The actual attenuation of external RF signals provided by a shielded room depends on the nature of the source , its location , and the degree to which the shielded room is compromised by imperfections . Generally speaking , low frequency magnetic fields will have the lowest attenuation i . e . most penetrating . For RF fields which are in the farfiled limit , the radiated plane waves are usually attenuated to the point that they can no longer be detected inside the room . Exceptions come up when there are cracks , openings , or penetrating conductors in the walls of the room . Those imperfections will allow some measurable RF to get into the room , and the higher frequencies will generally be harder to attenuate completely . For near field RF sources i . e . the source is less than a wavelength or so from the room , its a little more complicated due to the decoupling of electric and magnetic fields . As in the case of low frequencies , the magnetic fields will penetrate more readily than the electric fields . So reception of signals leaking into the room under those conditions would depend on the mode coupling or gain of the antenna .