Todays dumb question

["idd" <idd () indepthdefence plus com>]

There used to be a name for a technology (or way of implementing technology)
to prevent electronic eavesdropping.  I think it was along the lines of
fitting wire mesh into the walls of the building so that it would defeat
eavesdropping equipment.  And for the life of me I cannot remember what it
was called.
Apparently it was back in the 80's/early 90's and there may have been one or
two research projects on it.

Apologies for my stupidity, but I just can't find it.

Any ideas?  

Many thanks,

R