Magnetic Field - Any Insights (Please)

Adam Gold · November 8th, 2009, 11:59 AM

Quote:

Originally Posted by Benny Sampson

My simple assumption was that with all of the collective knowledge on this forum a concrete consensus could be attained.

Ha! Good one!

Ask 20 DVInfoers their opinion and you'll get 40 different answers. That's what makes this place great.

Benny Sampson · November 8th, 2009, 12:17 PM

:) Good one! A man with a watch knows the time, one with two is never sure........

Benny Sampson · November 9th, 2009, 10:58 AM

Okay folks, it's time for the engineers/scientists to pipe in; Just how strong are speaker magnets?

Richard Crowley · November 14th, 2009, 12:24 PM

Many of the more recent speakers made for table-top (or "computer") use were "magnetically shielded" because the mag field would affect CRT monitors. Now that CRT monitors are dying off in favor of flatscreen monitors, its not as much an issue anymore. Much of the "shielding" came from simply gluing a second magnet on the back of the speaker in the opposite direction (to cancel out external magnetic fields.)

If I had a bunch of reel-to-reel (or cassette) analog tapes 2 ft away from an unshielded speaker, I would not be worried about the effect on the recordings. I might move them to the other wall (or another room) for future reference, but nothing to freak out about. Remember that it takes a MOVING magnetic field to achieve any significant erasure.

Anyone who remembers early days of color TV CRTs knows that the earth's own magnetic field is a significant force, and it is the reason most CRT color TVs have automatic degaussing coils (the loud buzz when you first turn them on.) Nobody worries about the DC (constant) magnetic field of the earth erasing mag tape, and a 10-inch speaker 2 feet away is not significantly more powerful than the earth's own magnetic field. Now if you were waving the reel back and forth in front of the speaker for hours at a time, THEN you might have something to worry about.

Note that simple metal boxes, even ferrous metal (steel, etc.) offer no significant magnetic shielding because magnetic fields pass through them, mostly undiminished. Special materials (such as "mu-metal") are required for significant magnetic shielding, and it is very expensive and very difficult to make things out of mu-metal because merely bending or cutting it greatly reduces its magnetic shielding properties. Mu-metal is typically used only in very special cases where the advantages warrant the difficulty and expense. (For example, shielding tape playback heads, etc.)

Degaussing (erasing) magnets are AC (alternating current like from the power mains) and are quite strong. Many people have lost their old-fashioned mechanical wrist-watches to the strong magnetic field of these things. But that is what it takes to throughly erase a mag analog tape. Digital tapes (both audio and video) are much more difficult to degauss and most of the time it is cheaper to buy new tape than it is to try to completely erase a digital mag tape. This is also the reason that 99.9999% of all this planet's most valuable data is archived on digital mag tapes.

Reel to reel tapes in cardboard boxes 2 ft from the speaker magnet would not be a great concern, IMHO. If you haven't moved them away already (WHY?), then do it today for peace of mind, and get on with your life.

November 14th, 2009, 12:24 PM	#49
Richard Crowley Trustee Join Date: Aug 2007 Location: Portland, Oregon Posts: 1,238	Speaker mag fields Many of the more recent speakers made for table-top (or "computer") use were "magnetically shielded" because the mag field would affect CRT monitors. Now that CRT monitors are dying off in favor of flatscreen monitors, its not as much an issue anymore. Much of the "shielding" came from simply gluing a second magnet on the back of the speaker in the opposite direction (to cancel out external magnetic fields.) If I had a bunch of reel-to-reel (or cassette) analog tapes 2 ft away from an unshielded speaker, I would not be worried about the effect on the recordings. I might move them to the other wall (or another room) for future reference, but nothing to freak out about. Remember that it takes a MOVING magnetic field to achieve any significant erasure. Anyone who remembers early days of color TV CRTs knows that the earth's own magnetic field is a significant force, and it is the reason most CRT color TVs have automatic degaussing coils (the loud buzz when you first turn them on.) Nobody worries about the DC (constant) magnetic field of the earth erasing mag tape, and a 10-inch speaker 2 feet away is not significantly more powerful than the earth's own magnetic field. Now if you were waving the reel back and forth in front of the speaker for hours at a time, THEN you might have something to worry about. Note that simple metal boxes, even ferrous metal (steel, etc.) offer no significant magnetic shielding because magnetic fields pass through them, mostly undiminished. Special materials (such as "mu-metal") are required for significant magnetic shielding, and it is very expensive and very difficult to make things out of mu-metal because merely bending or cutting it greatly reduces its magnetic shielding properties. Mu-metal is typically used only in very special cases where the advantages warrant the difficulty and expense. (For example, shielding tape playback heads, etc.) Degaussing (erasing) magnets are AC (alternating current like from the power mains) and are quite strong. Many people have lost their old-fashioned mechanical wrist-watches to the strong magnetic field of these things. But that is what it takes to throughly erase a mag analog tape. Digital tapes (both audio and video) are much more difficult to degauss and most of the time it is cheaper to buy new tape than it is to try to completely erase a digital mag tape. This is also the reason that 99.9999% of all this planet's most valuable data is archived on digital mag tapes. Reel to reel tapes in cardboard boxes 2 ft from the speaker magnet would not be a great concern, IMHO. If you haven't moved them away already (WHY?), then do it today for peace of mind, and get on with your life. Last edited by Richard Crowley; November 14th, 2009 at 12:38 PM. Reason: fixed confusing double-negative

November 8th, 2009, 12:17 PM	#47
Benny Sampson New Boot Join Date: Oct 2009 Location: Quebec Posts: 22	:) Good one! A man with a watch knows the time, one with two is never sure........

November 9th, 2009, 10:58 AM	#48
Benny Sampson New Boot Join Date: Oct 2009 Location: Quebec Posts: 22	Okay folks, it's time for the engineers/scientists to pipe in; Just how strong are speaker magnets?