[Al Skierkiewicz]

Magnetic fields are interesting little beasties. You cannot block them with lead, wood, or copper shielding. Ferrous metals work to a certain degree but combinations of nickel, steel and other metals due tend to work in conjunction to shield the fields by bending them out of the way. A sandwich of high nickel steel, steel, and copper in alternating layers works best. There are a number of materials that are available for purchase but they are expensive and heavy. (Mumetal, guassshield and magalloy were some of the names I have used. I am not sure whether they were manufactured by companies now defunct or not.)
Fields to tend to reduce with distance but depending on the surrounding materials may actually be focused into a particular direction. Remember, magnetic fields, like elctricity, must flow from pole to pole so think of your classic bar magnet picture with the field curving from North to South poles.
Magnetic fields become particularly annoying to electronics when they are changing or moving. Remember that a wire will have current induced in it if it is in the presence of a moving or changing magnetic field. (Sometimes referred to as "cutting the (magnetic) lines") But remember that creating a magnetic field with electric current is what drives our motors. The field generated in the armature works against/with the filed in the magnet of the motor.