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Old October 30th, 2009, 12:18 PM   #1
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Magnetic Field - Any Insights (Please)

In anticipation of your kind replies, allow me to extend my gratitude. I have as of yet not been capable of securing a relaively definitive answer to this scenario; Evidently, magnetic fileds (such as those produced by an audio speaker) can produce irreparable damage to recording tape of many formats. Albeit that the formulation of digial video tape appears to be more resistant to its negative effects, my understanding is that traditional audio tape is necessarily not. As such, I may have inadvertanly done some harm to my reel to reel music collection. My wood speaker cabinet, which houses two 10 inch woofers, a midrange, and tweeter, sits approximately 1 feet from one of the shelves where the reel to reel music tapes are stored. Is this distance, coupled with the fact that the speakers are in an isolated structure (which would add another 12 inches of distance for a total of 2 feet) be collectively sufficient for shielding? I am most concerned. Thanx Very Much
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Old October 30th, 2009, 12:48 PM   #2
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I don't know how to calculate a safe distance, but that sounds way too close for speakers that size. I'd move 'em pronto.
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Old October 30th, 2009, 02:02 PM   #3
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faraday's cage (I think)

If you keep the tapes inside something metal (as long as it is not itself magnetic) then the magnetic field cannot penetrate it so the tapes should be OK.
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Old October 30th, 2009, 02:08 PM   #4
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Actually, all of the tapes are in their original cardboard boxes. I wonder if lining the inside with aluminum foil would do the trick?
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Old October 30th, 2009, 02:16 PM   #5
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Aluminum is non-magnetic so it wouldn't help. Thin sheet iron or steel or something called mu-metal (a magnetic alloy) would. Easiest and cheapest - move the tapes to another shelf a little farther away.
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Old October 30th, 2009, 02:20 PM   #6
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You know, on the first reading of your post I missed that the total distance is more like 2 feet, rather than one foot. You might be okay with that. Magnetism drops off exponentially with distance. I'd probably just get a magnetic compass, hold it next to the tapes in their present location, and if the needle is unaffected by the speaker's magnet, call it a day and not worry about it, as the magnetic pull would seem to be less than that of the earth's.
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Old October 30th, 2009, 02:56 PM   #7
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Well, that actually is a good idea. The distance itself is approximately 1 foot + the distance of the driver in the cabinet is another 12 inches so, yes, it is more like 2 feet overall. Just how radically does the magnetic potency dimishes over distance?
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Old October 30th, 2009, 03:18 PM   #8
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Inverse square - double the distance, 1/4 the strength
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Old October 30th, 2009, 07:38 PM   #9
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Thanx Folks :) I searched the forum and read with interest an older thread from the Thin Black Line forum where a similar topic was discussed;Numerous memebers chimed in but, again, no definitive consensus was attained. I am certain that some empirically established criteria exists somewhere.
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Old October 30th, 2009, 09:12 PM   #10
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From my experience with commercial bulk tape degaussers and recorded 1/4, 1 and 2 inch magnetic tape, I'd say you'd have to literally drag your analogue tapes over your 10inch speaker coils to even partially erase them.

To *see* your speaker coils mag field, at the back slowly move a suitable small coin into range, take your watch off first.

I'd be more concerned with analogue tapes decomposing over time if they're the 'matt back' variety. Shiny Polyester backed tape doesn't decompose and carefully stored has a 30yr+ life. Either way if they're important I'd look at getting them xfered to CD.
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Old October 31st, 2009, 09:11 AM   #11
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In other words, if I take a dime and pass it along the rear panel of the speaker cabinet and it doesn't "tug" (feel the attractive force) would it be safe to assume that there will not be any negative impact on my tapes?
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Old October 31st, 2009, 03:28 PM   #12
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Not quite. To feel the 'tug' IMO you'd have to use a suitable coin and move it in close to the rear of an actual 10" woofers speaker coil, *inside* an average cabinet. And different coins would have different results depending on their composition.

One way to prove there's no effect on your tapes would be to take into consideration how long they've been stored in their present location as you described in post one. I'd guess at least a year or 2 before this thread and they play Ok? Magnetic recording tape erasure is instant if the mag field is large enough.

BTW do you regularly degauss your analogue tape players heads? They can build up residual magnetism through much use and can eventually partially erase tapes. You'd notice a lack of high frequencies developing. I don't mean to scare you but you say you're concerned. Please contact me if you need more info.
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Old November 1st, 2009, 12:58 AM   #13
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Back in my days of producing radio ads (for which I forever apologise) we were taught if we wanted to kill a tape and ruin a days work, store it on or near a speaker cabinet.

Six foot minimum was the go...

Years later I used to store VHS tapes about six foot from the TV. Yes... they are now dead tapes, as the degauss mechanism in the TV created such a strong field on startup.

The storage location depends on the strength of the magnetic field in the first place... then work with the inverse square law..

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Old November 1st, 2009, 03:18 AM   #14
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Quote:
Originally Posted by Benny Sampson View Post
In other words, if I take a dime and pass it along the rear panel of the speaker cabinet and it doesn't "tug" (feel the attractive force) would it be safe to assume that there will not be any negative impact on my tapes?
If you have a dime that gets attracted by a magnet, then you have a very rare collector's item.

dimes are made of non-ferrous metals. If you have one that's made of a ferrous metal then you either have a dime that's one in a billion or a counterfeit coin! :-)
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Old November 1st, 2009, 01:34 PM   #15
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Greetings to All & Thanx Much for your insights to date, I do appreciate them :) In direct response to Allan, yes i do degauss the heads on both my players as was outlined in he original owner's manuals which accompanied the recorders; I use a standard wand type device to accomplish this task. I;m just wondering if using the "safe better than sorry" approach now wold save me a whole lot of trouble - Just move everything away 6 feet like Ben adopted and be done with it. My biggest issue is space and reconfiguring the layout is going to be next to impossible. And, good point about the dime Dean! I guess I could always just try to use a small nail of something of that nature. I think I'm going to run out right now and buy a cheap compass and attempt the compass experiemnt as was suggested by Marco. I'll report back as to what develops. Can anyone recommend if there is a cheap way with which to measure the actual field (a Radio Shack meter for instance)?
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