Subwoofer Box building contest entry by Jim85IROC

Challenge:
This subwoofer enclosure was designed as part of a budget oriented stereo system for my girlfriend’s 1992 Toyota Camry. In addition to the constraints set forth by the contest, I had additional constraints put in place by my girlfriend. She had a $600 budget, she wanted good sound quality, and she didn’t want to lose any more trunk space than she had to. In addition to all of that, I wanted to challenge myself by working with some new materials, and by coming up with a design that would allow me to quickly and easily change the tuning frequency of the enclosure.
Idea:
Because I needed a small enclosure that sounded good with pop music (for her) and rock/country music (for me), and because I had a tight budget, I decided to try a pair of Tangband W8-740C woofers in a ported enclosure. When I bought them they cost $35 each (they have since increased to $50 each), and had a great reputation for sounding good and getting loud. After plotting many different alignments in WinISD Beta, a .5 cubic foot enclosure tuned to 35hz was chosen. Since my design called for the subs to be mounted in an inverted method, driver displacement was not factored into the overall internal volume. The added displacement of the concave cone area is minimal due to the small cone and large dust cap.

This, when coupled with the vehicle’s cabin gain, will provide an in-car response that rises smoothly as frequency decreases. This type of response with an exaggerated bottom end is perfect for overcoming road noise while driving. Typically systems that are tuned for a flat response sound very thin and lifeless when listened to while driving. Simply cranking the bass to compensate doesn’t usually result in a natural sound. This alignment, while a bit bass heavy when doing critical listening while parked, is ideal during real-world conditions, especially for the pop, rock, and country music that will be played through it. It has the added benefit of extracting considerably more output from these small woofers than a sealed enclosure would provide.

Planning:
Now that my drivers and my alignment were decided on, I needed to design an enclosure that would provide adequate air space, the proper port length, and the ability to tune the enclosure easily incase I was not happy with the in-car response. On top of this, I also wanted to incorporate some new materials, and have a finished product that really stood out from the typical trunk boxes.

I spent a considerable amount of time trying to decide how to fit such a long port into this enclosure. A 3” round port would need to be over 28” long. Fitting that inside of a box that only needs .5 cubic feet, would be difficult and impractical. I considered putting the ports outside the enclosure, but couldn’t decide on a method that would be aesthetically pleasing. Eventually I concluded that a slot-port would be the most practical choice. After a lot of sketches, I arrived at my final design. This design takes advantage of the least useful space within the trunk (directly below the rear deck), provides a great location for mounting the amplifier, and also gives me the ability to change the tuning frequency of both enclosures simply by replacing one piece of wood (TOP 4). The port length was determined using methods that fzaba has outlined here: http://forum.sounddomain.com/forum/ultimatebb.cgi?ubb=get_topic;f=11;t=060743. End Correction was calculated, and plugged back into WinISD. Frequency response varied by less than +/- 0.5 dB, so it was not factored into the final design. Since I intend to tune this by ear anyway, the calculated port length simply serves as a good starting point. If it were necessary for this system to extract maximum SPL from the enclosure, it may have been more worthwhile to include. The slot port has the added benefit of helping to brace the enclosure. Since this enclosure is so small, no additional bracing is necessary.

Materials:
.75” MDF – 1 sheet
.75” Acrylic (Plexiglass) – 3 pieces, 12” x 12” each.
.125” Mirrored Acrylic – 1 piece, 36 x 12”
2 yards of stretchable vinyl – 1 yard gray, 1 yard graphite
2 yards of black fleece
1 yard of graphite carpet
20 5MM red LEDs
#8 1 5/8” drywall screws
6-32 stainless machine screws & nuts
10 x 3/4 stainless screws
#8 Washers
Loctite wood glue (PVA)
Clear silicone
3M Super spray adhesive (PN: 08088)& Super 77
Masking Tape
24AWG 2 conductor wire
Flat black spray paint

The enclosure is made from .75” MDF (Medium Density Fiberboard) that was purchased from a local hardware store. Personally I feel that the quality of MDF can vary, and I find that typically, the darker the color, the heavier and more dense the material is. The MDF from home Depot has a lighter yellow or beechwood color. The sheet that I used has a darker, tan color. I find that the higher quality MDF cuts smoother, but also puts more strain on my tools.

