Build Log: 2006 Audi A4 Avant Stealth 24" sub build.

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    Build Log: 2006 Audi A4 Avant Stealth 24" sub build.

    Funny story to how this came to happen.

    A couple of years ago I was at the Virginia State Finals for MECA ( Mobile Electronics Competition Association) at the Sundown show in North Carolina. Had the '66 Chevelle there competing in Extreme (single seat sound quality judging) and SQ2+, which is where both seats are judged at the same time. Won state championships in both classes.

    Had a guy come up and introduce himself. "Hey, I'm Nick from Stereo Integrity"

    "Oh, hey man. How's it going."

    That was pretty much our discussion. I had no knowledge of his products. Had only seen them on the forums. He never had the chance to listen to the Chevelle because people were spending 20-30 per person listening.

    Fast forward to last November. We bought the new place in March of last year and I finally started building my own work shop. I decided to have a group meet for all SQ competitors on the property because we had space. Like a PBB meet & greet, just a bunch of car audio guys listening to cars, eating BBQ (I cooked), and bullshittin'.

    Nick contacted me and asked if he could bring stuff to show to the guys that came to the meet. Told him he could.

    Everyone comes and Nick see the van that was sent here from Pittsburgh. Dude was unhappy with it and sent it to me to unfuck. Complete tear down and redbuild. Dash and all.

    So Nick asks what the plan is, I tell him that the dude had four JL 13W7s on four JL HD1200/1s and it only did 140.9dB.

    The van had 14 amplifiers, 52 channels of amplification, four digital signal processors, and 52 speakers.

    So Nick is listening and I tell him that I was going to put the 13W7s in a 4th order C-pillar wall.

    He suggested his HST-15s instead.

    I told him I had no experience with them and before I would entertain the idea of telling the customer to switch subs I needed to get my hands on one to test.

    So I drove to his place in North Carolina, tuned one of his cars (another Sound Quality car) and blew him away with what is half of my normal tuning process. He had issues with his processor so I didn't get to spend the time on the car that I wanted. I was limited to one day to drive there, tune, and drive back.

    I brought a HST-15 back with me. Did some testing and found that would it not only stomp on the 13W7s, it would do it in a smaller enclosure and have more output in the lower frequencies.

    For a 4th order enclosure that means going from four 13W7s to four HST-15s would save me about 18ft^3.

    So I was convinced. Told the customer that this was the better route. The HSTs sound better than the W7s, perform better, and do it all in a smaller enclosure than the 13W7s. It's an all-around win.

    And then Nick mentioned that he could build some 24s for the van that would work in 3ft^3 sealed.

    "I need to play with some 24s then."

    So we worked out a deal for me to build/install a complete stealth system in his 2006 Audi A4 Avant. Completely hidden system. Full stock appearance aside from the Kenwood head unit.

    Fortunately, that's exactly what I like doing.

    The build plan was to use one of his new SQL-15 in 2ft^3 sealed in the spare tire well of the wagon, his M25 tweeters, M3 midranges, and TM65 MKIII midbasses.

    Power would be coming from some prototype Sundown SALT multi-channel amplifiers. Class D full range units.

    Here's the two 24s he dropped off for me. HS-24s. 290lbs of subs here.

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    And here's the patient. Good looking Audi wagon.

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    The first thing I did was get to work removing this hump in the spare tire well. Under the car this is where the evap canister is housed. That will get relocated so it was removed.

    Then it was time to drill out the spot weld, cut through the seam sealer, and pop this big hump out so I could weld in a plate and get started on the sealed enclosure for the SQL-15.
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    So after getting that hump out and seeing the huge area in that spare tire well I immediately started measuring to see if I could get a 24 in the car and still make it stealth. I went the the Stereo Integrity website, pulled up the IB-24 specs, looked at the mounting depth, and knew this shit was going to work.

    For those that don't know, IB means infinite baffle. Basically the sub has no enclosure. You mount it to separate front wave from back wave, send power to it, and let it eat.

    With an infinite baffle install if you have the doors and hatch open you'll have no output to speak of. Once you close the hatch the cabin space of the vehicle effectively becomes the enclosure and you get ridiculous amounts of low end output on very little power. There's one thing that's king with an infinite baffle install, and that's cone area. A 24" sub has gobs of cone area.

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    So I called Nick the next morning, told him that I couldn't put his 15 in the car like he asked because it deserves an IB-24. I could hear him smiling through the rest of our conversation.

