Subwoofer enclosure designed and built by Besundale for the SoundDomain.com Box Building Contest

Introduction

Building a subwoofer enclosure for a boat presented some very unique challenges.  As with any box, the most important thing is the sound.  I wanted good SPL with decent SQ.  Because of the fact that a boat is an open air environment, I needed to somehow figure out what kind of enclosure would yield the best results without the benefit of cabin gain.  Size was also a big consideration.  Normally you would just go with several large drivers and a huge box.  While it's true that I have the entire bow (front area) to work with, due to state laws I had to leave room for six life jackets and a fire extinguisher.  I also had to save space for a wakeboard and a kneeboard.  All of that stuff gets tossed under the front deck by my sisters and dad.  To make matters worse, that stuff is usually wet.  In other words, the box must be as waterproof as possible, space efficient and still fill an open air environment with good solid bass.  Yes, it was quite a challenge and quite a learning opportunity.

 

The vehicle:  1989 Correct Craft Ski Nautique.  We use this boat at least 2-3 times per week during the summer.  We had a band pass box from Best Buy with a 12" Audiobahn 1200Q subwoofer in it but it took up too much space and only sounded okay so we took it out and have been tuneless ever since.  That's not good.
 
 
This is a picture of the area under the bow of the boat.   As you can see, a wedged shaped box would fit the best and be most space efficient.

  I designed this bandpass enclosure for two Rockford Fosgate PCH-88’s. The net volumes of the three chambers are as follows: chamber 1 = .73 ft^3 tuned to 40Hz, chamber 2 = .80 ft^3 tuned to 80Hz, and chamber 3 = .39 ft^3. The ports are all 2" in diameter; the smaller chamber has one port with a length of 5.75", and the larger chamber has two ports with lengths of 1.9". I used WinIsd to calculate ports volumes. For our particular application, we don’t need a low tuning because we listen to strictly rock and metal music, there won’t be any rap or techno going on in our boat. Most 4th order boxes have a sealed chamber and a ported chamber, but mine is different because two speakers that play out of phase with each other share the sealed chamber. This will hopefully cause the frequency response to be far less peaky and make the sound a little deeper. This is all very much experimental on my part, so if it doesn’t suit my tastes, then I’ll simply try again. The plan on the left is the original design. As I was building I canged it from one straight baffle and one angled baffle to both straight baffles. I orignally wanted to be able to clam shell two sets of two 8's but finally decided that it just wasn't going to work with the dimensions that I had to work with.
This is a drawing of what I hoped the box would look like when it was done.  Notice the port locations.  After some further thought and design I decided to switch the tuning of the ported chambers.  I didn't like the idea of having one port closer to the front driver than the other.  Now the back chamber has the two ports while the front has just one.

 
The whole project required 1-4'x8' sheet of 3/4" MDF.  There's also some real pine, fiberglass cloth and resin, all stainless steel hardware, a sheet of polycarbonate, some 2" PVC pipe, lots of liquid nail, lots of drywall screws, bondo, 12 spray cans of automotive paint, primer and clear coat, 1 can of  brush on satin black oil base paint, 1-8 terminal jack plate, 3' of speaker sealant, 8' of speaker box gasket material, 3' of 12 gauge wire, 1 set of Mad Lights, 8' of 3/4" pinstriping, 8' of 1/4" pinstriping and about 6 square feet of custom vinyl lettering.
 
 
Some of the angles got pretty steep.  I used a table saw for all of my cuts except for the speaker mounting holes in the baffles/braces.  To cut these angles, I tilted the blade as far as it could go and then used blocks under the piece that I was cutting.  By doing this I was able to angle the MDF as much as I wanted.  It wasn't easy, but it worked.
   
 
This is the front of the box where the 2 steep angles meet.  Originally it came to a sharp point.  I didn't like the idea of having a sharp point anywhere near our boat so I cut it off and rounded the tip.
 
Here you can start to see the basic shape of the box along with a driveway that is filling up with sawdust.

