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Comprehensive Glossary of Common Terms in EMI Shielding

Bookmark this page. We know, probably better than anyone, just how vast and complex the EMI shielding industry can be to understand. To help you navigate, we've updated our extensive collection of glossary terms to nearly 100, with new definitions spanning from absorption loss to thermal conductivity -- and everything in between.


Absorption Loss: Attenuation of an electromagnetic wave or energy encountered in penetrating a shield caused by the induction of current flow in the barrier and the resulting I2R loss. Usually stated in dB (decibels). 

Ambient Electromagnetic Environment: That electromagnetic field level existing in an area and emanating from sources other than the system under test.

Attenuation: A reduction in energy. Attenuation occurs naturally during wave travel through transmission lines, waveguides, space or a medium such as water, or may be produced intentionally by inserting an attenuator in a circuit or a shielding absorbing device in the path of radiation. The degree of attenuation is expressed in decibels or decibels per unit length.

Attenuator: An arrangement of fixed and/or variable resistive elements used to attenuate a signal by a desired amount.


Cross Coupling: Coupling of the signal from one channel to another where it becomes an undesired signal.

Conductivity: Capability of a material to conduct electrical currents.


Decibel (dB): A convenient method for expressing voltage or power ratios in logarithmic terms.

Degradation: An undesired change in the operational performance of a test specimen. Degradation of the operation of a test specimen does not necessarily mean malfunction. 

Dielectric Loss Tangent: This quantifies a materials inherent dissipation of electromagnetic energy to heat.  The dielectric loss tangent is defined by the angle between the capacitors impedance vector and the negative reactive axis.


Electromagnetic Compatibility (EMC): A measure of an equipment’s ability to neither radiate nor conduct electromagnetic energy, or to be susceptible to such energy from other equipment or an external electromagnetic environment.

Electromagnetic Interference (EMI): Undesired conducted or radiated electrical disturbances, including transients, which can interfere with the operation of electrical or electronic equipment. These disturbances can occur anywhere in the electromagnetic spectrum.

Emanation: Undesired electromagnetic energy radiated or conducted from a system.

Electromagnetic Waves: Electromagnetic waves come in three forms – Magnetic, Electric and Plane Wave as follows

Magnetic Field or H-Field: An induction field caused predominantly by a current source. Also called a low impedance source, such as may be generated by a loop antenna.

Electrical or E-Field: A field induced by a high impedance source, such as a short dipole.

Plane Wave: An electromagnetic wave which exists at a distance greater than a wavelength from the source, where the impedance of the wave is nearly equal to the impedance of free space – 377 ohms.

Electromagnetic Pulse: A short burst of high electromagnetic energy


Gasket-EMI: A material that is inserted between mating surfaces of an electronic enclosure to provide low resistance across the seam and thereby preserve current continuity of the enclosure.

Ground: A reference plane common to all electronic, electrical, electromechanical systems and connected to earth by means of a ground rod, ground grid, or other similar means.


Hertz: An international designation for cycles per second (cps).


Insertion Loss: Measure of improvement in a seam, joint or shield by the addition of a conductive gasket. Usually stated in dB.

Interference: Any electromagnetic phenomenon, signal or emission, man-made or natural, which causes or can cause an undesired response, malfunctioning or degradation of performance of electrical or electronic equipment.

Immunity: The ability of a device or equipment to resist malfunctioning when exposed to external electromagnetic interference.

Impedance: The measure of the opposition that a circuit (seem interface) presents to a current when a voltage is applied.


Malfunction: A change in the equipment’s normal characteristics which effectively destroys proper operation.


NRL Arch: The Naval Research Laboratory (NRL) developed test method for reflectivity of flat absorber materials


Permeability: The capability of a material to be magnetized at a given rate. It is a non-linear property of both the magnetic flux density and the frequency of wave propagation.

Permittivity: The ability of a substance to store electrical energy in an electric field.


Radio Frequency (RF): Any frequency at which coherent electromagnetic radiation of energy is possible. Generally considered to be any frequency above 10 kHz.

