I'm developing a type of enclosure that provides EMR shielding. It's constructed of two layers of shielding material. I don't have access to professional test equipment, so my dynamic range is limited. Therefore, I'm only able to measure the attenuation of a single layer of material to any degree of confidence. With two layers, the shielded signal strength is at or below the noise floor. Is there a formula for estimating overall attenuation given two layers of material of known attenuation?