 

 
This photo shows one of the two baffle/braces.  I predrilled holes for the t-nuts because I figured it would be impossible once the baffle/braces were installed.  You can also see the liquid nail in this photo.  I went through 2 tubes.  Every joining surface has a bead like you see in the photo.  Every joint also is screwed with drywall screws every 3".
 
This picture shows the process of drilling, countersinking and then screwing the panels together.  I used a 18 volt cordless Black and Decker drill.  The nice thing about this drill is that you can have a drill bit and a screwdriver bit on it at the same time.  You switch back and forth simply by pulling the chuck off.  It's still nice to have a second drill for countersinking, but this $60.00 tool saves a lot of time.  
  Here I have four of the five sides together, holes filled with wood filler and sanded smooth.  I also rounded over every corner with a router and sanded the entire box as smooth as possible.
This photo shows the most complicated part of the entire project.  I needed to be able to get at the drivers for potential maintenance so I put in a window.  I didn't trust screwing into MDF without also using Liquid Nail so I built a pine frame for the window.  This also let me use a thicker window than I otherwise could.  That should help with keeping it very stiff.  More on that later.  
  Here's the top with the pine window frame all ready to be drilled, countersunk, Liquid Nailed and screwed down.  Notice the baffles/braces are lower than the frame.  That was by design so that I could add pine to the top of them as well.  That helps in giving the wood screws something to really bite into rather than just the MDF.
   
 
Here you can see the polycarbonate window getting a trial fit before attaching the top of the box.  While it's more expensive and exotic than plexi-glass, I chose to go with polycarbonate because of it's strength.  It's a very strong and very hard material that hopefully will work out well.  I never heard of anyone using it before, but since this box is experimental in just about every other way, I figured why not!  The polycarbonate is 3/8" thick and due to it's incredible clarity should show off the drivers and Mad Lights really well.
 
 
Here's a photo just before starting to fiberglass the box.  I didn't want to just cover it with resin.  I wanted to go the full route and use the cloth and resin both because of the gain in strength.  Keep in mind, this box is going in the front of a boat.  Boats get bounced around pretty hard sometimes so strength and waterproofing are very important.
 
If you look close, you can see that I accidentally caught a fly.  It's on the upper right hand side of the pic. You can also see a few air bubbles.  Those had to be all sanded out and refilled.  Big job.
Here's the box all glassed with one pass of sanding and Bondo.  Notice that I even wrapped the window frame and glassed that as well.  Little did I know then, just how much work that would add later on.  That area had to be perfectly smooth in order to properly seal the window. Sure the fiberglass added strength and made it so the wood screws would get an even better bite, but it was a ton of work getting it smoothed.  A Dremel came in very handy.
 
Here I am during the sanding phase.  This was just after spraying the entire box with gray primer and then sanding it off.  Notice you can see some red paint...  That's from testing the smoothness.  I wasn't satisfied at that point so I sanded it completely smooth and did it again.  This was the longest and most tedious phase of the entire project.  I used Plasti-Kote automotive primer and paint for everything.  It's kind of expensive, but works very well for sanding and smoothing.  I went through 4 cans of primer and many hours of wet sanding before I was happy.
 
Here's the box after the first few coats of paint.  I sprayed 5 coats of white first.  After the white was dry I taped off the areas that were to stay white and then sprayed 4 coats of red.  Each coat had to be wet sanded with 600 grit paper to ensure a nice finish. Once the color was on, I sprayed an addional 5 coats of clear. With all that paint, the finish should hold up, even with all the abuse that it will have to live through.
 
Here's a pretty good photo of the front port.  All three of the ports were hand made from 2" PVC pipe.  I used epoxy to glue them in and then routed the top of each one to get a nice flared opening.  After that they were Bondo'd and sanded to a perfectly smooth finish.   This picture was taken after the second coat of red.  I like the way they blend into the box without looking obtrusive.
 