Radio Frequency Interference (RFI): Used interchangeably with EMI. EMI is a later definition which includes the entire electromagnetic spectrum, whereas RFI is more restricted to the radio frequency band, generally considered to be between the limits 10 kHz to 10 GHz.

Reflection Loss: Attenuation of the electromagnetic wave or energy caused by impedance mismatch between the wave in air and the wave in metal

Relative Conductivity: Conductivity of the shield material relative to the conductivity of copper.

Relative Permeability: Magnetic permeability of the shield material relative to the permeability of free space.


Shield: A metallic configuration inserted between a source and the desired area of protection which has the capability to reduce the energy level of a radiating electromagnetic field by reflecting and absorbing the energy contained in the field.

Shielding Effectiveness: A measure of the reduction or attenuation in electromagnetic field strength at a point in space caused by the insertion of a shield between the source and that point. Usually stated in dB.

Shielding Increase: The difference of an electromagnetic field amplitude emanating through a seam (measured under fixed test conditions) with and without the gasket in the seam, with the force joining the seam remaining constant. The difference is expressed in dB based on voltage measurements.

Skin Depth: Distance which a plane wave must travel through a shield to be attenuated 1/e, or approximately 37 percent of its original value. It is a function of the shield’s conductivity and permeability and the wave’s frequency.

Skin Effect: Increase in shield resistance with frequency because of crowding of current near the shield surface because of rapid attenuation of current as a function of depth from the shield surface.

Surface Treatment: Coating or plating of mating surfaces of a junction.

Susceptibility: Measure of the degradation of performance of a system when exposed to an electromagnetic environment.

Shielding Effectiveness: The difference of an electromagnetic amplitude emanating from a source within an enclosure, and that from a source in free space. The difference is expressed in dB based on voltage measurements.


Volume Resistivity: A resistance measurement which takes sample thickness into account.  Units of measurement are typically ohm-cm.


Wave Impedance: The ratio of electric field intensity to magnetic field intensity.

Wavelength: The wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave’s shape repeats.



Abrasion Resistance: The resistance of a material to wearing away by contact with a moving abrasive surface. Usefulness of standard tests very limited. Abrasion resistance is a complex of properties: resilience, stiffness, thermal stability, resistance to cutting and tearing.


Cold Flow: Continued deformation under stress.

Compression Set: The decrease in height of a specimen which has been deformed under specific conditions of load, time, and temperature. Normally expressed as a percentage of the initial deflection (rather than as a percentage of the initial height).

Compression Strength: The capacity of a material or structure to withstand loads tending to reduce size.

Compression Modulus: The mechanical property of linear elastic solid materials where you measure the force that is needed to stretch a material sample.

Conversion Coating: A protective surface layer on a metal that is created by a chemical reaction between the metal and a chemical solution typically applied in accordance with MIL-DTL-5541.  Type 1 Conversion Coating is Hexavalent CR3 Type.  Type 2 Conversion Coating is Trivalent CR6 Type.

CBRN: Chemical, Biological, Radiological and Nuclear exposure.  Current term for NBC.

CVCM: Collected Volatile Condensable Materials – A measurement of Outgassing.


Durometer: An instrument for measuring the hardness of rubber. Measures the resistance to the penetration of an indentor point into the surface of the rubber.

Density: The relationship between the mass of a substance and how much space it takes up (volume).


Elasticity: The property of an article which tends to return to its original shape after deformation.

Elastic Limit: The greatest stress which a material can develop without a permanent deformation remaining after complete release of the stress. Usually this term is replaced by various load limits for specific cases in which the resulting permanent deformations are not zero but are negligible.

Elastomer: A general term for elastic, rubber-like substances.

Elongation: Increase in length expressed numerically as a fraction or percentage of initial length.

EPDM: Ethylene Propylene Diene M-Class Synthetic Rubber used for gaskets in harsh environments – NBC Military applications.


Fatigue Strength: The amplitude or range of cyclic stress that can be applied to a material without causing fatigue failure – endurance limit.

Ferrex: Tin plated, copper clad stainless steel wire.

Flammability Rating: The classification of plastics according to how they burn in various orientations and thicknesses.  The standard for Safety of Flammability is UL-94 published by Underwriters Laboratory.

