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130Hz Lo Cut, Effect on Cooling?

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Centauri View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Centauri Quote  Post ReplyReply Direct Link To This Post Posted: 20 December 2009 at 1:56pm
Originally posted by Kaizen Kaizen wrote:

I've often heard that over excursion cooks the ends of coils, due to them leaving the magnetic gap; why do you think the ends of this coil cooked when the cone motion was lower (and presumably the coil within the gap)?


An overhung speaker (the majority) normally only has part of its voice coil within the gap as the coil is physically longer than the gap height, and the ends of the coil will run hotter than the centre at excursions below physical xmax (coil length minus gap height divided by two).  When overdiven beyond xmax, each end will be within the gap each half cycle.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 20 December 2009 at 3:57pm
"I've often heard that over excursion cooks the ends of coils, due to them leaving the magnetic gap; why do you think the ends of this coil cooked when the cone motion was lower (and presumably the coil within the gap)? "
 
See below.
 
"Because the part of the coil that is in the gap has heat taken away from it by the close proximity of the metalwork acting as an heat-sink. 

Its still not that effective at taking the heat away because of the air gap (however small) but it is in a better position than the wire dangling in free air."
 
Correct.
 
"An overhung speaker (the majority) normally only has part of its voice coil within the gap as the coil is physically longer than the gap height, and the ends of the coil will run hotter than the centre at excursions below physical xmax (coil length minus gap height divided by two).  When overdiven beyond xmax, each end will be within the gap each half cycle."
 
Correct.
 



 
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Kaizen View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kaizen Quote  Post ReplyReply Direct Link To This Post Posted: 20 December 2009 at 4:56pm
Ahh of course, I was assuming the coil was the same length (or less) than the gap height. Why are most drivers overhung, would the magnet be too big for a PA driver otherwise (i've seen the odd underhung Hi Fi driver).

Does XBL^2 tech help to overcome this prob?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 21 December 2009 at 3:16am
"I was assuming the coil was the same length (or less) than the gap height. "
 
The majority of all drivers are overhung.
 
"Why are most drivers overhung, would the magnet be too big for a PA driver otherwise (i've seen the odd underhung Hi Fi driver)."
 
In the past it has been very expensive to do underhung, they can get real heavy too.
 
"Does XBL^2 tech help to overcome this prob?"
 
Yes, to a large extent it does. Not a large number of these types of drivers to choose from however.

djk
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Tony Wilkes View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tony Wilkes Quote  Post ReplyReply Direct Link To This Post Posted: 21 December 2009 at 7:52am
The only under-hung drivers that I know for P.A. are made by ATC.

Tony


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Post Options Post Options   Thanks (0) Thanks(0)   Quote IanD Quote  Post ReplyReply Direct Link To This Post Posted: 21 December 2009 at 4:33pm
Originally posted by Tony Wilkes Tony Wilkes wrote:

The only under-hung drivers that I know for P.A. are made by ATC.

Tony




The Aura NRT18-8 is still available in theory, as are the successors from CGN and Worx based on the same technology:

http://www.worxaudio.com/product_desc_true.php?id=56
http://www.bhivemotor.com/

Whether you can actually get hold of one is an entirely different matter...

(I've got a Seismic 8196 which was another Aura 1808 spinoff, and is the driver referred to earlier which replaced the cooked McCauley 6174 in the photo)

Ian
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 21 December 2009 at 8:26pm
JBL K145/E145/2215/LE15
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Wayne Parham Quote  Post ReplyReply Direct Link To This Post Posted: 22 December 2009 at 7:15pm
See the thread on heat sink effectiveness below:

It seems counterintuitive that a woofer would become heated when driven with midrange and high frequency content.  The cone doesn't move as far, so that tends to make you think it wouldn't have any problem handling the power at higher frequencies.

But in fact, most woofers are designed to cool the voice coils by pumping air through the gap and vent(s).  They need the excursion because when excursion is reduced, the cooling vents stall.  This can be caused by horn loading or by higher frequencies.  Anything that limits excursion reduces the cooling vent's effectiveness.
 
When a driver is used in applications that require a lot of power but don't result in large excursions, again, then cooling vents are generally less effective.  This is often the case in hornsubs and midbass and midrange drivers.   This kind of driver should have a thermally conductive pole piece that wicks heat out of the core.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2009 at 12:07am
It would also help to have a copper thermo-inductive cap on the top plate like newer EV drivers.
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Tony Wilkes View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tony Wilkes Quote  Post ReplyReply Direct Link To This Post Posted: 23 December 2009 at 4:58am
Most decent driver manufacturers have also done a lot of work with under-spider ventilation that allows a lot of heat to escape if only through natural convection. Aluminium Faraday rings also help.

As Neo magnets become the norm more research will go into keeping the Motor assembly cool.

http://ldsg.snippets.org/motors.php

Tony


Edited by Tony Wilkes - 23 December 2009 at 8:55am
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