Formula for the Field Excited in a Cavity Sealed by a Plate Backed With a Conductive Elastomer

A formula is derived for the field excited in a cavity by an external field when the cavity has an aperture sealed with a plate placed over a conductive elastomer gasket. The predicted cavity shielding effectiveness (SE) when the cavity and the plate are both perfectly conducting duplicates published numerical results. The formula can be used to estimate the SE when only the attenuation provided by the closeout is known.