Tools:
Craftsman table saw with fine tooth carbide blade
Black & Decker Circular saw with carbide blade
Black & Decker Jig saw
Riobi plunge router & associated router bits
Home made circle jig for router
Framing square
Pipe clamps
C-clamps
Craftsman heat gun
Butane Torch (my idea of a hot glue gun)
Razor blade knife
Cardboard
Dremmel & various bits
Electric grinder

Preface:
First off, here is a picture of me, in the garage, holding my screen name. Although my screen name doesn't show up in every picture, this will at least show that all construction pictures were taken in the garage that I'm standing in.

The order in which this is presented is not exactly the same order in which I built the enclosure. I did many things in parallel to take advantage of down time while glue was drying or while I was waiting for additional materials. Since posting it in the order I did things would result in a hard to follow, fairly useless presentation, I've reorganized it for the sake of clarity. All relevant steps are here, and the procedure is grouped so that anybody can follow it and recreate this project if they desire.

Every attempt has been made to be thorough and complete with this description and pictures. The information here should be adequate for anybody to recreate this design in a similar car, or to incorporate certain design elements into their own project.

Anyway, on with the show...

Enclosure Assembly:
The first step was to visualize the pieces on the uncut sheet of MDF. I didn't bother marking them all out, because taking blade thickness of each cut into account, combined with the inevitable screw-up here and there makes it virtually impossible to be that accurate. I just took a mental picture of where they belong so that I could smartly cut the sheet with minimal waste. The framing square was used to verify that the MDF was square, and to draw cutting lines. I then cut along the lines with the circular saw, to provide manageable sized pieces that could then be cut on the table saw. When planning these cuts, try to put all pieces together that have the same dimensions. This will allow for more efficient cutting on the table saw, and more importantly, will provide consistent cuts from piece to piece. It’s not as important to have pieces that are exactly 7” wide as it is to have every piece be the same width. Taking your time here will make all the difference during assembly. Your glue will hold tighter because you’ve got two flat surfaces, and your panels will all line up much better, providing a better air-tight seal. If this is done well, the joints will be air tight even before you apply the silicone.

Once all of the pieces were cut, assembly began. I pre-fit each piece to ensure that all of the cuts were correct and that something didn’t wind up a hair too long or too short. I used a 3/8" roundover bit on my router to round the edges of the wood that was used to make up the port. This was done to minimize turbulence and port noise. Once I was satisfied with the fit, I put the glue on the edge of one piece, then used the pipe clamps to hold them together while I drilled then screwed. I do not countersink when I drill. I’ve found that it makes more work, especially if you countersink too far. When that happens, you don’t have as much wood to distribute the screw pressure through, and you wind up with a massive hole that you need to fill. If you use #8 sheetrock screws instead of #6, and you ensure that you’re not too close to the edge of the wood, the screw will easily pull itself flush without stripping the hole or puckering the edge. The result is a stronger grip than if you countersunk, or tried to use a #6 screw. If you're covering in carpet, this should be sufficient for a smooth finish when you're done. If you're covering in vinyl, you will still need to smooth out the area with filler, but it will be easier than if you've got the giant countersink hole to deal with. If all you have are #6 screws and for some reason you’re unable to buy the #8s, then I would highly suggest that you DO countersink, otherwise you’ll strip the hole trying to get the #6 to pull itself flush.

Keep in mind that ultimately, it’s the glue that holds this enclosure together and gives it it’s strength, not the screws. BUT, you need to use enough screws to maintain a high and even pressure between the pieces in order for the glue to adhere properly. I typically like to spread the screws 3-6 inches apart.

Once I got all of the framework glued and screwed together, it was time to attach the outer panels. At this preliminary stage, they were attached with minimal screws and no glue. In my design, it is necessary to remove them in order to continue working on the enclosure.

