Stupid speaker magnet question

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Hi! Quick, stupid question here.

Does sticking *another* speaker magnet to the back of a speaker do
anything horrible to it?

I replaced a hugeass woofer with a smaller speaker and the easiest way
to mount it was to tear out the paper of the dead speaker and just -
*plonk* - stick the new speaker in the frame of the old one (and then
cover the extra space left in front). Is this stupid?

I don't need high quality, it's a cheapass system anyway.. I was just
curious what that would do. I gather the part of the magnetic field
that matters is the part that extends over the speaker coil.. but I
don't actually know how sticking another magnet to the back would
affect that field.

Thanks!

-Lith

 

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Archived from groups: rec.audio.tech (More info?)

In article <a8a2949e.0408051620.19e5c940@posting.google.com>,
lithrael@hotmail.com says...
>
>Hi! Quick, stupid question here.
>

You must be blonde,

 

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Archived from groups: rec.audio.tech (More info?)

lithrael@hotmail.com (Lithrael) wrote in message news:<a8a2949e.0408051620.19e5c940@posting.google.com>...
> Hi! Quick, stupid question here.
>
> Does sticking *another* speaker magnet to the back of a speaker do
> anything horrible to it?
>
> I replaced a hugeass woofer with a smaller speaker and the easiest way
> to mount it was to tear out the paper of the dead speaker and just -
> *plonk* - stick the new speaker in the frame of the old one (and then
> cover the extra space left in front). Is this stupid?
>
> I don't need high quality, it's a cheapass system anyway.. I was just
> curious what that would do. I gather the part of the magnetic field
> that matters is the part that extends over the speaker coil.. but I
> don't actually know how sticking another magnet to the back would
> affect that field.

Speakers have external leakage fields whose flux density measure, in
close proximity to the magnet, in the realm of a few dozen Gauss at
most. In the gap of a woofer of the sort you're talking about, the
field density is on the order of 8,000-10,000 Gauss. The field density
needed to cause any significant change in the gap density, i.e.,
that needed to permantly change the total magnetization of the high
coercivity material in the magnet is itself on the order of 8,000
to 20,000 gauss. So, by itself, it's extremely unlikely you've
changed anything through the interaction with another magnet. In
fact, the voice coil generates a much higher level of reverse flux
density in normal operation than your external magnet could.

However, some of these hard magnetic materials CAN permanently lose
magnetic energy if subjected to shock, such as mechanical (you bang it
hard enough) or thermal (if it gets cold enough or hot enough). The
points at which this can happen are not so far out of the realm of
reality as to be ignorable. Stick a less than optimally designed
speaker magnet in a car and park it overnight some January in
International Falls, MN, and kiss goodbye to about 10% of the
total flux in the magnet. Drive a speaker hard enough that you raise
the temperature of the magnet structure hot enough that you start
to approach the Curie temperature (in the realm of 450C), and you
permanentl erase the magnet. Drop a woofer 3 feet onto 1/2" carpet
on concrete, and you might lose 5% of the total flux.