Promit's ultimate stealth audio build and trunk floor upgrade
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Promit's ultimate stealth audio build and trunk floor upgrade

This is a discussion on Promit's ultimate stealth audio build and trunk floor upgrade within the Focus RS Electronics forums, part of the Focus RS Garage category; It's been an agonizing wait to start this post but I wanted to make sure the project had a good chance of success before sharing. ...

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Thread: Promit's ultimate stealth audio build and trunk floor upgrade

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    Promit's ultimate stealth audio build and trunk floor upgrade

    It's been an agonizing wait to start this post but I wanted to make sure the project had a good chance of success before sharing. Here's the story of this build. I use my trunk cargo space a lot, enough that I am not willing to give up any space in any dimension. Not width, height, or depth. But I also love good quality audio and the stock speaker system is clearly nothing of the sort. Cutting the factory foam wasn't terribly appealing either, in case things went poorly and I wanted to rewind. I've spent the last couple months puzzling over how to square this circle and I've finally figured it out. With this build I get to install a real 10" subwoofer, add real audio amplification, preserve all of the normal cargo space, and increase the usable space under the trunk floor. And rather than deal with fiberglass, I've managed to construct this project entirely from wood! Just MDF and plywood, with none of the chemicals or fumes that come with glass.

    The work is not quite done, but I will keep this thread updated as the build progresses into its final stages of finishing and installation.
    Current Status: Subwoofer is completely finished and carpeted. Working on the storage section now, it's basically glued up but needs a lot of finish work.
    Latest photos:


    The subwoofer for this build is a Dayton Audio LS10-44 shallow mount. I looked at using the JBL Club WS1000 (also sold as an Infinity) but didn't like the simulation in WinISD quite as much and the build quality is worse. The maximum available depth is in the same location as the stock donut subwoofer, and clocks in right at 5". With a rear wall of 0.5", that leaves 4.5" to fit the subwoofer including all of its excursion. Box volume is about 0.45 ft³ gross. With the right build and some adjusted dimensions, it is probably possible to squeeze in a Stereo Integrity BM, but I didn't plan for that. If you raise the floor 0.75" (easy enough if you want to give up the space), a JBL 10TW3 will fit. Since my priority is cargo space and sound quality over raw output, the Dayton will do just fine.

    The speakers I've settled on are the Hertz HSK 165. These are nice because Hertz publishes exact dimensions and the tweeter is a dead match for size. We'll see how they mount up. I'll probably go with the NVX JAD 900.5 for the amp but I might switch over to the Pioneer 5ch.

    The build:

    If you've ever pulled up the trunk completely, you know that our sheet metal floor is unusually complicated and contoured, defying any obvious way to build a sub box/floor other than fiberglass. It turns out, though, that the contours can be simplified in such a way that it is essentially flat and level. The right cuts and a tiny bit of carving with a router and jigsaw will allow for wood sheets to sit stable on top of it. Once you have the basic layout correct, the rest of the build is a relatively straightforward woodworking project. Think of the whole trunk as having three different depths, each one exactly half an inch apart. By layering sheets of half inch MDF, you will end up with a flat floor.

    Here you see the main support for the storage section.

    Here, the style of layering I mentioned becomes visible. This is still a dry run test layout at this point, and will eventually get fully reinforced.

    Now the big layer that forms the central section of the storage compartment. I'm going to have a lot more space than before. The floor drops away under this panel towards the rear of the car; I will add some supports in at the end but there's plenty of space left for the cables underneath.

    The first supporting dividers are going in now. The rear right section will accommodate all of the factory tools.

    Clamps time! Dowels go in after this to reinforce the major joints.

    Everything except the back wall is attached. Note that the large center section has recessed walls and will get cover panels to support the trunk floor above it. Those panels will sit flush with the other walls, and should hold just fine with a few magnets.

    Had to ease the corner just a bit for everything to contour nicely. The right side compartment will get a curved face panel to close the rest of the gap.

    Also working on the subwoofer box shape and doing some dry fit. Ultimately decided against using this JBL.

    Side by side layout view, sub box is coming along. Added some panels to seal around the chassis brace, and attached side walls.

    Final subwoofer walls and a bracing post. No point bothering with elaborate bracing sections on this one. Silicone sealant on the joints too.

    Made a little face panel to follow the rear contour of the trunk.

    Checking alignment of the subwoofer hardware. These are M6 connector cap nuts in 12mm that go directly onto the factory studs. I found some sealing washers at Home Depot that fit perfectly to close up the hole from air leaks.

    Detail view of the mounting hardware. The cap nuts have 4mm hex heads.

    Final test fit of the subwoofer box.

    Whole trunk test fit, looks like I'm golden to finalize the back wall and top panels now. Fun fact: the new storage area is large enough to fit a cordless drill. If all goes well, that Sony amp will be swapped for a DSR1.

    Completed all the subwoofer work today.

    Final glue up to close everything up on the subwoofer.

    Final bottom contouring, for everyone following along.

    Final top with some roundovers and general cleanup.

    Primered the sub box. This is really about moisture sealing rather than finishing, so I didn't beat myself up on the finish quality.

