This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
Despite a lot of folk lore and some very impressive sounding wisdom
here on the net and in showrooms, there is effectively nothing that
you can do to the speakers or the monitor, short of moving them away
from one another, that will solve this problem.
The problem comes from the magnetic field created by and surrounding
the magnets in the loudspeaker. It's possible to design a magnet that
has very little external field, but it can be an expensive proposition
for a manufacturer. If the magnets do have large external fields, the
only technique that works is by solving the problem at the source: the
magnet. Special cancelling magnets are used, sometimes in conjunction
with a "cup" shield directly around the magnet.
You'll hear suggestions from people about placing a sheet of iron or
steel between the speakers and the monitor. That might change the
field, but it will not eliminate it. As often as not, it will make it
worse.
You'll also here from people about shielding the speaker by lining the
enclosure with lead or copper. This method is absolutely guaranteed
to fail: lead, copper, aluminum, tin, zinc and other such materials
have NO magnetic properties at all, they will simply make the speaker
heavier and won't solve the problem at all. There is but one material
that has a shot at working: something called mu-metal, a heavy, very
expensive, material designed for magnetic shield that requires
extremely sophisticated and difficult fabrication and annealing
techniques. Its cost is far greater than buying a new set of speakers
that does not have the problem, and it may not even work if the
majority of the offending field is radiated from the front of the
speaker, which you obviously can't shield.
Try moving the speakers relative to your monitor. Often, moving them
an inch or two is enough to cure the problem or at least make it
acceptable. Sometimes, placing the speakers on their sides with the
woofers (the major offenders in most cases) farthest away from the
monitor works. [Dick]
 
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