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Old 16th June 2004, 11:53 AM   #1
tuneman is offline tuneman  Australia
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Default voice coil vent size?

i have finished creating a hybrid sub consisting of various parts including a cone, surround and spider from a dd9515, but with a 4" former and different magnet and frame.
i believe i have created my own weapon of mass distruction as it was designed for extreme power applications and is very stiff, so much so i could stand on it! but unfortunatly its also probably the worlds most inefficiant sub as it moves about 10mm with 5000wrms!! but that aside'

i want to know if theres a formula or some way i can work out the correct magnet vent size?
it must be small enough to have back pressure that will be forced down beside the former and windings and out of the vents in the side of the basket but be big enough not to cause any undue restriction the the cones motion!
do i work out the volume of air space between the underside of the dustcap and the pole piece and use the vent sort of like a port? or is there a way to work out how much resistance a certain sized vent will have on the cone? or perhaps a way to work out what percentage of air would go to each vent! back and the sides
ih and the reason i ask this after its been built is that it normally has a vent that is way to big and i have to work out what sized restrictor to put in the hole.

anyone care to tackle this one
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Old 17th June 2004, 12:21 AM   #2
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Pics please!

-Casey Walsh
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Old 17th June 2004, 12:59 AM   #3
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Default Re: voice coil vent size?

Quote:
Originally posted by tuneman
i have finished creating a hybrid sub consisting of various parts including a cone, surround and spider from a dd9515, but with a 4" former and different magnet and frame.
i believe i have created my own weapon of mass distruction as it was designed for extreme power applications and is very stiff, so much so i could stand on it! but unfortunatly its also probably the worlds most inefficiant sub as it moves about 10mm with 5000wrms!! but that aside'

i want to know if theres a formula or some way i can work out the correct magnet vent size?

anyone care to tackle this one
Wow a 4 inch former is massive! what gauge of wire did you use? This size of coil would need a very high power magnet system and a large plate and shockingly big yoke, drilling the yoke reduces the iron in there, so dont go crazy with the pole yoke venting, I think a 20 mm vent is fine, but saying that without knowing the magnetic value of your gap and yoke and seeing how your WMD is not the most sensitive speaker in the world, its hard to say thats ideal. I have never been a big believer in more power for a louder sound, there is a cut of point, 5000 RMS watts sound very impressive but who makes a 5000 watt amp that sounds nice? besides how loud would it be? as loudness is a function of x max, would it be louder than the next woofer moving the same volume of air? i dont know for sure, but wouldnt think so. I would love to see a picture of your WMD even if its not finished,air cooling is fine for a domestic driver but if you really want to cool the coil then a Co2 or water vain system is needed,
Its not as difficult as its seems but its messy and expensive, can you tell i work with cars to,LOL, your designs sound proper mental so i guess this is not far from your door step!


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Old 17th June 2004, 06:09 AM   #4
tuneman is offline tuneman  Australia
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effeciency is my biggest problem, due to large 4" formers being quite heavy, 3inch is a better compromise between cooling and weight but its what i had lying around also probably made of the worst stuf for a former, its made of brass!

the magnet is unfortunatly from an old rockford power series sub so i wouldn't consider it very strong in a magnetic sence!, the original vent is way to big with very little air flow being forced down the gap because there is no restriction at all' all the air just blows in and out of the hole achieving not a lot.

the sub is usable but i did some trial runs with music for a laugh and this thing plays subsonics i mean there was no bass in my car only stuff falling of shelves 10m away, it was in a approx 3cf open ended tube i had from a previous transmission line stile setup.

it was originally designed to play low freq(30hz and below) just for demonstration purposes at car shows while being fed 5000wrms + i am also able to power it with 10,000wrms in my extreme spl car from 4xconcept d2400a's, the sub is a dual 2ohm as well
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Old 17th June 2004, 08:02 AM   #5
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Quote:
also probably made of the worst stuf for a former, its made of brass!
Depends on two things:

1.) Weight: When you say brass then I assume it is thicker than the material used for the average aluminium coil former !?

2.) Conductivity: When you use electrically conducting coil formers you end up with lower Qms. But that's not the worst part. It must never ever be made of a closed ring since this would work like the modern brakes used on heavy trucks. It then works like a shorted dynamo !!!! Leave a small gap open !!

Regards

Charles

BTW: Which middleofnowhere in Australia are you from since there are many of them ?
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Old 17th June 2004, 01:21 PM   #6
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Quote:
Originally posted by phase_accurate


Depends on two things:

1.) Weight: When you say brass then I assume it is thicker than the material used for the average aluminium coil former !?

Brass has to much mass compared to aluminium or a kapton or teflon former, a 4 inch coil is the best for extrem power and low infrawaves! no wonder things are falling around you, I would use smaller high power magnets in a helical arrangment for more flux, neodymium is best as you dont need a high iron content yoke plate, you can make a small spacer for the vent as this will allow you to make adjustments to the venting pressure, until you feel there is a real air colling effect taking place, some say a mesh is better in a large vent as this adds a drag or shakes the air, You know 007, Shaken not stirried, well apparently shaking is better than stirring! for cooling.
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Old 17th June 2004, 03:42 PM   #7
markp is offline markp  United States
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If your brass former is something like a pipe you must slit it or you have a shorting ring that is causing your low output.
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Old 18th June 2004, 12:52 AM   #8
tuneman is offline tuneman  Australia
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yes im aware of there being a gap!
i actually go it wound by someone else, im not setup to wind my own coils' its actually cheaper especially with regard to the high temp resin, and making up the winding bobbins etc.
i knew the weight would be a problem but i said give it a go anyway

helical arrangement! could you give me a brief description of what that entails? i it where there placed in a circle?
i have a large collection of various magnets but no neo unfortunalely, i have a quad stacked magnet that i was considering attatching to the back with the polarity reversed to mabe try to up its efficency would this have any benifits?
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Old 18th June 2004, 01:36 AM   #9
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Quote:
Originally posted by tuneman

helical arrangement! could you give me a brief description of what that entails? i it where there placed in a circle?
i have a large collection of various magnets but no neo unfortunalely, i have a quad stacked magnet that i was considering attatching to the back with the polarity reversed to mabe try to up its efficency would this have any benifits?

Placing the magnets around the coil is best for a big coil, Brass is strong but its heavy, Very few people have the ability to make a 4 inch coil! thats rare indeed, still I want to see this beast?
I have attached a picture of the magnets on a Goliath subwoofer.
Its amazing the force from this infrawoofer, it weighs more than 28.5Kg and has a 3 inch coil, it was to be a 4 inch coil but the yoke plate would be larger than the woofer. We will make a 4 inch coil on the next run. so am interested in your WMD.

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Old 18th June 2004, 01:47 AM   #10
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This is the 3 inch quad wound coil, over 210 feet of high grade copper, its heavy but it has a hugh electrical field, so the magnets have to be equal to the task.
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