130Hz Lo Cut, Effect on Cooling? |
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Centauri
Old Croc Joined: 29 November 2004 Location: Newcastle Aus Status: Offline Points: 1792 |
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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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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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"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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djk
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Kaizen
Registered User Joined: 22 February 2009 Status: Offline Points: 124 |
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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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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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"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.
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djk
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Tony Wilkes
Old Croc Joined: 02 August 2004 Location: West Midlands Status: Offline Points: 4840 |
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The only under-hung drivers that I know for P.A. are made by ATC.
Tony |
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IanD
Registered User Joined: 17 January 2009 Location: London Status: Offline Points: 400 |
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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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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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JBL K145/E145/2215/LE15
PA Sub (XBL^2) http://pasub.com/PASUB460.pdf
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djk
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Wayne Parham
Registered User Joined: 07 November 2006 Location: United States Status: Offline Points: 34 |
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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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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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It would also help to have a copper thermo-inductive cap on the top plate like newer EV drivers.
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djk
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Tony Wilkes
Old Croc Joined: 02 August 2004 Location: West Midlands Status: Offline Points: 4840 |
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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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