The Reconstruction Finance Corporation (RFC) was established during the Hoover administration with the primary objective of providing liquidity to, and restoring confidence in the banking system. The banking system experienced extensive pressure during the economic contraction of 1929-1933.
Typical materials used for electromagnetic shielding include sheet metal, metal screen, and metal foam. Any holes in the shield or mesh must be significantly smaller than the wavelength of the radiation that is being kept out, or the enclosure will not effectively approximate an unbroken conducting surface.
Mechanical waves use the substrate to move along (the medium). Electromagnetic 'waves' are a combination of a particle and a wave (they have the same properties of both) == photons. Electromagnetic waves encompass light, microwaves, radio waves, and gamma waves (to name a few). Since EM particles act as waves, but also have a mass they actually exist in the Space-Time continuum and can travel without a medium.