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Old 29th July 2009, 12:47 PM   #1
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Default Home built subwoofer driver

A project that I am working on is constructing a large subwoofer driver. My aim is to have a large Sd, and a low Fs of between 15-25 Hz. I plan to use it in a large quarter wave tuned pipe for response below 20 Hz.

What I have thought of so far is using the motor structure (basket,magnet,coil, and spider from a high power, high excursion car sub by carefully removing the existing cone.

The diaphragm will be about 18" in diameter, and for the outer suspension I plan on using a 20" bicycle inner tube thats been cut along the outer and inner most circumference, giving a highly compliant singe roll type suspension. This would be mounted on a specially made metal frame, and the frame of the car sub would be bolted to this using its existing mounting flange.
The voice coil would be coupled to the diaphragm by means of a simple lightweight extension piece.

For the diaphragm, I thought of using a round piece of plastic corflute. The surface of that would be covered in ~1cm long piece of cardboard tube in an arrangement similar to a honeycomb pattern. Then all cavities filled with adhesive foam, and a final layer of something strong, thin and light like maybe fiberglass or formica. This would be clamped under moderate pressure between a couple of sheets of plywood while it cured.

The sensitivity will most likely be significantly lower, but thats not an issue as I have plenty of amplifier power to drive it, and i would use a car sub that had a high power capability and preferably a high BL figure. And the added mass of the larger diaphragm and looser suspension should help to lower the fs.

I would like to hear any thoughts or suggestions any of you may have, especially on the construction side. It is an experiment, not a fully planned and calculated full on project and most of all something fun for me to work on in spare time. Any advice or tips welcome, but please keep any negative comments to yourself
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Old 29th July 2009, 04:43 PM   #2
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Hmm, dual low Fs/high Xmax 15" subwoofers should yield more performance than what you're contemplating. As a DIY project though and considering its intended sub only BW.........

How do you plan to use an existing basket unless it's at least a 20" unit, ergo what do you plan to gain?

For a 'sub' only BW alignment, a flat, void free plywood disc is all you need for a diaphragm, but until I have a better understanding of the situation, no point in further musings.

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Old 29th July 2009, 06:02 PM   #3
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that could be an idea, using a plywood disc. Just thin 3-ply?

What I plan to do is mount the 20" driver onto a specially made metal frame, and just bolt the original basket of the car sub onto the back of the metal frame. I dont know yet what style frame it will be, but that will be how the 12" basket attaches to the back, effectively just becoming part of the overall frame.

All I want it for is for low end sub bass, covering as low as possible up to about 40 Hz, past that I will use a conventional sub in a vented box that I have that works very well, just drops off past 35 Hz.
It will see most use in a home theatre setup to give more depth to the LFE channel, although it will be used for music as well.
I understand that i could buy a couple of low fs/high xmax 15's probably would have better performance, but I want to make something myself rather than just buy. More likely to learn something and I don't have much spare cash.
What would be best type of enclosure be for extended bass response? My first idea was a quarter wave pipe, but there may be more suitable boxes. If its still unclear what i'm thinking of, then I will draw a simple diagram.
Still looking for any suggestions or any feedback.
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Old 30th July 2009, 01:11 AM   #4
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Some years ago there was a brief fashion for drivers with flat diaphragms made of aluminium honeycomb. Some were round, some were square. The most expensive ones had a large voice coil mounted directly on the back of the diaphragm, but most used a conventional speaker construction - a normal voice coil attached to a normal (but small) cone, the edge of which was glued to the back of the flat diaphragm about half way out from its centre.

So instead of removing the cone completely from the "driver", you could just remove the surround and part of the basket. Mount the remainder of the basket to the back of your new frame/basket, and glue the edge of the cone to the back of your new diaphragm. If you're planning an 18" diaphragm, perhaps cut your donor driver cone down to about 9" diameter.

From a mechanical point of view, a donor driver with a pressed steel basket will be easier to modify. Cutting off the edge of the basket will leave the flat steel "spokes" coming up from the magnet assembly. You can bend the ends of these out flat to mount against the new basket / frame.

