GB 12190 specifies the methods used in China to measure the shielding effectiveness of electromagnetic shielding enclosures, the screened rooms and chambers in which EMC testing is carried out. A shielding enclosure is only useful if its attenuation is known, and the standard therefore defines the measurable quantity, the ratio between the field measured with and without the enclosure in the path, expressed in decibels. It describes the frequency ranges over which the measurement is made, extending from the low-frequency magnetic field region where the attenuation depends on the material of the screen, through the resonant region where the enclosure dimensions determine the behaviour, to the microwave region where aperture leakage dominates. The test sources, antennas, probes and the signal generator and receiver arrangement are specified for each band, together with the measurement of the reference level, the positioning of the source and the probe, and the points at which the enclosure is tested, including any doors, seams and penetrations. Acceptance criteria for a specified shielding performance class are given.