Once the framework was completed, I cut out the woofer holes. To do this properly, you need to have a circle jig, or have a perfectly round template that you can trace with the router. Since I had neither, I made my own circle jig.

The circle is cut by first drilling a small hole in the center of the circle. The adjustable slider on my circle jig has a screw that will sit in the hole, and allow me to rotate router around the center of the cirle, providing a complete cut all the way around. This needs to be done 3 times for each hole, because in order to get the best cut possible, no more than .25" should be cut on each pass.

Once the hole was cut, I used a 1/4” roundover bit with the router to round the edge. Then a rabbet bit was used to cut a 1/2” rabbet on the backside for the woofer to slide into. This was done so that the woofer wouldn’t appear to be “behind” the wood, but instead would appear to be almost flush with it. The rabbeted hole, along with a mounted woofer, can be seen in the following pic:

Because the cosmetic gasket on the Tangband woofer was so thick, the woofer still appeared to be “sunken in”. I decided to trim the gasket off with a sharp razor blade. The modified driver can be seen here:

Once this was done, I was very happy with the appearance of the woofer in the enclosure:

Here is the progress up to this point:

The next step was to take some cardboard and begin coming up with a rough shape for the amp hole. I traced the amp onto the cardboard, cut it out, then carefully modified it until I was happy with the fit. This was transferred to the wood, then cut out with the skill saw. I made sure to cut it a hair small, then hand sand my way to the proper fit. A dremmel was used to shape the notches for the heat sinks, as well as clearances for the cast pieces on the sides of the amp. This can be seen here:

After the picture was taken, it was sanded a bit more to provide clearance for the vinyl that will cover this panel.

The next step was to figure out how the amplifier would be held in place. I decided to make two MDF holders, and cut them approximately 1/4” thinner than they needed to be, in order to prevent rattles from having the amp too loose.

Here is a picture of the enclosure mounted in the trunk of the car:

This is the picture from inside the car with the seats down:

Next I decided to take a break from breathing all of that MDF dust so that I could breathe some spray adhesive instead. (DISCLAIMER: Wear proper respiratory protection.) I decided to cover the inside of the enclosure in vinyl. The first piece to be done was the board that the woofers and amp mount to. The board and the vinyl were laid out, and sprayed with the 3M spray adhesive:

After the glue got tacky, the vinyl was pressed onto one end of the board, then worked smooth while pulling it tight until I got to the other end.

The vinyl was cut at the woofer and amp openings and stretched around the back, then trimmed. The final piece can be seen here:

Now it was time to vinyl the inside of the enclosure. The same basic procedure was followed to adhere the vinyl, but care was taken in the corners so that the vinyl didn’t pull away. In this pic, you can also see a temporary brace installed so that pieces TOP 2 and TOP 3 had support at both ends.

Here is a picture with the vinyl work completed inside the enclosure:

Next I cut the mirrored plexiglass for the bottom of the enclosure. I put tape where I was going to cut so that the plexiglass didn’t chip. I used the table saw with the fine tooth carbide blade all the way up. The plexi was glued to the bottom of the enclosure with the spray adhesive. The pieces fit snugly, and the glue was used just to ensure that the pieces would not rattle.

The next step was to make the other panel that will complete the enclosure, and be visible from the trunk. A cardboard template was made that fit the trunk location, and then transferred to the MDF. The MDF was cut a bit large, then cut & sanded down to the perfect size. Due to the contours in the trunk, it was necessary to sand the back side of the board.

The next task was to cut the 3/4” plexiglass down to size. In order to protect the surface of the plexiglass, I covered both sides with masking tape. The cutting was done on the table saw with a very fine toothed carbide blade. In order to get the best possible cut, the blade was raised as high as possible. Extreme caution needs to be taken when cutting something with the blade like this. Fortunately, I still have all of my fingers. Cutting plexiglass this thick is not easy, and you should make sure that you have extra to practice with. I found this out the hard way and had to buy one new piece. Since I wanted rounded corners on the plexiglass, after they were cut to size, I cut an MDF template with the radius that I wanted. I set it up and used a flush trim bit on my router to duplicate the radius on the three pieces of plexiglass.

