speakers, magnets, hard drives question

[joe h]

Hello,

I've been looking at a Macbook computer with built-in speakers. I'm
used to keeping magnets away from magnetic media like tapes and disk
drives.

I would think the magnet elements in the small built-in speakers are
not good to be just a few inches from the hard drive. Maybe the
speakers could create a magnetic field that could produce data loss on
the hard drive.

Any thoughts on this?

[Don Pearce[_3_]]

On Tue, 5 Apr 2011 10:16:59 -0700 (PDT), joe h >
wrote:

>Hello,
>
>I've been looking at a Macbook computer with built-in speakers. I'm
>used to keeping magnets away from magnetic media like tapes and disk
>drives.
>
>I would think the magnet elements in the small built-in speakers are
>not good to be just a few inches from the hard drive. Maybe the
>speakers could create a magnetic field that could produce data loss on
>the hard drive.
>
>Any thoughts on this?

It would appear not.

d

[hank alrich]

joe h > wrote:

> Hello,
>
> I've been looking at a Macbook computer with built-in speakers. I'm
> used to keeping magnets away from magnetic media like tapes and disk
> drives.
>
> I would think the magnet elements in the small built-in speakers are
> not good to be just a few inches from the hard drive. Maybe the
> speakers could create a magnetic field that could produce data loss on
> the hard drive.
>
> Any thoughts on this?

Never had an issue with anything like that in ten years of using Mac
laptops, first a Titanium Powerbook and now a 17" Macbook Pro.

I doubt there is much magnetic energy emanating from the little built-in
speakers.

--
shut up and play your guitar * http://hankalrich.com/
http://armadillomusicproductions.com/who'slistening.html
http://www.sonicbids.com/HankandShaidriAlrichwithDougHarman

[Scott Dorsey]

joe h > wrote:
>Hello,
>
>I've been looking at a Macbook computer with built-in speakers. I'm
>used to keeping magnets away from magnetic media like tapes and disk
>drives.
>
>I would think the magnet elements in the small built-in speakers are
>not good to be just a few inches from the hard drive. Maybe the
>speakers could create a magnetic field that could produce data loss on
>the hard drive.
>
>Any thoughts on this?

There are folks at Apple who do nothing but sit around with field modelling
software, modelling magnetic field interactions inside the box. Note that
not only is the permanent magnet field from the speaker interacting with
the drive, but there are very small but powerful permanent magnets in the
drive voice coil assembly and the stray fields from those affect the speakers.
A whole hell of a lot of engineering has gone into making this stuff work
right.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Mike Rivers]

On 4/5/2011 1:16 PM, joe h wrote:

> I would think the magnet elements in the small built-in speakers are
> not good to be just a few inches from the hard drive.

Hard drives have magnets, too. It's actually very hard to
erase a digital recording. That's why you don't really erase
a file, you just make it so that it can't be found - and if
you can't find it, neither can the firmware that tells it
where to write data, so the space becomes available for new
data. When you "erase" a whole drive you overwrite all the
data with a pattern since you can't just erase it reliably.

There are hard drive erasers that are used as the first step
in "declassifying" a drive, that have large magnets, about
the size of coffee cans. The second step of
"declassification" is usually to punch a big hole in the
platter. Like this:

http://i.imgur.com/epiMi.jpg

Then, sometimes for good measure, they grind the drive into
a coarse powder that they send to the metal recycler.




--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson

http://mikeriversaudio.wordpress.com - useful and
interesting audio stuff

[Arny Krueger]

"joe h" > wrote in message



> I'm used to keeping magnets away from magnetic
> media like tapes and disk drives.

Pretty much a waste of time. Modern high-corecivity magnetic media takes
much more than stray magnetic fields to change its magnetization.

I don't think it has been possible to damage data on a floppy disc with
stray magnetism since the days of 3.5" floppies, if not before that.

The leading problem area with stray magnetization has been CRTs, and now
they are going away as well.

[William Sommerwerck]

> It's actually very hard to erase a digital recording.

Not at all. You simply overwrite it with zeroes, rather than data.

[Mike Rivers]

On 4/5/2011 6:25 PM, William Sommerwerck wrote:
>> It's actually very hard to erase a digital recording.

> Not at all. You simply overwrite it with zeroes, rather than data.

You'd probably be happier if we used the proper term for
what we're actually talking about - degauss.

I don't think speakers are capable of writing zeroes on a
disk drive.


