Faraday cages of the conducting type are not effective for lower frequencies; https://en.wikipedia.org/wiki/Electromagnetic_shielding#Magnetic_shielding mentions a frequency of 100 kHz. The electromagnetic spectrum is composed of both an electrical, and a perpendicular magnetic field. If a faraday cage works by 'earthing' the electrical field, would the cages effectiveness depend on its size? For lower frequencies the wave length is longer, so does the electrical field 'dodge' the cage? hence the need to saturate the magnetic field with Mu-metal.