The plexiglass can be seen in the following picture with the radiused edges. You can also see my name appear for the first time as proof that this is indeed my project:

After the plexiglass was radiused, I used my rabbet bit to notch a “shelf” into the plexiglass. This was done in two stages, because cutting plexiglass with a router can be very, very difficult if not done properly. I set the rabbet bit up to cut in 1/4”, then went back and did the full 1/2” cut. This was done on all 3 pieces.

In order to cut an exact hole in the board for the plexiglass, it was necessary to make a template first. The template was made by following the directions found here: http://www.exquisiteaudio.ca/baffle.html. The template was also used to locate the mounting holes in the plexiglass. The template is shown below:

The template was used to locate the mounting holes, which were then drilled on a drill press. After the holes were drilled, the pieces were sanded and painted black:

The next step was to mark out the locations for the plexiglass windows on the finish piece.

The template was traced onto the board, then cut 1/8” small with the skill saw. The template was re-attached, and the board was flipped over. The router was used with the flush-trim bit to cut the hole identical to the template:

The holes that were drilled into the template were then transferred to the wood. The template was removed, and the rabbet bit was used to cut a shelf around the hole.

This rabbet, combined with the rabbet on the plexiglass, allowed for the plexiglass to sit perfectly flush in the hole:

It was then necessary to modify the template for the center piece of Plexiglass, because that piece is slightly wider than the outer two. The template was cut right down the center, and the appropriately sized block was added at the center. I then went though the same procedure for the center piece as I did with the two outer ones.

The next step was to wrap this outer board in vinyl. Spray adhesive was sprayed on the panel, and on the vinyl. Once it tacked up, the vinyl was pressed on to the board, starting at one end, and working to the other end while stretching and smoothing. The vinyl was cut in the plexi holes, and pulled around with additional adhesive. The corners were VERY difficult to work with, because the plexi didn’t want to stretch enough. It was necessary to heat and stretch the vinyl multiple times. The 3 plexi windows were then installed backwards to hold the vinyl in place because the corners were being very stubborn and didn’t want to stick. This can be seen here:

Here’s the picture after the glue dried and the plexi windows were pressed into place:

The plexiglass windows were screwed to the MDF with the 10 x 3/4 stainless screws. The holes were pre drilled in the MDF so that it would not split. Washers were used on the screws to distribute the force over a larger part of the plexiglass to make sure it did not crack. No sealant was used. This plexi fit so tight that it was almost impossible to install it. The vinyl can compress and works as a very good gasket.

The next step was to attach the board with the plexi windows to the enclosure. Because the piece already had the vinyl on it, it wouldn’t look too good if I used screws to hold it. Instead, I glued it up, then put over 300lbs of weight on the piece by filling two trash barrels with water. This provided more than enough force to ensure that the glue held.

The next step was to wire up the LEDs. This had to be done before the panel with the woofers & amp was attached. There is 1 LED glued in each of the 4 corners of the woofer enclosure, and 2 strips of 3 LEDs each in the amp enclosure. Small holes were drilled through the enclosure for the wires. The LEDs and wires are positioned with hot glue, then held in place with 2-part epoxy. Here you can see the two LED strips, as well as a longer one that I wound up not using:

14 LEDs were used inside the enclosure. Four LEDs were used in each subwoofer enclosure, and 6 in the amp area. 6 more will be installed to illuminate the trunk. In the future I hope to also put a few LEDs behind the amplifier so that the heat sinks will glow red. Each of the two sets of four LEDs will have a 270 ohm resistor in series with them, and each of the sets of six LEDs will have a 100 ohm resistor in series. All of the LEDs will be powered from a line tapped into the original trunk light, which will be removed. Final LED wiring is shown at the bottom of the page.