Take two!

--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson

http://mikeriversaudio.wordpress.com - useful and
interesting audio stuff

[Scott Dorsey]

In article >,
flatfish+++ > wrote:
>
>it is VERY difficult.
>I have an IBM degausser circa 1975 that was used to erase 9 track r2R
>tapes used on IBM 3420 tape drives.

Later generation (6250 bpi) 9-track tapes ran around 360 Oersteds
coercivity, so it didn't take a lot of field to wipe them, or record
on them for that matter. That's only a little bit higher than Ampex 406
which comes in around 290 Oe, and they have been doped with cobalt to
make them more nonlinear; they magnetize and demagnetize more abruptly
which is good for digital recording.

Ampex 406 has been known to get self-erased sitting on the floor of
subway trains in NYC, just from the stray fields of the traction motors.
So a degausser doesn't have to be very powerful to wipe them. The old
9-track degaussers were hardly strong enough to ruin your watch.

>The ones you see in the typical scifi flicks with the spinning reels
>which are usually spinning the wrong way.

Should have got a Pertec drive, they're bidirectional.

>It does nothing obvious for hdisks in my limited testing over the years.
>although it will clobber a floppy drive or a cassette tape etc.

Metal particle tape like DAT or Hi-8 runs in around 1500 Oe, so figure
it needs about five times the field to wipe it compared with a 9-track.
Older hard drives (Eagles and the like) used similar media.

Modern hard drives use plated media that is even higher, mostly
cobalt-chrome alloys in the 2200 Oe and up range. But the thing
about wiping hard drives with external fields is that the actual
drive media are often buried inside a nickel-alloy box intended to keep
stray fields out, and even without it on larger drives the platter may
be some distance from the base so you need that much stronger a field
to wipe it.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

William Sommerwerck > wrote:
>> It's actually very hard to erase a digital recording.
>
>Not at all. You simply overwrite it with zeroes, rather than data.

I work with people who do it all the time, accidentally.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Arny Krueger]

"Scott Dorsey" > wrote in message

> Later generation (6250 bpi) 9-track tapes ran around 360
> Oersteds coercivity, so it didn't take a lot of field to
> wipe them, or record on them for that matter. That's only a little bit
> higher
> than Ampex 406 which comes in around 290 Oe, and they
> have been doped with cobalt to make them more nonlinear;
> they magnetize and demagnetize more abruptly which is
> good for digital recording.

The coercivity of 3.5 disks of various densities is:

DD: 2 µm magnetic iron oxide (Coercivity approx. 300 OE Oersted)
HD: 1.2 µm cobalt doped iron oxide (Coercivity approx. 600 OE)
ED: 3 µm Barium ferrite (Coercivity approx. 750 OE)

[William Sommerwerck]

>>> It's actually very hard to erase a digital recording.

>> Not at all. You simply overwrite it with zeroes, rather
>> than data.

> You'd probably be happier if we used the proper term
> for what we're actually talking about - degauss.

That wasn't what we were talking about.

[Mike Rivers]

On 4/5/2011 7:24 PM, William Sommerwerck wrote:

>> You'd probably be happier if we used the proper term
>> for what we're actually talking about - degauss.
>
> That wasn't what we were talking about.

Really? Wasn't the original poster concerned about erasure
from loudspeaker magnets? How about "Gauss" then?

"Degauss" is a pretty well accepted term for removing
useful information from recorded media by the application of
an AC or DC magnetic field.



--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson

http://mikeriversaudio.wordpress.com - useful and
interesting audio stuff

[Arny Krueger]

"Arny Krueger" > wrote in message

> "Scott Dorsey" > wrote in message
>
>> Later generation (6250 bpi) 9-track tapes ran around 360
>> Oersteds coercivity, so it didn't take a lot of field to
>> wipe them, or record on them for that matter. That's
>> only a little bit higher
>> than Ampex 406 which comes in around 290 Oe, and they
>> have been doped with cobalt to make them more nonlinear;
>> they magnetize and demagnetize more abruptly which is
>> good for digital recording.
>
> The coercivity of 3.5 disks of various densities is:

> DD: 2 µm magnetic iron oxide (Coercivity approx. 300 OE
> Oersted)

> HD: 1.2 µm cobalt doped iron oxide (Coercivity
> approx. 600 OE)

> ED: 3 µm Barium ferrite (Coercivity
> approx. 750 OE)