    And now it's carpeted! I used carpet from Crutchfield, and this was my first time doing carpet on a box. I was worried with how complex the shaping of this box is, but there's plenty of stretch in the carpet which allowed things to go fairly smoothly. I made a complete mess of the seams but hey, you can't see them.


    I've gotta clean up a bunch of dog fur, but it's looking pretty good otherwise! I didn't bother taking pictures of the jack plate, it doesn't take a genius to figure out.

    The final construction steps are to cut and glue the back wall for the storage section, make the top panels, and add some support underneath the sloped section so that it sits fully level and flat. Then I'm going to shoot everything with a primer sealant, and finally I get to see how difficult it is to carpet this.
    Last edited by Promit; 05-19-2019 at 06:39 PM.

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    Because the storage area is enlarged so much, something needs to sit on top of it to provide support to the flexible trunk floor overhead. I decided to make a pair of panels that would be strong enough bear weight of whatever is loaded in the trunk. I'm also making then mitered frames with captive metal mesh for aesthetic reasons.


    Basic miter cuts of oak pictured for the first frame. They fit together "okay".

    Starting to glue things up, and testing a channel cut along the side that will be in the middle of the storage unsupported.

    Epoxied a 3/8" steel bar into the channel, and some magnets in the corners to keep things in place. I didn't want to take any chances with the weight capacity of these.

    Stained the oak black, and rattle canned the mesh in a nice hammered gray finish.

    Test fit of the top panel, and laying out the bottom panel. The bottom is cut to contour with the trunk plastic.
    Last edited by Promit; 05-19-2019 at 06:46 PM.

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    The Hertz HSK 165 components are great! I have them in front and the Hertz HSK two way speakers in the rear doors.

    Excellent build! If you can duplicate or provide step-by-step directions, I'm sure others will throw some cheddar your way!

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    I'll be watching this majestic jigsaw puzzle !

    Seems like high speakers deserve better than those middle-of-the-road amplifier choices you present: consider an Audison amps as well! AP1D = mono 540 watts (into 2 ohm load), AP4D = underrated 70x4 at 4 ohms, 140x4 at 2 ohms

    I picked up both my amps off an eBay seller for a touch more than $600 shipped.

    The Audison Prima AP F8.9 bit would be a solution to power all speakers with more than enough power behind them all.
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    Interested in seeing where this goes. Sub'd. Also PM'ed.

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    Alright it took a good couple hours this weekend but the subwoofer is now completed! I've updated the top post but here's a picture for everyone jumping to the bottom. Don't mind the dog fur.

    I've gotta finish up the storage section now, which is a more of a pain but hopefully won't take too much longer.
    Focus112 likes this.

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    Nice work!

    Where is your amp going? You're not planning on driving your sub with the factory amp are you?

    Are you going to swap out the factory door speakers, and/or do any soundproofing in the doors?

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    Quote Originally Posted by fiST2RS View Post
    Nice work!

    Where is your amp going? You're not planning on driving your sub with the factory amp are you?

    Are you going to swap out the factory door speakers, and/or do any soundproofing in the doors?
    I'm going to install the DSR1 and amp in the newly built storage section, towards the back (but not in the rearmost compartment, it's not wide enough). Hertz HSK165 in the front doors, and just the normal sound proofing material that is commonly added. I'll probably put some spare Infinity coaxials I have in the rear doors and kill the rear tweeters.

    Quote Originally Posted by MorningZ View Post
    .
    I'll be watching this majestic jigsaw puzzle !

    Seems like high speakers deserve better than those middle-of-the-road amplifier choices you present: consider an Audison amps as well! AP1D = mono 540 watts (into 2 ohm load), AP4D = underrated 70x4 at 4 ohms, 140x4 at 2 ohms

    I picked up both my amps off an eBay seller for a touch more than $600 shipped.

    The Audison Prima AP F8.9 bit would be a solution to power all speakers with more than enough power behind them all.
    I hear what you're saying, but I struggle to pay for high end amplification in a car that has a pretty substantial noise floor as it is. I was thinking of picking up a JL amp (missed a used RD that I'm kicking myself over) but in general I'm having trouble finding a used amp right now that hits a good price/performance ratio for me.

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    Promit's ultimate stealth audio build and trunk floor upgrade

    Quote Originally Posted by Promit View Post
    but I struggle to pay for high end amplification in a car that has a pretty substantial noise floor as it is
    Just want to note, because it's perplexing to see these higher end speakers get installed w/o the considering of a higher end amp to complete the system: if you haven't bought the DSR1 yet, then keep in mind the $630 Audison 8.9 Bit amplifier will directly plug into the Sync/Sony system with a $50 iDataLink cable. So that'd be $200 saved by not buying the DSR1 and able to be applied towards a proper amplifier.

    Sorry, I don't know if you are asking for advice in the above posts for amp recommendations/ideas of not, but it would be a shame to ruin all that hard work and great speaker setup with an underpowered or terrible sounding amplifier... /2-cents

    I promise that the RS isn't as terrible a platform as you may seem, there are certainly a lot worse cars out there to start with!
    Last edited by MorningZ; 05-02-2019 at 06:02 AM.
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    .,,
    Last edited by fiST2RS; 05-01-2019 at 10:57 PM.

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