National Technical Systems of Boxborough, Massachusetts:

• Shielding effectiveness rating for the enclosure of 70 db from frequencies of 20 megahertz to 3.29 gigahertz. (1) This capability to dissipate charges through eddy currents greatly enhances the ability to ground the structure. A well-designed enclosure can handle transient charges or currents like lightning through the rise in earth potential, without the need for an optimal ground.

• (1) The use of decibels is commonly associated with sound level, the ratio of sound strength to a reference value. The concept is also useful in measuring the relative strength of electro-magnetic phenomena such EMI and RFI. The calculation is A (attenuation in db) = 20 log (V/V’), where V is the voltage of the initial current applied or EMI and V’ is the residual current or EMI. Thus, a 70db reduction in an EMI current represents a reduction in voltage of the EMI current of approximately 3162:1. A 40db reduction represents a ratio of initial to residual voltage of approximately 100:1.