To continue, the coercivity of modern digital recording tapes are given as:

AIT-2 8 mm 1380
AIT-3, AIT-4, S-AIT 1-1/2 1400
D8: 8 mm 1600
HI-8 Metal Particle 1700
MII 1500
DV Cam 1500
BetaCam SP Small/Large 1500
DTF-1, Small/Large 1579
3590, 9840 1625
DLT III 1700
4mm, DAT 1500
DDS1 4mm 1500
Super DLT I 1900
Super DLT II 2600
DLT S4 2650
DTF-2 Small/Large 2300
DDS2 4mm 1750
DDS3 4mm 2250
DDS4 4mm 2350
DAT-72 4mm 2350
DVC Pro 2300
3592 2500
LTO-Ultrium 1 1850
LTO-Ultrium 2 2150
LTO-Ultrium 3 2650
LTO-Ultrium 4 2710

Other sources suggest that the magnetic media used in modern PC hard drives
is in the 2000 range.

[William Sommerwerck]

>>> You'd probably be happier if we used the proper term
>>> for what we're actually talking about - degauss.

>> That wasn't what we were talking about.

> Really? Wasn't the original poster concerned about erasure
> from loudspeaker magnets? How about "Gauss" then?

> "Degauss" is a pretty well accepted term for removing
> useful information from recorded media by the application
> of an AC or DC magnetic field.

As far as digitial media are concerned, "erase" has a different meaning than
it does for analog media.

With analog media, the magnetization is randomized -- what we normally call
"degaussing" -- removing the net magnetization.

With digital media -- hard drives, anyway -- "erasure" is performed by
converting everything to ones or zeros. The medium retains its
magnetization -- only the information is destroyed. *

Running a huge magnet over a hard disk obviously takes much less time. It
also presumably wipes out the base formatting and the servo tracks.

[Arny Krueger]

"William Sommerwerck" > wrote in
message
>>>> You'd probably be happier if we used the proper term
>>>> for what we're actually talking about - degauss.
>
>>> That wasn't what we were talking about.
>
>> Really? Wasn't the original poster concerned about
>> erasure from loudspeaker magnets? How about "Gauss"
>> then?
>
>> "Degauss" is a pretty well accepted term for removing
>> useful information from recorded media by the application
>> of an AC or DC magnetic field.
>
> As far as digitial media are concerned, "erase" has a
> different meaning than it does for analog media.
>
> With analog media, the magnetization is randomized --
> what we normally call "degaussing" -- removing the net
> magnetization.
>
> With digital media -- hard drives, anyway -- "erasure" is
> performed by converting everything to ones or zeros. The
> medium retains its magnetization -- only the information
> is destroyed. *
>
> Running a huge magnet over a hard disk obviously takes
> much less time. It also presumably wipes out the base
> formatting and the servo tracks.

I dunno. At the level we're talking, the media *is* analog. Please explain
your thoughts.

[Scott Dorsey]

Arny Krueger > wrote:
>"William Sommerwerck" > wrote in
>>
>> With digital media -- hard drives, anyway -- "erasure" is
>> performed by converting everything to ones or zeros. The
>> medium retains its magnetization -- only the information
>> is destroyed. *
>>
>> Running a huge magnet over a hard disk obviously takes
>> much less time. It also presumably wipes out the base
>> formatting and the servo tracks.
>
>I dunno. At the level we're talking, the media *is* analog. Please explain
>your thoughts.

After you erase a hard drive, you can still use it.

After you degauss a modern hard drive, it is scrap metal.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[William Sommerwerck]

> After you erase a hard drive, you can still use it.
> After you degauss a modern hard drive, it is scrap metal.

I think Scott has said it perfectly. Good show!

[joe h]

Thanks everyone, and thanks Mr. Dorsey.

I've never even heard of an Oersted. Your knowledge of electrical/
mechanical stuff is phenomenal. I'll take your word for it that Apple
has this stuff figured out.

I was thinking that a general de-gaussing effect could occur over
time. Like you wouldn't know the speakers are compromising your drive
on day one. But after two years your drive starts to produces errors
due to stray magnetics from the speakers. I grew up making four-track
tape recordings, and I was always careful to keep magnetic things away
from the tapes. And I would also store the tapes "vertically" rather
than flat so the edges wouldn't get squashed over time.

If you ever go on Jeopardy, let rec.audio.pro know. I'll bet
thousands that you will win if there is a place that takes bets for
things like that.

