal. It's crucial to use tools to check the SE across your target frequency band and adopt an approach of selecting materials and thicknesses that are adequate and necessary.
Next is the tendency to think that "material property values are always constant." The conductivity and permeability used in simulators are values for ideal purity and annealing states. Actual materials like extruded aluminum or steel sheets vary due to alloy composition and processing history. For example, the conductivity of A5052 aluminum alloy is about half that of pure aluminum. In real-world design, you must account for measured values from datasheets and incorporate safety margins.
Finally, "calculations that ignore the effects of seams and apertures" are the biggest pitfall. Tools calculate the performance of a uniform flat plate, but actual enclosures have gaps, screw holes, and display windows. Electromagnetic waves leak easily from these points. Even if a flat plate has an SE of 100dB, a 1mm slit can dramatically reduce the overall SE. Treat simulation results as the "potential capability of that material." In a real product, designing for shielding continuity (using gaskets, etc.) is where the real work lies.
For a copper shield at 100 MHz with 2 mm thickness: skin depth δ = 0.0633 mm, absorption A = 25.3 dB (exponential attenuation through material), reflection R = 94.7 dB (impedance mismatch loss at surface). Total SE = 120 dB, sufficient for medical device enclosures. Mu-metal at same frequency/thickness yields δ = 0.159 mm, A = 18.6 dB, R = 112 dB, SE = 130.6 dB—superior for shielding low-frequency magnetic fields in precision instrumentation.