800W on Lab 12 woofers ? |
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_djk_
Old Croc Joined: 23 November 2004 Location: United States Status: Offline Points: 6002 |
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Posted: 08 November 2006 at 6:25am |
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The Eminence Lab 12 is a fine woofer for only $115 or so (in the USA), but it only handles about 400W long term.
The problem is not x-max limiting, but heat.
A good read from a new (to this forum) member:
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djk
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Peter Moller
Old Croc Joined: 26 December 2005 Location: Denmark Status: Offline Points: 2531 |
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Phildekked brought this up a few weeks ago... Very nice idea.. Only thing that'd worry me is the restriction of airflow through the vent, even with the holes drilled in the pipe.. The magnet vent hole is reduced in diameter and this may affect performance in some way?
I think Tom Danley had thermal issues in mind when he designed the labsub access panels using aluminum, but this trick certainly increases thermal transfer
/peter
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real phone no: +45 four zero six two four four nine eight ( using obvious anti phone spam encryption )
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moray james
Registered User Joined: 07 September 2005 Location: Canada Status: Offline Points: 141 |
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Wondering if using a large diametre aluminum pipe with lots of holes in it and a doughnut shaped flange welded to it would not work better? This heat sink could be bolted onto the magnet assembly back plate with some thermal grease via a few tapped holes (3). If the pipe had a fairly large diametre and a flange that went close to the vent hole you could suck a lot of heat off the back plate and the pipe could also fit into a hole on a metal access cover plate and be sealed off from the outside. This would also provide a clamp between the driver magnet and the cabinet. The aluminum could be anodised to increase thermal capacity of the assembly. The main thoughts here are to increase the thermal mass of the sink and to keep it away from the vent hole to minimize interaction there. The bonus as I see it is the added benefit of clamping the speaker to the cabinet. Thoughts? Regards Moray James.
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moray james
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odc04r
Old Croc Joined: 12 July 2006 Location: Sarfampton Status: Offline Points: 5483 |
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Spring loaded contact heatsink with thermal paste on it that compresses
onto the magnet when you bolt on the access panel? Could work well.
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Wayne Parham
Registered User Joined: 07 November 2006 Location: United States Status: Offline Points: 34 |
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I measured T/S parameters and found no issues. There is the potential for shifting parameters, so that is a very good thing to consider. You were right to bring it up.
The thermal resistance is much too high for the aluminum plate to do any good without a conductor between the pole piece and plate. But with the cooling plug, it becomes a very effective heat sink.
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Wayne Parham
Registered User Joined: 07 November 2006 Location: United States Status: Offline Points: 34 |
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Most heat is radiated into the center pole piece and front plate, since they are closest to the voice coil. What you want is as much contact surface area as possible. Contact with the center pole piece is best, either by press fit or thread fit within the center pole. Use heat conductive grease to improve the thermal junction.
Edited by Wayne Parham - 10 November 2006 at 2:35pm |
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godathunder
Old Croc Joined: 19 July 2004 Location: wicklow Status: Offline Points: 1833 |
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the problem is that as soon as you stack a couple of cabs next to each other they insulate each other and theres nowhere for the heat to dissipate
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LOUDER THAN LOUD
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Wayne Parham
Registered User Joined: 07 November 2006 Location: United States Status: Offline Points: 34 |
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Heat isn't trapped in the cabinets so much as it is trapped in the motors. That's where the problem lies.
I initially thought that heat passing through the cooling vent was trapped in the box, continually heating the air inside and making the cooling vent less and less effective. So I made a device that ducted heat out through a heat exchanger, like shown below.
I found that the air passing through the cooling vent wasn't heated very much, even at high power levels. What was getting hot was the pole piece and magnet. Heat is radiated from the voice coil into the pole piece. It is a heat transfer mechanism that can be used to reduce voice coil temperatures, if pole piece heat is dissipated.
When the pole piece is cool, it acts as a radiant cooler. Heat is wicked away from the voice coil by the cool pole piece nearby. But once the pole piece gets hot, it is no longer is effective as a radiant cooler. In fact, it re-radiates heat back into the voice coil, cooking the glue and causing it to fail.
It is relatively easy to remove heat from the pole piece. Simply conduct heat away with a cooling plug, as shown below.