[alex]

Il 05/04/2011 22.12, Mike Rivers ha scritto:
> Hard drives have magnets, too. It's actually very hard to erase a
> digital recording. That's why you don't really erase a file, you just
> make it so that it can't be found - and if you can't find it, neither
> can the firmware that tells it where to write data, so the space becomes
> available for new data. When you "erase" a whole drive you overwrite all
> the data with a pattern since you can't just erase it reliably.

true, harddisks have magnets for motor and the moving arm, very close to
the plates with no problem.

The file erasing is done by the OS very fast, marking as "deleted" the
file entry in the table that map every sector (grouped in blocks) used
for every single file. (FAT do this by removing the very first character
of the filename in the table)
That way the blocks becomes available again and they can be overwritten
but they are actually not overwritten until the OS is requested to have
access to that space again by an application. This is the reason why
data can be "unerased" if not phisichally overwritten, here i mean
overwrite block of sectors, not just file names.
The reason of this is speed. Overwritting all the sectors can be a very
long task depending on drive speed, fragmentation, and filsize, just
marking an entry in a table is much faster.
Erasing the whole disk or partition is actually the same as
reformatting, except that in reformatting the table is simply
overwritten with an empty one instead to mark as deleted, one by one,
every single entry in it. Reformatting DOESN'T really erase data from
the sectors. To reconstruct files without a table will be very long and
tedious but this is still possible and can be done by forensic technicians.
Formatting can involve the "sector verification" option (slow). in that
case the surface is checked and if a bad sector is found, the containing
block will be marked as unusable in the new table.
"wipe" is the word used to be forensically sure the data are erased.
This is accomplished by overwritting every single sector with a specific
value. "Paranoid" option involves the multiple passages, every one with
a different value.

[Arny Krueger]

"Scott Dorsey" > wrote in message

> Arny Krueger > wrote:
>> "William Sommerwerck" > wrote
>> in
>>>
>>> With digital media -- hard drives, anyway -- "erasure"
>>> is performed by converting everything to ones or zeros.
>>> The medium retains its magnetization -- only the
>>> information is destroyed. *
>>>
>>> Running a huge magnet over a hard disk obviously takes
>>> much less time. It also presumably wipes out the base
>>> formatting and the servo tracks.
>>
>> I dunno. At the level we're talking, the media *is*
>> analog. Please explain your thoughts.

> After you erase a hard drive, you can still use it.

You must be talking about "an early technology hard drive". Factory-written
servo tracks go back at least to the early days of IDE, if not earlier. It
seems to me that the early MFM drives could be reformatted in the field from
the lowest level. They relied on the drive's own mechanical indexing to
establish the reference points. Mechanical indexing across the drive surface
went away in order to simplfy the hardware.

> After you degauss a modern hard drive, it is scrap metal.

For a hard drive made in the past 20 years, yes it is.

[Scott Dorsey]

Arny Krueger > wrote:
>
>> After you erase a hard drive, you can still use it.
>
>You must be talking about "an early technology hard drive". Factory-written
>servo tracks go back at least to the early days of IDE, if not earlier. It
>seems to me that the early MFM drives could be reformatted in the field from
>the lowest level. They relied on the drive's own mechanical indexing to
>establish the reference points. Mechanical indexing across the drive surface
>went away in order to simplfy the hardware.
>
>> After you degauss a modern hard drive, it is scrap metal.
>
>For a hard drive made in the past 20 years, yes it is.

You have totally missed the distinction being made. Please go back and read
my message again. Degaussing and erasing are not the same procedure here.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Arny Krueger]

"Scott Dorsey" > wrote in message

> Arny Krueger > wrote:
>>
>>> After you erase a hard drive, you can still use it.
>>
>> You must be talking about "an early technology hard
>> drive". Factory-written servo tracks go back at least
>> to the early days of IDE, if not earlier. It seems to me
>> that the early MFM drives could be reformatted in the
>> field from the lowest level. They relied on the drive's
>> own mechanical indexing to establish the reference
>> points. Mechanical indexing across the drive surface
>> went away in order to simplfy the hardware.
>>
>>> After you degauss a modern hard drive, it is scrap
>>> metal.
>>
>> For a hard drive made in the past 20 years, yes it is.
>
> You have totally missed the distinction being made.

No.

> Please go back and read my message again. Degaussing and
> erasing are not the same procedure here.

Right.