The other day, I had a nice chat with Paolo on the science behind these products that shields electro-magnetic (EM) waves. He recalled the formula that is used to calculate the skin depth of a conducting material in order to understand whether a certain EM signal can penetrate it or not.
An electromagnetic wave entering a conducting surface is damped and reduces in amplitude by a factor 1/e in a distance ∂ given by:
where ω is the angular frequency of the radiation and σ the electrical conductivity of the metal. This distance is referred to as the skin depth of the conductor. The effect is caused by electromagnetic induction in the metal which opposes the currents set up by the wave E-field, and holds for oscillations below about 1012 Hz.