The cooling plug works by conducting heat away from the pole piece so it can continue to act as a radiant cooler. Heat radiated from voice coil into the pole piece and magnet in turn radiates into the surrounding air. Without a cooling plug, the heat transfer is very slow, causing heat build-up in the pole piece and magnet. The speaker is like a several hundred watt soldering iron with its heat trapped inside a metal and ceramic insulated thermos bottle. With a cooling plug and some kind of radiator surface, the heat transfer is much faster. It wicks the heat out of the motor, allowing the pole piece to stay cool.
The radiant cooling provided by the cooling plug augments forced air convection cooling from the vents. It is not intended as a replacement mechanism, rather it is an additional mechanism used to remove heat generated by the voice coil.
The next step, of course, is to get the heat out of the box. One would think that having boxes stacked together would reduce the radiating efficiency of the panels. This is true, but in practice, using panels with large surface area is sufficient even if obstructed. Some of the heat radiates into the air outside the cabinet, some inside the cabinet, some is conducted into the cabinet walls. The main thing is that the heat is removed from the motor.
Without cooling plugs, aluminum access panels don't make sense. There is too much thermal resistance between the access panels and the hot surfaces inside for the panels to do any good. They might as well be made of wood.
When cooling plugs are used, aluminum access panels work as very effective heat sinks. It is great if you can leave a little air space between cabinets for convection. But even without any space between them, the panels will work as heat sinks when cooling plugs are used.
You can take a speaker having aluminum access panels but no cooling plugs and heap dry ice on the panels and blow fans on them too. It won't work as well as a speaker with cooling plugs even if the cabinets are completely blocked from airflow across the plates. The weak link is heat in the motor, not heat in the box.
Edited by Wayne Parham - 11 November 2006 at 6:24pm |
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Peter Moller
Old Croc Joined: 26 December 2005 Location: Denmark Status: Offline Points: 2531 |
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One way that would affect it even less ( I think ) would be to weld a flange on both ends of the pipe, cut some threads in the magnet assembly and bolt it on... That way it shouldnt affect airflow coming out of the magnet vent at all, but OTOH it wont put the thermal conductor as close to the voicecoil as your trick does..
The horn shape in the labsub does make it pretty much impossible to do any neat tricks, so I would expect that Mr Danley decided that the aluminum access panels were better than nothing - they do conduct heat better than birch ply.. Also he did warn builders that it could be a problem using this design at full power because the rear chamber was as small as it is..
This reminds me of a demo I did a month or 2 ago.. had 4 Void Impulse 3T running off a pair of Lab FP3400... At the end of the night the oputside of the cabs was warm to the touch :-)
/peter
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real phone no: +45 four zero six two four four nine eight ( using obvious anti phone spam encryption )
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Wayne Parham
Registered User Joined: 07 November 2006 Location: United States Status: Offline Points: 34 |
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I designed the cooling plug to be 100% compatible with the LABhorn. It was my contribution to their project, and I posted blueprints for the device on the LAB messageboard.
I had originally planned on a different layout, but at some point I realized that I could make the cooling plug compatible, making it a bolt-on improvement for the LABsub. All a person needs to install them is to drill one hole in each access panel.
Most any machine shop is capable of making cooling plugs, or they can be purchased from me. We have them made on CNC lathes and maintain a large inventory.
Edited by Wayne Parham - 11 November 2006 at 6:39pm |
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Peter Moller
Old Croc Joined: 26 December 2005 Location: Denmark Status: Offline Points: 2531 |
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Very cool...
I built the first design available, but havent been hanging out around that forum for some years now, so I havent kept up to date with the ongoing improvements on the design..
Right now I am envisioning regular vented cabinets w. the bracing / internal rigging reinforcements made with an aluminum structure inside.. Think Dual purpose :-)
Guess its time to get some drivers and start playing with the TIG welder..
/peter
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real phone no: +45 four zero six two four four nine eight ( using obvious anti phone spam encryption )
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Peter Moller
Old Croc Joined: 26 December 2005 Location: Denmark Status: Offline Points: 2531 |
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This thread is strange tho... Everytime I post to it, I get a Http 500 server error, the post does show up, but not on the index and email noticing doesnt work... Browser or server problem ?
/peter
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real phone no: +45 four zero six two four four nine eight ( using obvious anti phone spam encryption )
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