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Old 6th March 2013, 04:09 AM   #21
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re:"One thing we learned in my college classes " - you had to go to college to learn that???...
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 6th March 2013, 04:24 AM   #22
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PeteMcK,
Well I guess you didn't you used that as your source of information, I didn't! That cost you at least a grade in any class I had if you used that as a reference source, but it is amazing how many do use that for their information. Who would have thought that would ever be used for anything, I never use that source for anything.
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Old 6th March 2013, 06:29 AM   #23
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Don't rush to accuse companies like Mission and KEF (lots of reports of sticking tweeters from their products) of using counterfeit product.
Ferrotec themselves hint at lifetime issues in this article Ferrofluid: Magnetic Liquid Technology - both carrier oil evapouration and particle chains forming
The carrier will be a mineral or silicone oil, all of which have finite vapour pressure
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Old 6th March 2013, 06:44 AM   #24
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From Ferrotec
Audio Ferrofluids
Selecting the Correct Audio Fluid
APG 300 Series Audio Ferrofluid
The material safety datasheet http://ferrofluid.ferrotec.com/downloads/apg300msds.pdf
which states that evapouration is 4e-7 gm/cm2/sec at 175C which corresponds to 12 gm in a year - not too good.
To get back to OP, these pages also have a lot to say about use of ferrofluid in woofers, venting and spiders replacement.
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Old 6th March 2013, 02:01 PM   #25
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Okay,
I take back what I have said and I have been informed by the lot of you that I was the one who was misinformed. The evaporation will be increased at elevated temperature so in worst case heating conditions where you would have wished for the optimum protection in fact that is the opposite! Calculation of the life expectancy would be dependent on voicecoil heating, gap width would affect volumetric amounts and heat transfer removing heat would be a secondary factor to localized heating of the fluid. Back to the drawing board it would seem on the carrier fluids used in these applications. Perhaps the case for the electrodynamic shock absorbers with be the fact that the fluid is contained in a closed container which could counter the losses through evaporation.
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Old 7th March 2013, 09:50 PM   #26
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Quote:
Originally Posted by Kindhornman View Post
The evaporation will be increased at elevated temperature so in worst case heating conditions where you would have wished for the optimum protection in fact that is the opposite!
I would assume, however, that this failure of the ferrofluid cooled woofer occurs at a significantly higher power level than the one where a comparable driver designed for non-ferrofluid use goes belly up.


Does anyone have a rough, ballpark estimate of how much more power a ferrofluid cooled low frequency driver can absorb than a similar driver not designed for ferrofluid use? Twice as much, three times as much.....?

And another question, the changes in the driver necessary to accommodate ferrofluid does not affect the driver's sensitivity, does it? There would be greatly reduced benefit in using ferrofluid to boost power input limit 3x if the changes necessary to accommodate the ferrofluid lower sensitivity by 3 dB.
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Last edited by kelticwizard; 7th March 2013 at 09:56 PM.
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Old 7th March 2013, 09:57 PM   #27
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Kelticwizard,
I don't know why I want to remember this but I think it is about 4 times the power as the dissipation of heat is so much greater. But 4 times the power would normally push the voicecoil right out of the gap so it isn't a realistic increase in power handling. It is just the instantaneous peak power that is better handled but the physical limitations of the driver would still apply.

ps. The most common reason that ferrofluid is used in a driver is for voicecoil centering and the lessening of rubbing and buzzing of the voicecoil assembly making assembly tolerances wider and QC inspection simpler. Power handling is a secondary issue to most manufacturers, they just can get sloppier in construction. Pro audio would use it for power handling but even then it has a limitation once the entire assembly reaches a critical point and further dissipation can not be achieved due to the limited surface area of the motor assembly.

Last edited by Kindhornman; 7th March 2013 at 10:01 PM.
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Old 7th March 2013, 10:13 PM   #28
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Kindhornman:

Thank you for the very prompt answer.

I am not now contemplating adding ferrofluid to an existing non-ferrofluid woofer, (well maybe I'll mess around a little bit with a cheapie speaker but I'm not expecting much). I just pose the question because most boxes that DJ's and even small bands lug around are 3 cu ft internal volume, (5 cu ft external volume) with a cutoff of 60 Hz or so, and with music having more low frequency content the boxes either have to get larger, (which would cause problems from a transportation standpoint), or you've got to put more power into an existing size box.

Just trying to see what the possibilites are.

Four times more power means that you can put in a ferrofluid driver with twice the voice coil length and with 4x the power can put out either an additional 6 dB SPL or the same SPL but output down to a half octave lower. The designer might want to back off that a little since, as you stated, when the end comes with ferrofluid it comes quickly.
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Last edited by kelticwizard; 7th March 2013 at 10:19 PM.
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Old 7th March 2013, 10:47 PM   #29
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Is it possible that ferrofluid is used to make drivers fail after a few years, safely after the warranty has expired? It would sell more speakers, while reducing returns due to playing loud during warranty.
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