This picture shows the back ports as well as the cut-out for the control panel (more on that later).  I tried spray painting the inside of the box, but as you can see, it didn't work very well.  Because of the confined space inside, it built up air pressure and the paint would just come up and out of the box rather than sticking to the sides.  So, I bought a can of brush on oil based paint satin black and brush painted it.  That was a messy job.
 
This picture shows the inside of the box under the window frame.  I drilled holes for the speaker wires and Mad Lights ribbon cables.  After running the wires, I filled the holes with Bondo, sanded the areas smooth and then painted them satin black as well.
 
Here's a picture of the Mad Lights.  Mad Lights were designed by an electronics company here is Wisconsin.  They were built for computer cases.  Mad Lights are made up of 4 small circuit boards with 3 extremely bright and intense LED's each. They can be set to change color and put on quite a light show for your computer case.  After calling the company, I found that they run on 12 volts and should work in my application.  So, I busted open my computer and took them out. The center chamber houses 2 sets of lights while the outer 2 chamber has 1 each.

Click here for a QuickTime Video 47 sec. 1.7 MB

 
This photo shows the stainless steel speaker bolts and t-nuts. The bolts have allen heads for added looks and ease of use. Also, you can see the polycarbonate without the protective paper. At this point it's all drilled, the edges are all sanded smooth and polished and it's ready to go in. The polycarbonate is 3/8" thick and extremely strong.
 
    This shows one of the two Rockford Fosgate Punch Subs. I'm adding speaker sealent in this photo. The speaker sealant is very sticky and pretty easy to work with. If you look close you can see the old and very thin built in gaskets. I didn't trust that they would do the job so I added the sealant.
 
  This shows the rear chamber with both the speaker wires and Mad Lights wires.  
This is a close up of the gasket I used to seal the polycarbonate. It's sticky on one side and specifically made for subwoofer enclosures.
At this point, I'm closing in on completion.  I have access to a vinyl lettering machine so I made my own lettering.  I used the exact same font and style as the lettering on the boat.  Above the "E" you can see the power rating for the Rockford subs.  I put the lettering in that position to give the idea of boat registration numbers. 
This shows the speaker cup that I converted to a control panel. You have to look close, but you can see the gasket that seals the cup to the box.
  Here's the box on the boat. My goal was to make a box that sounded good and looked like the boat. At this point I didn't hear it yet, but as you can see it definately looks like the boat.  
This is a photo from the other side.
This is view from the top looking down. You see the number of screws and decorative washers that I used to secure the polycarbonate. There is one every three inches.
Here's a photo of the Bass Nautique in it's final spot under the boat's bow. It shows the control panel. I took out two of the speaker terminals and added the push buttons for the Mad Lights. I also converted two of the terminals into positive and nagetive "12 volts in" jacks.
A view from the left side under the bow.
A view from the right side under the bow. This was before I wired everything up.
Conclusion

The sound this box produces is just about perfect for the music I’m interested in. The kick drums and bass guitar sound very tight, and for only 200 watts, it has a decent amount of SPL. There is a major drop off around 40Hz. This was expected and I personally don’t have much of a problem with it. Just for fun, I put the box on the backseat of my car to test if I succeeded in designing it for more of an open-air environment or if cabin gain would have helped the overall SPL. I found that as soon as I opened the doors of my car, the sub got a lot lower and louder. At this point I'm thinking I might open it back up, reverse the driver that's firing into the lower tuned chamber and wire both drivers in phase. That will get me a gain in chamber volume and should lower the tuning somewhat. I'm not sure if I'll like it that way, but it would be fun to experiment. I put the box into the boat to make sure it fit correctly and worked as designed. Everything came out as expected. The fit is perfect and the sound it very good. Possibly it could go a little lower, but for now I'll stick with it. The Mad Lights are a cool touch. I won’t be able to find out until next spring, but as an added bonus the box should increase the size of the wake that the boat produces due to the extra 65 pounds in the bow. Think Summer!