EMI Shielding Effectiveness Calculator

Estimate plane-wave shielding effectiveness of a conductive barrier with optional aperture leakage. Calculates absorption loss, reflection loss, aperture degradation, and effective SE vs. frequency. See also: EMI/EMC Basics | EMI/EMC Testing

Barrier Material

Uses the Schelkunoff plane-wave model for a uniform, infinite shield. Aperture SE uses the waveguide-below-cutoff model. Real enclosures have seams, gaskets, and connectors that further degrade SE.

Material

Thickness

Thickness Unit

Frequency Range

Apertures (Optional)

Include aperture leakage

Aperture Type

Hole Diameter

Slot Length

Slot Width

Bolt / Fastener Spacing

Overlap Depth

Contact Quality

Approximate model — Holm contact theory with asperity screening correction. Estimates the effective EMI gap from surface roughness, material hardness, and contact pressure. The √Ra dependence accounts for contact bridges that partially screen the gap. Order-of-magnitude accuracy; does not account for oxide layers or contamination.

Flange Material

Vickers Hardness (MPa)

Surface Finish

Bolt Pressure

Contact Pressure (MPa)

                                weff = 19 × √1.6 × √(1000 / 10) = 240 µm
                            

Effective Gap Width (µm)

Aperture Dim. Unit

Number of Apertures

Aperture Depth

Holes & Slots: SE = 20 log₁₀(λ/2l) − 10 log₁₀(N) + 27.3 t/l
Seams: SE = 20 log₁₀(λ/2l) + 10 log₁₀(l/w) + 27.3 t/l, where l = bolt spacing, w = effective gap width, t = overlap depth. The narrow-slot correction 10 log₁₀(l/w) captures the reduced radiation efficiency of a tight seam.

Skin Depth vs. Frequency

Frequency	Skin Depth (δ)	t/δ	Absorption (dB)	Reflection (dB)	Total SE (dB)

Formulas Used

Skin depth:

                        δ = 1 / √(π f μ0 μr σ)
                    

Absorption loss (signal attenuation traversing the shield):

                        A = 20 log10(e) × t/δ = 8.686 × t/δ  (dB)
                    

Reflection loss (impedance mismatch at both air–shield interfaces):

                        R = 20 log10[(η0 + ηs)² / (4 η0 ηs)]  (dB)

                        where η0 = 377 Ω (free space) and ηs = √(ωμ0μr / 2σ)

This is the exact two-boundary expression. For good conductors (η_s ≪ η₀), it simplifies to the common approximation R ≈ 20 log₁₀(η₀/4η_s). The exact form is always ≥ 0 dB, which correctly handles poor conductors (e.g. water) where η_s can exceed η₀.

Total barrier SE (plane-wave, Schelkunoff model):

                        SEbarrier = A + R  (dB)
                    

Note: The re-reflection correction term B is neglected here; it is significant only when A < ~15 dB.

Aperture leakage (waveguide-below-cutoff model for holes and slots):

                        SEaperture = 20 log10(λ / 2l) − 10 log10(N) + 27.3 tap/l  (dB)

                        where l = max aperture dimension, N = number of apertures, tap = aperture depth

Seam/joint model (narrow-slot with waveguide):

                        SEseam = 20 log10(λ / 2l) + 10 log10(l / w) + 27.3 t/l  (dB)

                        where l = bolt spacing, w = effective gap width, t = overlap depth

The 10 log₁₀(l/w) term accounts for the reduced radiation efficiency of a narrow slot versus a square aperture of the same length. A tighter seam (smaller w) provides more shielding. The seam between fasteners is modeled as a narrow rectangular slot: the bolt spacing defines the slot length, and the effective gap width represents the electrical discontinuity at the mating surface.

Note on welded/brazed joints: A continuous metallurgical bond eliminates the seam entirely. Use the barrier-only SE model (no aperture) for welded or brazed enclosures.

Gasket presets are rough estimates. Real gasket SE depends on transfer impedance, compression, and gasket material — use the manufacturer's published data for precise analysis. Surface finish on the mating flange also affects gasket contact quality; a machined or ground surface is recommended.

Holm contact model for bare-metal seams:

                        weff ≈ k · √Ra · √(H / P)    (k ≈ 19)
                    

where Ra is surface roughness (µm), H is Vickers hardness (MPa), and P is contact pressure (MPa). The physics: softer metal deforms more at asperity tips, reducing the effective gap; smoother surfaces have smaller asperities to begin with; and higher pressure crushes the asperities further.

The √Ra dependence (rather than linear Ra) accounts for asperity screening: rougher surfaces have taller gaps but also more contact bridges per unit length, partially screening the gap electromagnetically. The net dependence on roughness is sub-linear.

The empirical constant k ≈ 19 is calibrated so that standard machined aluminum 6061-T6 (Ra = 1.6 µm, H = 1000 MPa) at 10 MPa bolt pressure gives w_eff ≈ 240 µm, matching the “tight bolts” preset.

EMI gasket, good compression — ~25 µm (conductive elastomer, proper bolt torque)
EMI gasket, typical — ~75 µm (average installation)
Tight bolts — ~250 µm (clean, flat surfaces, high bolt pressure)
Loose screws — ~1 mm (fewer fasteners, lower pressure)
Poor / corroded — ~3 mm (oxidized or warped surfaces)

Limitations: This is an untested empirical approximation intended for order-of-magnitude estimation only. It applies to bare metal contacts and does not account for oxide layers, contamination, or gasket physics. The constant k = 19 is a calibration fit, not a measured value. The “effective gap” is an electromagnetic equivalent (the slot width giving the same SE), not a physical air gap — the actual surface separation is typically only a few × Ra. You can always override the gap width directly for your specific geometry.

Effective SE (with apertures):

                        SEeffective = min(SEbarrier, SEaperture)
                    

Important disclaimer: These are simplified analytical models intended for preliminary design estimates only. Real-world shielding effectiveness depends on many factors not captured here — enclosure geometry, resonances, cable penetrations, gasket aging, surface finish, assembly tolerances, and polarization. Measured SE in actual enclosures frequently differs from analytical predictions by 10–20 dB or more, especially above 1 GHz. The seam gap-width model provides physically reasonable trends but has not been validated against measured data for specific configurations. Always verify critical shielding requirements with measurement per IEEE 299 or MIL-STD-461.

References

S. A. Schelkunoff, Electromagnetic Waves, Van Nostrand, 1943. — Original derivation of the absorption + reflection shielding model.
H. W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009, Chapter 6 (Shielding). — Plane-wave SE formulas, aperture models, seam degradation factors.
C. R. Paul, Introduction to Electromagnetic Compatibility, 2nd ed., Wiley, 2006, Chapter 8. — Shielding effectiveness derivation and aperture coupling.
MIL-HDBK-419A, Grounding, Bonding, and Shielding for Electronic Equipments and Facilities, 1987. — Seam/joint degradation guidelines and contact quality classifications.
NIST, Electrical Resistivity of Pure Metals, CRC Handbook of Chemistry and Physics, 97th ed. — Conductivity values used in material database.
R. B. Schulz, V. C. Plantz, and D. R. Brush, “Shielding Theory and Practice,” IEEE Trans. EMC, vol. 30, no. 3, pp. 187–201, Aug. 1988. — Comprehensive review of shielding models including aperture arrays.
R. Holm, Electric Contacts: Theory and Application, 4th ed., Springer, 1967. — Contact resistance and effective gap theory for metal-to-metal interfaces under pressure.