What happens when you place an iron plate near that magnet?

Let's see the picture below.  The field on the other side of the plate is almost nil.  This is because the plate has diverted the field, causing a lot of it to flow within the plate itself instead of in the air. 

This is sometimes used to help shield a speaker from your TV set.  Placing the speaker in a box lined with sheet steel prevents the speaker magnet from messing with the colors on your TV screen.  Or, the speaker manufacturer may place shielding around the speaker magnet itself - much smaller and more effective.