Fiberglass Sub Box Fabrication - Bimmerfest - BMW Forums

Welcome to Bimmerfest -- The #1 Online Community for BMW related information! Please enjoy the discussion forums below and share your experiences with the 200,000 current, new and past BMW owners. The forums are broken out by car model and into other special interest sections such as BMW European Delivery and a special forum to voice your questions to the many BMW dealers on the site to assist our members!

Please follow the links below to help get you started!

Go Back   Bimmerfest - BMW Forums > BMW Model Discussions > Z Series > E36/7 Z3 (1996-2002)

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

Thread Tools Display Modes
Old 03-05-2012, 05:35 PM
Blacklane Blacklane is offline
Officially Welcomed to the 'Fest
Location: Springfield, Ohio, USA
Join Date: Jun 2011
Posts: 1,977
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 I got 100lb XX high-density foam blocks in sample size from Poly-fill came from a fabric store. Flower foam came from a craft store.
Click image for larger version

Name:	Fiberglass Sub Box.jpg
Views:	3860
Size:	255.3 KB
ID:	315219

Click image for larger version

Name:	First and Second Shape.jpg
Views:	557
Size:	86.8 KB
ID:	315220

Click image for larger version

Name:	First Fiberglass Layer.jpg
Views:	408
Size:	88.3 KB
ID:	315221

Click image for larger version

Name:	Foam Moulds.jpg
Views:	456
Size:	75.4 KB
ID:	315222

Click image for larger version

Name:	Moulds dug out.jpg
Views:	432
Size:	79.8 KB
ID:	315223

Click image for larger version

Name:	Foam Shape.jpg
Views:	530
Size:	82.8 KB
ID:	315224
* 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 07:05 AM. Reason: Additional Info
Reply With Quote
Old 03-05-2012, 07:13 PM
jmanscotch's Avatar
jmanscotch jmanscotch is offline
is a Texan
Location: Colorado Springs
Join Date: Aug 2009
Posts: 1,400
Mein Auto: '04 530iA
Well done, sir! I'm going to mark this for my future attempt to make one for my Z4.

2004 325Ci - 5MT - SOLD & MISSED
1967 1602 - 4MT - STOLEN IN KY
2008 328XI - 6MT - LOST IN DIVORCE
1995 325i - A/T - RUSTED BACK TO EARTH
2009 135i - 6MT - COPS IMPOUNDED
Reply With Quote
Old 03-05-2012, 08:21 PM
dkindig dkindig is offline
Officially Welcomed to the 'Fest
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 08:26 PM.
Reply With Quote
Old 03-06-2012, 09:06 AM
tohbi tohbi is offline
Officially Welcomed to the 'Fest
Location: arizona
Join Date: Aug 2011
Posts: 413
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
Reply With Quote

See More Related BMW Stories

Forum Navigation
Go Back   Bimmerfest - BMW Forums > BMW Model Discussions > Z Series > E36/7 Z3 (1996-2002)
Today's Posts Search
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

All times are GMT -7. The time now is 04:44 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
© 2001-2015 performanceIX, Inc. All Rights Reserved .: guidelines .:. privacy .:. terms