Perforated metal is being used to enclose electrical devices to attenuate the EMI/RFI radiation they emit and to ventilate them at the same time. Many questions have been asked about which perforated pattern should be used to satisfy both of these design requirements and the shielding effectiveness of various perforated patterns and materials. To answer these questions, the l.P.A. contracted with Intertek Testing Services, testing laboratories in Boxborough, MA to evaluate the shielding effectiveness of 16 perforation and material combinations at 9 frequency levels.

The test results show that a Shielding Effectiveness of 40 dB provides 99.000% attenuation of the electro-magnetic (EMI/RFI) radiation while a Shielding Effectiveness of 92 dB, the highest Shielding Effectiveness found in the tests, provides 99.997% attenuation. A Shielding Effectiveness of 40 dB is the targeted minimum in most applications.

Very effective shielding was provided by most samples up to frequencies of 7 GHz. Above that frequency, some of the samples dropped below 99.000% effective, but most samples stayed comfortably above 95% effective even at the highest frequency level of 10 GHz. The obvious conclusion to be reached is that there are many perforated patterns that designers can choose from to meet their design requirements.

The largest single source of leakage is along contact surfaces between two parts. If a tightly sealed electrical connection is not made, the leakage through the interface can be greater than through the structure.