When you look for a suitable donor driver, look for one with a high power rating (big, high temperature coice coil), a powerful motor (high Bl spec), and high xmax. These figures are more important than the size of the cone, so look at 10" and 12" drivers too.

You're likely to end up with a driver with a high qts and Vas, so consider mounting it in IB (Infinite Baffle) style - in a wall between two rooms.

You can use existing box modelling software to predict the performance of your driver if you take measurements, such as diaphragm mass, as you construct it.

Lastly, think outside the circle. If you can't find a single suitable driver, consider using two (with an oval diaphragm) or 4 (with a square diaphragm).
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Old 30th July 2009, 02:30 AM   #5
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Thanks for your reply. About cutting the basket and using the cone, I want to keep the basket intact, as the original mounting flange will provide a more secure mount and is guaranteed to be square. I will instead just make a straight cone that tapers from the size of the voice coil, up to about 9" as you say.
And yes, earlier on I said that i wanted a high power high BL car sub with a large voice coil. I have an amp capable of 1000W RMS into a 2 ohm load, or 650W into a 4 ohm load. I will try and stick with the 4 ohm load, as its still plenty more power than i will ever need, and it will have a better damping factor value
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Old 31st July 2009, 06:02 AM   #6
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Quote:
Originally posted by flyingtele
that could be an idea, using a plywood disc. Just thin 3-ply?

What would be best type of enclosure be for extended bass response?
If it was braced to get rigidity up enough to ensure its Fs is well above the sub's HF BW, so that it can't be excited enough to be perceived.

It will depend on the specs it winds up having, but I'm guessing a large tapped TQWT.

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Old 31st July 2009, 06:32 AM   #7
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Quote:
Originally posted by flyingtele
Thanks for your reply. About cutting the basket and using the cone, I want to keep the basket intact..........
Well, if you're going to do it this way, no point in the flat disc. Instead, just treat the cone on both sides with epoxy to make it rigid enough. Build/measure to see what its specs are and go from there, i.e. adding mass to lower Fs if required with the understanding it raises Qts, so will probably have to compromise. Or run it sealed and use a Linkwitz-Riley Transform (LRT) to electrically alter its specs to get the desired Qtc, Fb.

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Old 31st July 2009, 06:39 AM   #8
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Quote:
Originally posted by GM


Well, if you're going to do it this way, no point in the flat disc. Instead, just treat the cone on both sides with epoxy to make it rigid enough. Build/measure to see what its specs are and go from there, i.e. adding mass to lower Fs if required with the understanding it raises Qts, so will probably have to compromise. Or run it sealed and use a Linkwitz-Riley Transform (LRT) to electrically alter its specs to get the desired Qtc, Fb.

GM

I considered something along the lines of that, but I would rather remove the original cone and use the flat disk as it will have a much bigger sd (18" compared to 12" diameter) and therefore move more air.
But also I want to experiment with building a diaphragm and suspension setup from easily obtainable materials etc at the same time.
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Old 31st July 2009, 06:50 AM   #9
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If you're making a new cone, then the disc is superfluous since the cone will be as deep as it needs to be to grow to the desired diameter at whatever angle you choose. If you use a disc, then in lieu of a cone you extend the VC. Either way it ideally needs dual spiders.

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Old 1st August 2009, 10:53 PM   #10
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flyingtele,

Here's what I would probably do,if I were to attempt such a project.

First,leave the woofer cone and surround intact,the surround will be like a second spider.

Glue extension pieces(cardboard tubes,foam,etc) to the cone and to the new and larger cone,which could be either a flat disc,or a cone,which should be stronger.then attach the bicycle tube to the new cone and frame,which is fastened to the original woofer.put it in a suitable enclosure,and Robert is your father's brother...

The trick is to make it strong enough to not fall apart,but not so heavy that it won't work well.

If it doesn't work,you should be able to take it all apart,and still have your original woofer.

What ever you wind up doing,we want to see pictures!!!

Good Luck!
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