# Shielding? **URL:** \ **Category:** Factual Questions\ **Created:** [June 4, 2000, 11:16am UTC](https://boards.straightdope.com/t/shielding/21360 "2000-06-04T11:16:44Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![chief](https://avatars.discourse-cdn.com/v4/letter/c/df788c/32.png) [@chief](https://boards.straightdope.com/u/chief)\ **Post date:** [June 4, 2000, 11:16am UTC](https://boards.straightdope.com/t/shielding/21360/1 "2000-06-04T11:16:44Z")
what do they use to magnetically shield speakers? ---
**Author:** ![funneefarmer](https://avatars.discourse-cdn.com/v4/letter/f/9fc348/32.png) [@funneefarmer](https://boards.straightdope.com/u/funneefarmer)\ **Post date:** [June 4, 2000, 12:19pm UTC](https://boards.straightdope.com/t/shielding/21360/2 "2000-06-04T12:19:22Z")
From [Speaker Faq](http://www.unik.no/~robert/hifi/faq/faq-9.html#15) "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." ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 4, 2000, 9:29pm UTC](https://boards.straightdope.com/t/shielding/21360/3 "2000-06-04T21:29:06Z")
If you have an arbitrary (changing, perhaps, or unknown) magnetic field, the easiest way to shield it is with a layer of a strongly magnetic substance, such as iron or steel. It basically mimics whatever arrangement of fixed magnets would be needed to do the job. In a speaker, though, you’ve only got one fixed magnet you need to cancel, and you can describe the field due to that magnet pretty well, so it’s not too difficult to make a fixed magnet for the job, and typically more efficient than the plain iron.