Magnetic Shields

[neo4s]

I just got a pair of shielded woofers. It appears that there is simply a piece of thin sheet metal formed around the magnet acting as the shield. My question in... how does the metal prevent the magnetic field from radiating from the magnet? Does the metal absorb or deflect the flux? What types of metal can be used as a shield? Does shield thickness need to be proportional the field strength?

Thanks

 

[CTho9305]

My understanding is that the metal converts the magnetic fields to currents in the metal. Assuming that is correct, then it would only need to be thick enough so that the currents don't melt it.

 

[Smilin]

Uber Bump.


I've always been curious about this. For instance why does placing a plate between two monitors stop interference but a metal plate won't save a hard drive from a magnet. Is it just a matter of the strength of the field?

Vry super curious. Someone please spout some technobabble that I can digest on for a while.

Thanks.

 

[f95toli]

You need a proper shielding material in order to make good magnetic shield, usually an alloy. Look up for example mumetal and permalloy in a table and you will see what I mean. The shielding efficiency of a material also depends on the frequencies that you are trying to shield, is is usually a good idea to mix different materials.
The best shields you can get are usually layered structures of mumetal and type-I superconductors (such as lead), of course you have to cool it but sometimes it is worth it.

 

[Geniere]

Any material that can be magnetized such as iron can be used a a shield. As f95toli stated, some materials are better than others depending on their permeability. Mumetal is good magnetic shield whereas copper, being non-magnetic, is very poor. You would want to avoid using an alloy such as steel as it would become permanently magnetized by the field you're trying to shield.

It is possible that current may be induced in the shield but that is not why it acts as a shield. To induce a current in the shield, the magnetic field would have to vary. A speaker magnet generates a non-varying field and would not induce a current in the shield.

Regards