# Question on magnetic field effects from speakers/subs on electronics



## cshaun

I recently bought some new speakers. And the sub that came with them had a warning to keep it at least 60cm (2 feet) away from "PC's, Monitors, TV's, etc".

 Now I'm assuming this is still the case even when it's off? (seeing as it's likely to have a magnet in it)
 I assume the field gets stronger the higher the volume/power? (electromagnet powering up).

 Surely I don't need to keep ALL electronics away from it? What do I need to worry about?
 Obviously computer HD's...
 What about a routers, UPS's, LCD monitors, phones, mobile phones...?

 Reading this has got me paranoid... Just been upgrading all my gear over the last 2 months, so really don't want any harm to come to it. Anyway I'd rather be much more clued up on this and put myself at ease (been four days now and I'm driving myself crazy trying to decide were to position it).

 * Updated: 6cm to 60cm - typo.


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## error401

Your biggest worry is going to be anything with a vacuum tube, like a CRT monitor. These work by using magnetism to control an electron beam that excites phosphors on the screen. If you introduce external magnetism, the display will be messed up, possibly permanently. Keep magnetic storage media away too (credit cards, floppy disks, hard drives), but most other electronics should be fine, including all the devices you list. Hard drives should actually be okay for the most part, but of course avoid it if possible.

 I believe that the majority of the field strength comes from the permanent magnet, so yes it doesn't matter if the speaker is playing or not, or at what volume, the field will be relatively constant.


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## gjustice

IF they are saying 6cm then there is no big deal. Any form of magnetic storage device may be at risk(aka harddrives, any old tapes or floppy disks) If you have any CRT based technology(computer moniters or TV's) you should see color distortion long before any damage occurs. As it is there is a large magnet in the middle of the voice coil for the speaker so the field will be present any time regardless of if it is on or not. Turning it up or down should have only a minimal change as the field decreases in strength relitive to the cube(distance^3) of the distance from the magnet/voice coil. 

 Basically what it all boils down to is they want to make sure they are not going to be sued.... Just dont put anything sensitive to magnetic fields on top of it or touching it and your safe.


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## cshaun

mmm yeah that's kinda what I thought (before freaking out seeing the note), only really hard drives, floppy's, cathode ray tubes and those sorts of things that are really an issue around magnetic fields (credit cards - good one didn't think of that - though I'm sure it's have to be a very strong magnet to wipe those clean).
 Other electronics that aren't using magnetism should be fine... only possible issue is eddy currents - but I'm guessing the magnetic field would have to be quiet strong for that to be an issue (though yeah not sure 'how' strong).

 Ok so things to keep away:
 Hard Drives - 4 of them, 40cm above top of sub in computer case if I have it that side
 CRT Monitor (or TV) - using an LCD
 Floppies - don't have any
 Cards with magnetic strips (e.g. credit cards)

 Ok guessing the 40cm is sorta safe, but perhaps not worth risking (don't know how much extra buffer the lawyers put in the 60cm recommendation).

 So UPS and router/adsl modem would be save right next to it - correct?

 You guys don't think there's any issue having a mobile phone near?

 After mulling over it the past 4 days I think I can work around having it the other side of the desk which is just further away from everything. Good to know what's safe around it and what's not.

 Bastards putting a scary note like that on them and not giving more details. Should have a website address were it addresses all sorts of questions!

 Any more comments anyone?


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## cshaun

Oh yeah and I assume I should keep my soon to be arriving BEYERDYNAMIC DT 770 PRO/80 OHM well well away from the sub...


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## error401

I wouldn't worry much about hard drives. The field strength required to flip the bits is really rather astronomical. It can only be achieved by the write head because it's micrometres from the surface of the disk.


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## cshaun

Yeah just read a bit more about that (I even remember trying to damage an old floppy with a strong magnet with no success).
 But one additional piece of information I just found is that normal magnets aren't all that bad for hard drives and other magnetic storage media. What's more dangerous is changing/alternating magnetic fields. Which I guess can/would be the case with a speaker/sub.


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## error401

Quote:


  Originally Posted by *cshaun* /img/forum/go_quote.gif 
_Yeah just read a bit more about that (I even remember trying to damage an old floppy with a strong magnet with no success).
 But one additional piece of information I just found is that normal magnets aren't all that bad for hard drives and other magnetic storage media. What's more dangerous is changing/alternating magnetic fields. Which I guess can/would be the case with a speaker/sub._

 

Yup, only changing fields will cause the magnetic media to be corrupted. Static fields won't affect it. With a strong enough magnet though, just walking past it could potentially be 'changing' enough to cause corruption. I highly doubt this would be the case with any speaker.

 As was mentioned above, the field strength of the permanent magnet in a speaker dwarfs the strength of the field induced by the voice coil. As I understand it, the stronger you make the permanent magnet, the less current needs to pass through the voice coil for the same speaker extension - in other words, the more powerful the magnet, the more sensitive the speaker. I believe for the speaker to be usably efficient the permanent magnet must be orders of magnitude stronger.


