Perforated metal can do more than stop wind-borne debris. Properly connected to a building, a perforated metal barrier also attenuates electromagnetic and radio frequency interference (EMI/RFI) – a consideration for emergency operations centers, communications rooms and other sensitive facilities.
STORM SHIELD Shielding Specification
- The shutter system to have a shielding effectiveness per the Dash, Straus & Goodhue study.
- The shutter system shall have a shielding effectiveness of at least 93.69% attenuation effectiveness, when a tightly sealed electrical contact is made.
- The shutter system shall have a shielding effectiveness of 24 dB attenuation effectiveness, when a tightly sealed electrical contact is made.
Perforated Metal and EMI/RFI
One of the most common uses for perforated metal is 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 Industrial Perforators Association contracted Intertek Testing Services, testing laboratories in Boxborough, MA, to evaluate the shielding effectiveness of 16 perforation and material combinations at 13 frequency levels. The results of these tests, with descriptions of the test samples and the frequencies tested, are shown in the EMI shielding test charts. Results have been expressed in dB of shielding attenuation and in % of attenuation.
What the Tests Found
The test results show that a shielding effectiveness of 40 dB provides 99.000% attenuation of the electromagnetic (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 18 GHz.
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. The obvious conclusion is that there are many perforated patterns that designers can choose from to meet their design requirements.
Understanding Decibels and Percent Attenuation
Shielding effectiveness is measured on a logarithmic decibel scale. Every 20 dB of shielding reduces the strength of the field passing through by a factor of ten, so 20 dB corresponds to 90% attenuation, 40 dB to 99%, and 60 dB to 99.9%. That is why small differences in percentage at the high end of the charts represent large differences in shielding performance.
For project-specific shielding requirements, request specifications and describe the frequencies and attenuation levels your facility requires. See also the sun control study.