In order to define specific requirements and design the appropriate shielding strategy, it is helpful to understand some key concepts and terms. These definitions are common in the industry and will provide a valuable foundation for people looking to develop a deeper knowledge and understanding of magnetic shielding theory.
Magnetic Field Strength (H)
Magnetic Field Strength (H) describes the intensity of a magnetic field in free space. Field strength (H) is measured in Oersteds (Oe) and is a function of the intensity of the magnetic source and the distance from the source at which it is measured.
Magnetic Flux Density (B)
Magnetic Flux Density (B) describes the concentration of magnetic lines within a material. Flux density (B) measured in Gauss (G), describes the number of magnetic lines that exist in a given cross sectional area of a material. Flux density depends on the intensity of a magnetic source, the distance of the material from the magnetic source, and the material’s attractiveness, or permeability, to the magnetic field.
Magnetic Permeability (µ)
Magnetic Permeability (µ) refers to a material’s ability to attract and absorb magnetic lines of flux. Materials with a strong attraction for magnetic fields generally have a high permeability. Mathematically, permeability µ = B/H, which states that the permeability of a material can be determined by taking the ratio of the measured flux density (B) in the material at some point in space to the magnetic field strength (H) at the same point in space. Shielding materials are typically chosen for their unusually high permeabilities.
Saturation refers to a material’s limiting point for absorbing additional lines of magnetic flux within a given cross sectional area. Each permeable material has a specific saturation point. Once a shielding material becomes saturated, it will no longer attract lines of flux and will no longer function as expected. (Note: saturation and permeability characteristics of a material are inversely related: the higher a material’s permeability, the lower its saturation point.)
Frequency is measured in cycles per second or Hertz (Hz), and is the same as the operating frequency of a field’s source. For example, a 60Hz power line will yield a 60Hz magnetic field. Knowing a field’s frequency is imperative for determining the proper composition and thickness of materials to be used.
Attenuation is a ratio for measuring the effectiveness of a given shield and often defines the shielding objective. The ratio is expressed in field strength (H) at a given point versus the resulting field strength (H) at the same location with the introduction of the magnetic shield. For example, a shield that provides a field reduction of 100 times has an attenuation of 100:1.