Flexural Strength: Known as the modulus of rupture, bend strength or rupture strength.  The stress in a material just before it yields in a flexure test.

Flexural Modulus: The ratio of stress to strain in flexural deformation or the tendency of a material to bend.

Fluorosilicone: A silicone polymer chain with fluorinated side-chains for improved oil and fuel resistance.

FEA: Finite Element Analysis (FEA) – Mechanical modeling software.  Mechanical displacement formulations method to calculate component displacements, strains and stresses under internal and external loads.


Galvanic Corrosion: The process by which dissimilar metals in contact with each other oxidize or corrode.  Must have dissimilar metals, electrical conductivity between them and the conductive path must allow the metal ions to move from the anodic to cathodic metal.  

Galvanic Compatibility: Dissimilar metals that corrode little or none in a corrosive environment.

Gravimetric Weight Loss: The loss in metal created by corrosion when two dissimilar metals are in contact in a corrosive environment.


Hardness: Relative resistance of rubber surface to indentation by an indentor of specific dimensions under a specified load. (See Durometer). Numerical hardness values represent either depth of penetration or convenient arbitrary units derived from depth of penetration. Devices for measuring rubber hardness are known as durometers and plastometers. Durometers are used most commonly.  The higher the durometer number, the harder the rubber, and vice versa.

Hardness Shore A: Durometer reading in degrees of hardness using a Type A Shore durometer. (Shore A hardness of 35 is soft; 90 is hard).

HDT: Heat Deflection Temperature (HDT) – is the temperature at which a polymer of plastic sample deforms under a specific load.

Hygroscopic: The ability of a substance to attract and hold water molecules from the surrounding environment.  Also termed “wick”.


Izod Impact: The kinetic energy needed to initiate fracture and continue the fracture until the specimen is broken.


LOI - Limited Oxygen Index: Is the minimum concentration of oxygen, expressed as a percentage, that will support combustion of a polymer.


Modulus of Elasticity: The ratio of the stress applied to a body or substance to the resulting strain within the elastic limit.

Monel: A family of alloys primarily composed of nickel and copper with small amounts of Iron, Manganese, Carbon and Silicone.


NBC: Nuclear, Biological, and Chemical exposure.


Outgassing: The release of gas that was dissolved, trapped, frozen or absorbed in a material.


Permeability: A measure of the ease with which a liquid or gas can pass through a material.

Permanent Set, Stress and Strain Relaxation: Permanent Set is defined as the amount of residual displacement in a rubber part after the distorting load has been removed. Stress Relaxation, or Creep, is a gradual increase in deformation of an elastomer under constant load with the passage of time, accompanied by a corresponding reduction in stress level.


Resilience: The ratio of energy given up on recovery from deformation to the energy required to produce the deformation – usually expressed in percent.

RoHs Compliant: The Restriction of Hazardous Substances.  European Directive 2002/95/EC restricts the use of specific hazardous materials found in electrical and electronic products.


SAE: Society of Automotive Engineers maintain committees to develop EMI gasket test standards.

Shear Modulus: The coefficient of elasticity for a shearing or torsion force.

Surface Treatment: Coating, plating or conversions coating of mating surfaces of a junction.

Specific Gravity: The ratio of the density of a substance to the density of a reference substance; equivalency.


Tear Strength: The force per unit of thickness required to initiate tearing in a direction normal to the direction of the stress.

Tensile Set: The residual elongation of a test sample after being stretched and allowed to relax in a specific manner.

Tensile Strength and Elongation: Tensile Strength is the force per unit of the original cross sectional area which is applied at the time of the rupture of the specimen during tensile stress. Elongation is defined as the extension between benchmarks produced by a tensile force applied to a specimen, and is expressed as a percentage of the original distance between the marks. Ultimate elongation is the elongation at the moment of rupture. Tensile Stress, more commonly called “modulus,” is the stress required to produce a certain elongation.

TML: Total Mass Loss – A measurement of outgassing – loss of weight.

Thermal Conductivity: The property of a material to conduct heat.