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Author Topic: Questions about Lab subwoofer.  (Read 1722 times)

Helge A. Bentsen

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Questions about Lab subwoofer.
« on: January 01, 2006, 10:15:37 pm »

I'm considering building some Lab subs and have been trough the archives and using google extensively, but I still have some questions that need answer before I say go, so I hope you guys can help me:

1. On the Lab documentation page there are two available designs, Jeremy Bridge's for 18mm wood in metric, and Brian Fehrman's design for imperial measures. I've noticed some differences between the two designs, so I wonder if its two different designs, or reading error, or different versions? And if it's two different designs, witch one is "best"?

2. I've seen some fabrication photos, and for me it looks like people are only gluing the horns together. On my other DIY-subwoofers I've used glue and screws/nails in all my joints, but is this project designed to be glued only?

3. If I'm building 4 boxes, how do I stack them "correctly"? Standing side-by-side, 2x2 stack on their side bottom-to-bottom(all horn mouths in "center" of the stack), or 2x2 on side top-to-top? I know that it's no absolute truth on how to stack subwoofers because of the different locations and demands you meet on the road, but I believe that someone here has found a good working average solution.

4. If I were to build 8 and wanted to use them in a traditional LR-system with subs on each side, as you do commonly with ordinary front loaded subs, would the system work in the same way as an ordinary subwoofer system in terms of dispersion, power-alley etc.?

Edit:

Forgot a couple of questions Embarassed

5. Is the common HPF and delay setting 35Hz and 10,5ms?

6. Can I build them in MDF or do I need to use Birch plywood or another "high grade" material?

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Antone Atmarama Bajor

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Re: Questions about Lab subwoofer.
« Reply #1 on: January 03, 2006, 04:21:31 am »

    I'm not much of an authority on building lab subs (Since I havent done it) but I'll comment on a couple of questions.

    I have 8 Servodrive BT7's I usually lay them on their sides 2X2 so that they make one large horizontal horn mouth.  I like the way it looks.  I think similar results can be achieved by having them all upright Side by side (flair opening up).  I would think that it may give you slightly different polar response up to a certain frequency 60ish Hz or so but I've never measured it.

    I don't think you can escape Lobing (Power Alley) unless you can keep the driver spacing no greater than 1/4 wavelength of the Highest intended operational range (Hmm that raises an interesting question) If you have 8 of the Lab subs spaced at 1/4wave of the High Cut Range would they still mutually couple?).

    I wouldn't bother running the subs in Stereo.  Top boxes fine in stereo.  Sum the sub channels.

    Top Box Delay Should be ~ 1/(speed of sound/Horn Length)  ~1/(1130ftpersec/10.6ft)~=9.38ms.

    HPF If you have a cluster of 4 I think 30Hz should be fine, Maybe?

    I can't comment on glue only  That would be some serious glue.

    As far as construction materials.  What do you plan on doing with them.  Are you going to install 8 Labs in a movie Theater never to be moved again?  Are you going to tour with them?  MDF is fantastic and has great damping but will not hold up if you are constantly lugging the things around.  You will want to go void free with your ply.  I used Marine Ply for a project and it  is much to soft IMO.  The good 13ply Finnish or Baltic Birches are very strong hard and Heavy (And it smells good when you cut it).  I wouldn't use CDX or anything cheap like that.

Hope that helps a little

Antone-  



   
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Phil Pope

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Re: Questions about Lab subwoofer.
« Reply #2 on: January 03, 2006, 11:01:58 am »

Good glue is stronger than screws.  Get some polyurethane glue.  It is very strong and foams with moisture to fill gaps. I have been using dowels to make sure everything lines up properly and small screws rather than clamps to hold stuff together. The advantage of screws is that you don't have to keep clamps on whilst you are waiting for the glue to dry so you can get on with the next piece of work. I think some others have used biscuits or pocket-hole joinery

Phil
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