Once the LEDs were installed, the protective coating was pulled off of the mirrored plexi, and the plate with the subs & amp could finally be installed. Wood glue was not used because this piece is covered in vinyl. Clear silicone was sparingly applied to the box, then the plate was put into place. Since I had installed a few screws way back at the beginning, I was able to realign the piece perfectly. Additional screw holes were drilled and the screws were installed. The combination of the vinyl and silicone provided a perfectly air tight seal. Although the plate with the plexiglass was perfectly flat and glued well, I chose to put some silicone around the outside where I could reach as an extra precaution. This silicone, along with both the front and rear panel can be seen here:

So, what does the whole enclosure look like? Here it is on the work bench with the protective coatings removed from the clear plexiglass:

Here is the enclosure, installed in the trunk:

LED Wiring:
The last part of the main enclosure is the wiring and installation of the LEDs within the enclosure. As mentioned above, I installed 4 LEDs in each of the subwoofer enclosures, and 6 in the amp enclosure. The installation of the LEDs is covered in the Enclosure Assembly section above.

Wiring the LEDs was not complicated, but it was time consuming. Most of the wiring was done with solid-core 24 AWG wire from Radio Shack. If I were to do this again, I would absolutely NOT use solid core wire. You only get 1 or 2 bends out of it before it breaks. Make sure you find some stranded-core wire for this part of the project if you intend to do this.

In addition to the LEDs in the enclosure, I also added 6 LEDs to the trunk area to illuminate the trunk when open.

Since the LEDs will be destroyed if you don't limit the current flow through them, it is necessary to add a resistor to the string of LEDs. There are a number of websites out there that will do these calculations for you if you're not able to. The schematic that I used is shown to the right. I apologize for making it look so complicated, but these pictures are so small that too much detail would have been lost if I laid it out in a manner that's easier to follow. The wiring is simple. There are 4 paralleled groups. Each paralleled group has 4 or 6 LEDs and one resistor in series. In an attempt to keep the schematic as simple as possible, generic "lamp" symbols were used to represent the LEDs instead of the actual LED symbols, which most people probably would not recognize.

When wiring LEDs, remember that the annode is the positive side. The annode will be the longer of the two LED pins. The cathode is the negative, and is the shorter pin. If you wire an LED backwards, you will not damage it, but it will not illuminate. When wiring the LEDs in series, it is essential to keep track of the annodes and cathodes, because if you wire one backwards, none of them will light up.

The power source for the LEDs comes from the 12 volt feed going to the factory trunk light. This light comes on when the trunk is opened. Wire-taps were used, and the OEM light was removed.

Overall I'm happy with how the LED lighting looks, but unfortunately it is not as bright as I had hoped. In retrospect, it would have been a better option to use more LEDs or to use neon.

Here are the pictures. Unfortunately, I'm not a good enough photographer to get accurate pictures in such low light conditions. In the sub chambers, the LED light looks like it's very bright in 2 spots, and dim everywhere else. In person, the lighting is much more even.

Enclosure Cover Assembly:
Next are the pictures of the cover piece. The objective of this piece is to provide a stock appearance to passers-by, as well as protect the stereo during day-to-day use. Having a bunch of stuff in the trunk would take its toll on the plexi and the vinyl in no time. The plan was to use 1/2" MDF, with carpet on the visible side, and black fleece on the back side to prevent scratching when it's up against the plexi.

I built this by using the same template I used for the panel with the plexi windows. I cut it a hair large, and trimmed it to fit with the grinder. Here is a pic of the piece in the car:

Although I wanted this to maintain a mostly stock appearance, I also wanted to dress it up a little bit. I decided to incorporate the Toyota logo into the panel. I downloaded a picture of the Toyota logo, then printed it and blew it up on a copy machine to the desired size. I cut it out with a razor, and set it onto a piece of 1/4" thick scrap wood. I taped it into place, and used some spray paint to transfer the logo to the wood.

When I painted the logo outline onto the wood, I also painted along the edges of the cover board. This was done so that if any gap developed between the carpet and the fleece backing, it wouldn't be as obvious. Since this is going to be removed and reinstalled periodically, it's likely to suffer some wear and tear. In the picture, you can also see the 1/4" wood with the paper stencil removed.