    I went over how this would still be a stealth install and the 24 will fit and work properly.

    The very idea of a stealth 24" sub install is unheard of. We HAD to do it.

    So...simple decision. Fuck that 15. 24 it is.

    I told Nick to get started building the sub for the car and to clear coat the motor (it's either nickel/chrome plated) to protect it from corrosion due to humidity, and send it to me.

    In the meantime I worked with one of the HS-24s to make a template for the mounting baffle to go in the car.

    But first I had to remove stuff that was in my way and would need to removed anyway.Click image for larger version
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    The first step to getting this 24 in was to make a template of what will be the mounting baffle. I figured the best thing to do would be to make the baffle from either 3/16" or 1/4" steel plate.

    I grabbed some scrap whiteboard I had here and got to work making the baffle template. I didn't get it perfect (working through a migraine), but I did get it dialed in when I transferred the migraine attempt to a 2nd piece of whiteboard before I went to the metal shop.

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    I also trimmed the factory plastic trim panel to sit flush on the template. That means it'll sit flush on the new steel baffle once I get it made.

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    I then made a ring from MDF that is the footprint of the 24s basket and used that to locate the subwoofer cutout fore/aft in the car. It's centered on the car left/right, obviously.

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    Then it was time to hit the local metal shop.

    The guy there was intrigued by what I was doing and wanted to challenge himself laying out the complex outer perimeter of the baffle.

    He drew it up and we hit the CNC plasma.

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    I also had him cut a trim ring that I may or may not use. Was the same cost to have him do it so I figured it was a win if I was able to use it.

    His attempt to copy the template didn't work so I had him cut the hole for sub mounting and go outside of the footprint for the baffle on the plasma.

    After the plasma was done we put the template on the steel, traced it, and cut what we could on the shear.

    At that point I came home and did the detail work with my handheld plasma cutter.Click image for larger version
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      The initial cuts with the handheld plasma. I swear I had this fucking steel plate in and out of the car at least 30 times getting the fit right. Plasma, grind, fit, plasma, grind...
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      One of the first fits to the car. Also dropped the additional ring I had him cut into the spare tire well. May weld that in to open up the opening in the bottom.
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      Factory (OK, they're not factory, they're what the previous owner made for his suitcase sub) floor panels in place.
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      Nearest panel removed with the 24 sitting in the baffle.
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      Another view. The awesomeness of this big bastard being hidden is way f'n cool.
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        Man i missed reading these threads. Glad ya made it over.


          I have no idea of what half of this means, you are filling the bottom hole in?


            Needs more triangulation.

            This takes me back to my 20s, in the '80s.

            I had a 1kW stereo system.

            Looking good.
            μολὼν λαβέ
            '94 Ford Bronco XL / Sky's ORD 6"lift / Sterling 10.25 dually / HMMWV tires / Bilstein shocks


              This is awesome. You have any more details on the build? What head unit amps ect? How much power does it take to fire that thing? In Your infinite baffle setup is the bottom of the sub open to the atmosphere or is it sealed in the car but separate from the interior?


                Originally posted by Crimsen View Post
                Man i missed reading these threads. Glad ya made it over.
                This. I have no desire to have a car for DB, but your talent and attention to detail is inspiring.


                  i missed reading these threads, glad you came over here. Checked out your youtube channel and subd last night.


                    Will you be putting an aperiodic membrane at the bottom to reduce outside weather on the back of the sub?

                    Awesome build, please keep them coming.

                    I'd love it you would tune my car, I have done it myself but I'm sure I do shit tuning as compared to you.

                    I run a Dayton 4x8, 3 seperate amps, in a Lexus


                      That's badass. I have 4 of Nick's ht18 subs in 12hz LLT enclosures in my theater room. I can't imagine what a 24 in a car would be like in comparison.
                      Last edited by swtfman; 05-24-2020, 07:00 AM. Reason: spelling


                        I've got a powered sub behind the seat of my truck.

                        Love to hear that 24 coming down the road.


                          Holy hell that is massive. Nice work as usual


                            My bil owns THE high end shop in our area. It always blows my mind what some people will spend on thier stereo.
                            I'm assuming that is a made to order sub? What does it cost? with this type of sub that doesn't need an enclosure, does it only sound good on the deep slower stuff or will it hit fast like a smaller sub?

                            I remember back when I had 2 flea market 12s in the 2bigbronco and that it was the shit.


                              Is OK Boomer, an appropriate response?