I stumbled across this one on the net too. Its from URL: http://www.unik.no/~robert/hifi/faq/faq-9.html so you can get to the source. Its more or less correct. The next to last sentence in the first paragraph is correct; however, that doesn't mean shielding lining the internal walls with metal can't be done as an after thought. That being said, the last sentence in the first paragraph is the one I'm uncertain of. Hopefully the first link to links I provided helped some.
9.15 Can I magnetically shield my speakers for use near a TV?
You probably will need to buy speakers that are made with an integral magnetic shield. Magnetic shielding is usually done by either shielding the speaker magnet or by cancellation of the magnetic field very close to the magnet, or by both. Shielded speakers are NOT built by lining the enclosure with metal. While it sounds like a good idea, it doesn't work.
A common magnet shield is a mild steel cup around the magnet. This is the cheapest shield, and is usually fairly ineffective. It also will interfere with the speaker's critical magnet gap, so this type of shield can hurt speaker performance by shorting the magnetic field and reducing the magnetic flux density in the gap, which can reduce efficiency and affect the speaker's low frequency performance.
Cancellation is done using a reverse-polarized magnet glued to the back of the main magnet. If done right, it can almost completely cancel the rear stray field. In some cases it can also increase the magnetic flux density in the gap, which may or may not be desirable.
