Tesla Model 3 Stereo - Part 7: Subwoofer Installation

Previously, Part 6: Speaker Installation

Update 2019-05-31:
        A summary of this entire project is available in Part 9. This page covers the initial investigation and Page 9 has the final results, so if anything on this page disagrees with anything on Page 9 then Page 9 is the one you should follow because I don't plan to come back and correct my mistakes in Parts 1-8.


Subwoofer Installation

        I'm really glad to be done with this part. I hate working with fiberglass but it was the only way to go in this instance. And now I finally have a complete audio system. I achieved most of my design goals and the final result sounds glorious.

        I wanted an invisible or close-to-invisible installation. You could simply put a box in the well underneath the trunk, but I already have my three amplifiers and two DSPs there and I wanted to keep space to store my charger, roadside emergency stuff, etc. So I planned to make a fiberglass box to fit into the OEM subwoofer location. I'm my old Honda Civic, molding a box to the shape of the trunk was really easy to do. In the Model 3 it's a bit more difficult. The OEM subwoofer box is contorted to fit around the right rear wheel and matching that shape just wasn't going to be possible.

        In my previous car I simply stripped down the trunk, coated the body metal in masking tape, and painted fiberglass directly onto that to make a mold for my fiberglass box. The Tesla's trunk geometry is a bit more irregular, so I started out by making a cardboard mold instead.

        Unfortunately it's impossible to make full use of the available space, simply because there are many protruding shapes and you have to twist the box into place around all of these. This cardboard box gave me at least half an inch of clearance from the car body, several inches in some places, so I covered this box in masking tape and applied the first layer of fiberglass. After it dries, you can remove and discard the cardboard and you have the first, innermost layer of your box.

(Don't use the white masking tape seen here. Use 3M Original Blue Painter's Tape. It's much easier to separate from the cured fiberglass. It took hours to get all this tape off.)

        I didn't want to make the complete back side of the box and then add on a front. The structure will be stronger if it's made as a single continuous part, with each layer covering the entire box. So after making the back using the carboard mold, I made an MDF ring with insert nuts to mount the subwoofer to, mounted that in the box, and added the first layer of fiberglass to the front. I also added in tabs to mount the box to the car and internal ribs to strengthen the large, flat portions of the box. Then I wrapped several more layers of fiberglass, until the box was ~3/16-inch-thick all around.

        After some fitting, adjusting, and sanding, the box fits in place pretty nicely. There's less than a quarter inch of free space between the box and the car body in some places. I secured it with three bolts, two of which are used for the OEM sub and one of which I drilled myself in a welding flange on the right rear wheel well.

(data provided by Focal)

        I was aiming for 1.08 cubic feet, which would give me a nice response down below 40Hz, according to Focal. (The sub itself is 0.08 cubic feet) I ended up getting a little bit less than that, probably 0.8-0.9 cubic feet, but that doesn't change the result much. I thought about measuring the box by filling it with water, but no.

        Originally I hoped I could install the sub behind the existing trunk liner and just cut a window through, but unfortunately that just wasn't possible. It sticks out about an inch too far. I was concerned that a simple circular cover attached to the trunk liner wouldn't provide much impact protection, so I used some of my remaining fiberglass to make a larger cover that fits the shape of the trunk liner over a larger area. The carpet isn't an exact color match, but I don't plan on spending much time in the trunk so I don't care.

        The end result is outstanding. The JL 10w7 in my previous car was great, but the Focal Utopia 10WM is unbelievable. It blends seamlessly with my Utopia woofers up front, directional ambiguity of the bass notes is much better than with my previous systems, and the bass has a clarity and precision across its full range that I didn't realize was possible in a car. Bass drums are just as clean and clear and natural-sounding as snares and hi-hats. I am incredibly happy with this subwoofer.

        That's all there is on this subject. This was one of the easier and more straightforward parts of the project, aside from the mess of working with fiberglass. The system is complete, though not finished, and it sounds glorious. I've made a lot of changes to almost everything I've posted here, and I'm going to sum all that up in another post, hopefully sometime soon. I'm also going to write a reverse engineering post with all the information I've accumulated about the car's power and control systems for anyone who wants to do their own build, stereo or other.