Here is a picture of the Toyota logo that has been cut from the 1/4" wood. Since I was in a hurry (We needed the car for a weekend road trip), I used spray adhesive instead of wood glue to attach the logo to the panel. This worked very well, and since it only has to support its own weight, I don't think I'll have any problems in the future.

Once the logo was in place, the board and the back side of the carpet were both sprayed with adhesive and allowed to tack up. Once tacky, I worked the carpet into the crevaces around the logo, starting at the center and working my way out. Once the logo was carpeted, the carpet was pulled flat and pressed into place over the rest of the board.

Once the carpet was glued into place, it was trimmed flush along the back, and the black fleece was glued in. Working with the fleece was not easy, and in retrospect, trimming it with scissors instead of a razor blade would probably have provided neater results. The fleece isn't visible when the piece is installed, but I still would have preferred a cleaner appearance. Once installed, I ran a heat gun around the edges of the fleece. This melted the edges enough to give a smooth, uniform look, and should also prevent fraying.

Here is the finished piece. Getting the Toyota logo to show up in pictures proved very challenging. When the camera flash is used, it washes out the detail of the raised piece, so some of these pictures have some very unnatural lighting, or are fuzzy due to no camera flash.

The final piece came out very nice. It looks very much at home in the trunk, and to the casual onlooker it would probably go completely unnoticed. The embossed Toyota logo adds a tasteful customization without drawing unwanted attention. Most importantly, it protects the stereo behind it.

Subjective Listening
I listened to this enclosure both during and after completion. While I was building it, I wanted to ensure that I was happy with the sound before I got too far involved in the cosmetics. Once the subwoofers and amp were mounted, I built a temporary plate without the plexiglass viewing windows so that I could tune the system and listen. Overall I am very impressed both with the enclosure and the woofers themselves. These little 8s in the enclosure tuned to 35hz could easily fool you into thinking you were listening to a pair of 12s. They play fairly deep, and have a strong output at those deep frequencies. Their total SPL isn't going to compare to a good pair of 12s, but they get a lot louder than I thought 8s could get, especially considering that they're only being powered by two channels of a 4x50 amplifier. Best of all, the sound is clean and precise. This setup is very musical, and sounds very natural with rock and country music. With the powerful bass lines of pop & hip hop, the subs really provide authority and remains clean even at higher volume levels. When you stop to think that you're listening to $35 woofers, it really becomes quite impressive. I was happy enough with the sound that so far I haven't fooled around with the tuning frequency of the enclosure yet. Maybe over the winter if I get a little free time I'll cut up a couple boards to raise and lower the tuning and see how I like it.

UPDATE: After listening to this stereo for over a month, overall I am still very happy. However, after recently listening to some songs with very deep bass lines, I've found that this system is a bit lacking in the lowest octave. Thanks to my box design, I can very easily modify my tuning frequency. I may fool around with a lower tuning frequency, but WinISD modeling indicates that this may actually make it worse. It could be that I'm simply expecting too much from such small woofers.

UPDATE 2: It looks like my "no deep bass" issue was more of an amp-tuning problem. When re-installing the amp the other night, I mush have accidentally bumped the gain control knob. Re-setting the gains brought the system back to life, and that deep throbbing bass that I'm used to getting from a pair of sealed 12s is stronger than ever. :)

Conclusion
This box building project was a lot of fun. This was the first time that I built slot-ported enclosure. I learned a lot about using plexiglass, and I also learned a few new things about wood. Best of all, it got me out of the house, which is always good for lowering bloodpressure and maintaining sanity. Overall I'm very happy with how the box looks and sounds, and I'm satisfied that my end product looks so similar to how I envisioned it all along. The enclosure is very solid with no resonance. Although no internal bracing was used, the enclosure is small enough that none was necessary. The 3/4" plexiglass windows are much thicker than necessary for a low powered 8" woofer install, so they are equally as dead. I do have some port noise at higher volumes, but with the back seat up, it masks most of the sound. Since this is a low powered system that won't be boomed very often, it wasn't worth taking up extra space with larger ports just to eliminate a little bit of noise.