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## judas391

There's no need to worry when the speakers are off. The current flowing through them is the thing that causes the magnetic field


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## tbonner1

Quote:


 Your biggest worry is going to be anything with a vacuum tube, like a CRT monitor. 
 

I agree.


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## cshaun

Quote:


  Originally Posted by *judas391* /img/forum/go_quote.gif 
_There's no need to worry when the speakers are off. The current flowing through them is the thing that causes the magnetic field_

 

I magnetized a pin and played with it on top of the sub... definitely as a magnetic field over a certain portion of the top. Difficult testing it higher up (further up), but seems to get weak very quickly.
 Couldn't get the center channel speaker to have any effect on the pin while off.

 So anyway there is definitely a field when it's off, not sure how different it is when on.

 But anyway when off seems weak enough to be a non issue.


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## cshaun

Radiation? Er that message just looks like spam.


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## voxr3m

Your speaker has a persistent magnetic field from the permanent magnet regardless of whether it's on or off. It's the nature of the material.

 When you turn the speaker on, the speaker's voice coil becomes an electromagnet. With two magnets near each other, one of them is going to feel significant enough a force to move. You have a total of two magnetic fields here. The changing magnetic field due to the changing current flowing through it is what causes the voicecoil(and hence the cone, spider, etc.) to move within the permanent magnet.

 With a changing magnetic field, you also get a time varying electric field, and that radiates outward from the voice coil. It's not 'radiation' in the nuclear sense.

 Keep your sub/speaker away from sensitive objects, but HDDs as mentioned should be fine. You'll risk more danger from the vibration that the sub causes when it plays than with the actual magnetic fields.


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## badmonkey

Quote:


  Originally Posted by *cshaun* /img/forum/go_quote.gif 
_Radiation? Er that message just looks like spam._

 

Spam alert.

 The only "radiation" coming from your sub will be infrared as it heats up... hey well it'll make a nice foot-warmer.


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## Prozakk

Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_I wouldn't worry much about hard drives. The field strength required to flip the bits is really rather astronomical. It can only be achieved by the write head because it's micrometres from the surface of the disk._

 

But you do have to worry if the subwoofer causes too much vibration, which can ruin a hard drive. Been there, done that.

 I suggest vibrapods, or similar vibration isolation products.


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## infinitesymphony

Quote:


  Originally Posted by *Prozakk* /img/forum/go_quote.gif 
_But you do have to worry if the subwoofer causes too much vibration, which can ruin a hard drive. Been there, done that._

 

How can you ruin a hard drive with a subwoofer? There are low frequency sounds humming through computer cases 24/7 without ill effects...


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## Prozakk

You'd have to use a good sub to know. A good sub shakes the **** of everything in the room it's in.


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## infinitesymphony

Ah, I think our definitions of "good sub" are different.


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## Prozakk

And yours is....?


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## Prozakk

SVS PB12 Ultra/2 here.

http://www.svsound.com/news-reviews.cfm


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## infinitesymphony

I don't use a subwoofer in my two-channel system... The SVS PB12-Ultra/2 looks very nice. I just meant that not everyone demands room-annihilating bass. 
	

	
	
		
		

		
		
	


	




 But after looking at the sub, I see your point. It's not so difficult to imagine a hard drive being pulverized by the bass output from that woofer.


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## Prozakk

Quote:


  Originally Posted by *infinitesymphony* /img/forum/go_quote.gif 
_I don't use a subwoofer in my two-channel system... The SVS PB12-Ultra/2 looks very nice. I just meant that not everyone demands room-annihilating bass. 
	

	
	
		
		

		
		
	


	




 But after looking at the sub, I see your point. It's not so difficult to imagine a hard drive being pulverized by the bass output from that woofer. 
	

	
	
		
		

		
		
	


	


_

 






 I want the upcoming version of the ultra 2.

_So after nearly two years, this is the best of the best from SVS. It's a 13.5" wonder (edge to edge basket dim) which can displace far more air than its predecessor (the key to any subwoofer's performance). Yet it still fits into practical and efficient "form factors" we're famous for: both Powered Cylinders (PC), and Powered Boxes (PB). Sporting state-of-the-art materials, hand-assembly, and exclusives like the new SVS deep-cast aluminum basket, it barely breaks a sweat on tough stuff that would have pushed its predecessors to the wall. In short, the Ultra-13 is poised to provide you with two different kinds of depth, power and accuracy -- like nothing we've previously seen (heard, or measured) in our lab (at least not in price points vaguely similar). If you are as insatiable about technical details and product images as we are, you can read a bit more here. Both PB13-Ultra in 3D rendering and a new PC-Ultra mockup are pictured at right (a larger dual woofer PB13-Ultra/2 may debut sometime in 2008). _


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