The use of mesh and open work materials is only employed where a very economical construction is required and
only a low shielding performance is necessary. Likewise the high permeability foils are not normally employed,
although the low frequency performance of these can be extremely good when related to the foil thickness. A
more economical construction often results from the use of welded steel in thicker gauges, although high
permeability materials are required where the shield must provide high attenuation to extremely low frequency
or constant magnetic fields.
The most efficient practical shielding is provided by a continuously welded steel sheet clad enclosure. Standard
practice in Great Britain is to employ a 1.2 mm (0.048") thick electrogalvanized mild steel sheet continuously
seam welded along all edges using an inert gas shrouded electric arc welding process. This approach may
achieve the highest performance realizable at an economical price. Construction may either be supported by
the walls and ceiling of the parent room, or the shield may incorporate its own independent steel framework.
The shielding effectiveness of a shielded enclosure can be improved with the use of double shields. As indicated
in the earlier section on the theory of shielding, the shielding effectiveness of two parallel (but slightly
separated) shields is better than that of one double thickness shield but not twice as effective as a
single-thickness shield. The actual improvement in shielding efficiency is dependent upon the degree of
electrical isolation maintained between the two shields.
At least one manufacturer (8-20) of shielded rooms maintains that the isolated double shielded room is
substantially more effective than either the single-shielded or the "not isolated double shielded" room. The
same types of doors, ventilation apertures, and filters described for the modular rooms are used except that in
many cases an rf-proof access lock is provided; this may combine interlocks between the doors and completely
automatic operation either by electric, hydraulic, or pneumatic systems.
8.9.3 Foil Room Liners.
When the shielding requirement does not justify an all-welded steel room or a separate screen room, it may be
possible to use metal foils. For example, a copper foil nominally 5 mils thick with continuous soft soldered
seams may be employed. This copper foil can be glued to the walls, floor, and ceiling to provide a complete
lining to an existing room. If this construction is used in conjunction with gasketed metal doors, properly
designed vents, and electrical filters, performance, while not being good for low frequency magnetic fields, can
be comparable to welded steel at the higher frequencies. To achieve this performance, it is essential that all
seams and joints be carefully soldered to establish continuous bonds. The cost of construction is not as low as it
might first appear, especially when the additional complications which result from the need to provide fixtures
for internal decorative finish and equipment mounting within the shielded area is considered. In general, this
form of construction is only used where a relatively unsophisticated enclosure is required, e.g., in certain
electro-medical work. If even more economy is required, it is possible to omit the soldering of the joints
between the copper foils and use a conductive adhesive tape which is less expensive to install. If only electric
fields are present at low frequencies, then a copper foil shield constructed in this manner will probably be
When shielding is required only for microwave frequencies, a very economic shield may be constructed using
aluminum foil of approximately 5 mils thick glued to the walls, floor, and ceiling. An overlap between adjacent
foil sheets of approximately 5 cm (2 inches) should be allowed; these overlaps should be secured with
aluminum-backed contact adhesive tape. This type of shield is most effective at frequencies above several