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:

where:

M = maximum permissible field strength

R = measured field strength after shielding

This ensures the shield reduces field intensity to below allowable limits.