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What is field attenuation?

April 26, 2017attenuation field

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What is field attenuation?

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This is also known as the shielding factor (S) and is a ratio of the magnetic field strength outside of the magnetic shield (Ha) and the resultant field on the inside of the shield ie Ha/Hi (no units) or S = 20 x log(Ha/Hi) (Db). There are various formula based on the permeability of the material, the shape and size of the shield and the material thickness. In most cases these formulae are only approximate. For a closed shielding can : S = 4/3 X (Mu x d/D) where Mu : The permeability(relative) d : material thickness D : Shielding Diameter For a long hollow cylinder in a magnetic transverse field : S = Mu x d/D For a cubic shielding box : S = 4/5 X (Mu x d/a) a : box side length. In the case of multiple layer shields with air gaps provided by insulating spacers the shielding factors of the individual shields are multiplied together resulting in excellent shielding factors.