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E36/7 Z3 (1996-2002)
E36/7 Z3 Roadster, Z3 coupe, Z3 M Roadster and Z3 M Coupe talk with our gurus here.

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  #1  
Old 03-05-2012, 06:35 PM
Blacklane Blacklane is online now
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Location: Springfield, Ohio, USA
 
Join Date: Jun 2011
Posts: 1,122
Mein Auto: 1998 Z3 Roadster
Fiberglass Sub Box Fabrication

The 1999+ Z3 Roadsters came with a single 6.5-inch subwoofer that, with very little work, could be retrofitted with a Kicker Comp VT 65. Those of us with earlier versions with the premium Harman-Kardon system got two-speaker subwoofers that could not be retrofitted with anything on the market, and even if replacement speakers were available, it would still sound poor. After looking everywhere for a single-speaker sub box, I finally decided to borrow a line from Dr. Suess:

Did that stop the old Grinch...?
No! The Grinch simply said,
"If I can't find a reindeer, I'll make one instead!"

So I decided to make a subwoofer box that virtually matched the BMW HK single-speaker box. It wasn't too hard, but it did take some time. Basically, I made a plywood mount for the speaker (Fiberglass doesn't hold screws), formed three inner moulds out of green flower foam, fiberglassed them until they were strong enough to handle, dug out the foam, assembled the pieces into a box, and fiberglassed the entire assembly to the final thickness.

One of the biggest questions I had was how thick to make the fiberglass, and I had to decide that before making the moulds. I settled on 1/4 inch*, which is a reasonable planning factor. The final thickness was slightly less than that. I stopped adding layers when the box was rigid, and that turned out to be about four layers, two before assembling the halves and two after.

Kicker recommends a sealed volume of 0.2 cu ft. There really isn't enough space available to achieve that. I tried to use every bit of space available, but could only come up with 0.17 cu ft or so. The first thing to go was the mounting brackets. I'll be using some high-density foam blocks to hold it between the roll hoops and some sound-deadening foam sheets to keep it off the steel bar. I also sprayed the inside of the box with sound deadening spray ("Quiet Kote") and I filled the box with poly-fill in an attempt to make it sound acoustically larger.

I actually made two versions. Since I had never worked with fiberglass, I expected some mistakes. I also felt the shape would be too complex to get it right the first time. I was right on both counts. The first version was good to put into the space to understand what would fit and where to trim. The second version was much better. Another thing I did on the second version was to wrap the flower foam with plastic food wrap before fiberglassing. It took some extra time to glue the plasic wrap to the foam and trim it, but it made the foam come out much easier.

I could have made the box a bit prettier with the addition of a layer of Bondo or even just a coat of paint, but it is entirely hidden, so I didn't bother.

It was interesting to hear what a difference a sealed box made to the sound. At one point, I just mounted the speaker to try it out. While the box wasn't sealed, it was very tight. At moderate volumes, there was an objectionable cackle in the sound. Later, after I had sealed the wire hole and the speaker mounting flange with silicone, the sound was much nicer and accurate.

Fiberglassing materials are readily available at hardware stores and home centers. You'll also need to pick up measuring cups, disposable paint brushes, latex gloves, and a good respirator mask. I got sound-deadening foam sheet and Quiet Kote from PartsExpress.com. I got 100lb XX high-density foam blocks in sample size from BestwayFoam.com. Poly-fill came from a fabric store. Flower foam came from a craft store.
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* My apologies to the non-US members. Here in the US, we long ago rejected the arbitrary British notion of driving on the left, but steadfastly hold on to the nonsensical British measurement system, long after they abandoned it.

Last edited by Blacklane; 03-06-2012 at 08:05 AM. Reason: Additional Info
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  #2  
Old 03-05-2012, 08:13 PM
jmanscotch's Avatar
jmanscotch jmanscotch is offline
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Location: Colorado Springs
 
Join Date: Aug 2009
Posts: 1,282
Mein Auto: '09 135 - 6MT - TwinTs
Well done, sir! I'm going to mark this for my future attempt to make one for my Z4.
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2004 325Ci - 5MT - SOLD & MISSED
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  #3  
Old 03-05-2012, 09:21 PM
dkindig dkindig is offline
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Location: Murphy, NC
 
Join Date: Nov 2009
Posts: 314
Mein Auto: '01 Z3 3.0i Roadster
VERY nice job! I was lucky enough to find a new enclosure with HK sub on eBay 2 1/2 years ago for $150. I thought that was high then but my opinion has changed as I see others trying to even find a used box.

You obviously know what you're doing, as I calculated the volume of the stock enclosure right at 0.2 cubic feet. You might be able to make some money if you construct a mold for your enclosure. I see quite a few people stuck with the old enclosure that would love to have a way to mount a Kicker properly.

I'm a big fan of sealed enclosures as well. I like good, tight bass response and of course in the area we have to work in, a ported enclosure won't work anyway. As you found, with a sealed enclosure, the air in the box acts as a suspension mechanism for the cone. With a port the cone has too much flex and you can't drive it as hard. I've got 250W driving the Kicker and it makes a difference. If it wasn't sealed it would sound like crap in a hurry!

I have dissected the audio in the roadies pretty well. If you have any questions, ask away. I'll do my best to give you the benefit of my experience.

Last edited by dkindig; 03-05-2012 at 09:26 PM.
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  #4  
Old 03-06-2012, 10:06 AM
tohbi tohbi is offline
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Location: arizona
 
Join Date: Aug 2011
Posts: 387
Mein Auto: z3 '97 atlanta blue 2.8
i'll take advantage of your experience, dk. my hk unit turns itself off for a few seconds, then back on. seems like a power problem because the red "on" lights go off and on, too. they are also quite dim when the radio/cd is on.

where should i look? thx
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