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Old 25th March 2014, 11:00 AM   #1
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Talking Pole Piece Outside Box

I know this may seem like and obvious answer, but......

Im looking at building some MT-130's and would like to know whether it would be silly idea to recess the magnet structure into the back panel so that the magnet sits flush with the outside of the box, exposing the pole piece vent.

My only opinion against is that the back chamber would no longer be a sealed enclosure as the pressure would be released behind the diapham. The other possible problem being that there may be considerable noise from the vent.

Of course the benefits of cooling and air exchange would be an advantage.
Its mainly to loose 20mm from the back of the box in and attempt to make it fit nice and snug with my other box designs (more proffesional).

Any advice would be greatly appreiciated as there is not much info about this subject.

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Old 25th March 2014, 04:03 PM   #2
badman is offline badman  United States
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You'll basically form a damped dipole that way, aka a cardioid. The diaphragm loading from a sealed box would be reduced as well, so you'll lose bass response overall, between the cancellation and the unloading. One alternative would be to apply a heatsink to the rear of the magnet, just a flat plate on top of the magnet, which will cool the steel (Pi Speakers has found this method very effective) but also raise Qms somewhat, changing the alignment. I'd put some foam into the vent if you did this to prevent a cavity resonance. You will see some slight response changes from this method as well.
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Old 26th March 2014, 05:28 PM   #3
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So to review, the only issue I see with this Idea is that I would not be saving on the extra depth I wanted to get rid of as the cabinet would still have the magnet inside and adding a plate may just make it even longer.
As for stuffing the vent, I guess if there is a plate over the back and you ares tuffing the vent, then there will no longer be a vent ?
Would this not defeat the purpose of the manufacturer making a vent in the first place and create extra cooling issues. ?
Unless the design you mention is to offset the extra heat generated by closing the vent by using a thin (say 2mm) plate across the back of the entire magnet and box as a heat sink.
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Old 26th March 2014, 06:45 PM   #4
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I Have mocked up an idea that might be something usefull for this type of design.

https://www.dropbox.com/s/jn9yy05hqv...eat%20Sink.jpg

The only thing that I may need advice on is the design of actual vents. Supposing that of course the venting would be driver specific, but I wonder if it could be modeled with Akabak or something to produce a specific resonance to help the overall system.

What cannot bee seen in the picture is the fins that would attach the outer heatsink part to the inner magnet part (allowing for heat transfer and distribution).
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Old 26th March 2014, 06:54 PM   #5
DavidL is offline DavidL  United States
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Why worry about heat or power handling as long as you observe the manufacturer's power limits?
If it's a sealed box, try adding more stuffing to the box so it appears larger to the driver then you can make the box depth less. Also trying an aperiodic enclosure, which is simply a few holes drilled into the back of the enclosure so it's "leaky" will make the box appear larger to the driver.
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Old 26th March 2014, 07:14 PM   #6
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Its not really about the apparent size of the back chamber, as i say im going to build some MT-130's and well there are physically about 20mm to deep to match up with my mids/subs. Like I say im being pedantic (but this is sound and image).
The idea is the driver is pysically of a certain depth (with clearence for the pole piece vent) and the wood is of a certain thickness.

What Im trying to do is primarilly lose 20mm of the back of the box by recesing the drivers magnet either into the wood or having it go through (to allow the vent to still function).
If the vent is to be closed up completely then the thermal characteristics of the driver will be compromised. The idea of the heatsink design is to keep the pole vent open but divert the flow of air around and to the outside of the magnet, keeping it specifically as a sealed enclosure.

The actual heatsinking and protrusion from the back of the cab was purely and additive idea to be able to dissipate extra heat away from the magnet (extra cooling = better power handling). The amount I needed to save would probably be the same as the wood thickness anyway. Also it makes them unique in design as opposed to being another top cabinet. I have other ideas for the subs to attach the pole vent to the grill using copper.
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Old 26th March 2014, 07:21 PM   #7
DavidL is offline DavidL  United States
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I just addressed the box size issue. If you make the box "appear" larger to the driver using either more stuffing or an aperiodic vent, then you can make the physical size of the box smaller and the 20mm you wish to lose my be possible.

I think you are trying to solve a heat/power handling issue that has already been taken into account by the driver manufacturer. Use drivers that can handle what you want from them is the simple solution.
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Old 26th March 2014, 07:24 PM   #8
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My cooling system augments the existing vent by adding a conductive plug that wicks heat out of the motor core. The vent isn't blocked, so forced air convection cooling is still used. My system is used in conjunction with forced air convection, and doesn't usually replace it.

The cooling plug simply contacts the center pole with enough surface area to get a good thermal interface, and it conducts it out to a heat sink. Most of the heat removed is caused by eddy currents in the magnet and some is from radiated heat.

The whole purpose of the cooling plug/plate is to reduce the local ambient temperature. Without this device, the motor core of most speakers running at a few hundred watts or more reaches temperatures high enough to boil water, and this weakens the adhesive holding the voice coil to the former. The voice coil peaks temperatures twice that, but if the local ambient temperature is low, then the voice coil is able to cool off between power peaks. So the cooling plug/plate keeps local ambinet temperatures down.

I started off making a cooling system that improved forced air convective cooling. As I worked through that, I addressed all the issues around the air leak created by venting outside the box. I explored various methods. But I found that all to be something of a waste of time - The existing cooling vents are pretty effective, and what was really lacking was a cooling system for the motor core.

So rather than focusing on the vents, I turned my focus to the pole piece and the magnet structures surrounding the voice coil. It's sort of like a Thermos bottle, getting really hot and holding heat in. The cooling plug/plate arrangement fixes that problem.

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Old 26th March 2014, 07:34 PM   #9
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The physical depth from baffle to the back of the speaker PD.121 (including enough space for pole piece venting is too much. The idea is to decrease this length, i.e take away the space required for the pole vent to lose 20mm. As you have stated twice, i must reiterate you cannot physically make the rear chamber and shallower, the driver is a set physical depth and the pole piece vent would be completely closed up.

I include the plan for the MT-130 for reference.

MT-130

The only possible difference that I may not have included was a recessed piece as on the OHM design into the vent itself to add a bit more surface area for heat transfer into the backplate.

I like the Idea that OHM has got, a simple single plate.
This would allow for mounting the driver and having the back wall mount further forward than normal. The single plate would act as the seal for the hole cut into the back panel.
I suppose the idea is that I was using two sheets of metal with fins between and larger surface area on the outside plate, for heat and look.
Venting the air along the fins and out around the magnet, rather than straight out from the centre. the Idea being that the plate mounted to the magnet (heat paste) would allow for greater heat conduction.

Last edited by sketchyphish; 26th March 2014 at 08:03 PM.
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Old 26th March 2014, 08:23 PM   #10
DavidL is offline DavidL  United States
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Wayne,
How often do you hear of drivers failing due to heat issues? Most listeners will be putting maybe 2 to 5 watts average into their system with yes, 100 watt peaks to the bass. That's peaks not continuous. A 'few hundred watts" as you state is maybe for the professional venues not homes.

Sketchy,
I see you are building a horn loaded speaker. What is the sensitivity of the design? I am guessing along 95 to 100 Db? 2 watts will be very loud if so. Again and since seeing this is a horn design, heat isn't going to be an issue unless you are planning on putting on a concert for the neighborhood.
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