Attenuation is the logarithmic ratio between input and output signals. For power, we use 10 log; for voltage or current, 20 log. Since EMF shielding is measured as field strength (V/m or A/m), power relationship is proportional to the square of field. Thus the correct attenuation calculation for shielding effectiveness is:
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where:
* M = maximum permissible field strength
* R = measured field strength after shielding
This ensures the shield reduces field intensity to below allowable limits.
References: IEEE Std 299 (Shielding Effectiveness Measurement), IEC 61000-5-7 (EMC mitigation).