I recently published an Instructable for an open back guitar/bass cabinet (https://www.instructables.com/id/An-Open-Back-Cabinet-for-Guitar-and-Bass/), and it has received 200 views overnight! (thanks!). I thought while I'm at it, I should describe my other cabinet that is used for guitar/bass, and also for testing electronic music circuits. This cabinet features cylindrical enclosures and 2 speakers rather than one. The top is a sealed system with a 6" driver, and the bottom is a ported system with an 8" driver. The idea was stemmed from 2 totally unrelated things. First was talking with a friend who built a "kicker" for his car from a cardboard Sonotube (those concrete forms found at any hardware store). Second was doing some testing on Sonotubes for use as a telescope tube. The idea was straight forward--fiberglass the tubes for strength, cap the ends with 1/2" plywood, and install some drivers and wiring. The fiberglass finish is rather rough, and after a bit of sanding, I decided to kind of level it out with 80grit sandpaper, and cover it in some tie dye fabric (maybe that's another Instructable?)
I'll be honest--the project made a real mess! ANY project involving fiberglass is a messy undertaking. Before you do anything--get some old beat up clothes, some disposable latex gloves, and several layers of cardboard under your work space. Take it from me--sanding globs of epoxy or polyester resin off of the cement is no fun.
The enclosures work great. They are not as "room filling" as the open backed cabinet, but have a real tight, focused sound. If I was to do it again, I would have gone with an 8" and 10", or even a 10" and 12". Do some reading and suit the enclosure to your needs. 6", in my opinion, is too small for really good guitar reproduction, and not of much use for bass. For my electronic gizmos, it's perfect.
Step 1: Design Considerations
For the open back cabinet, the volume enclosed by the box is not real critical. Jim Marshall claimed that building a Marshall Stack was really a matter of cramming enough power handling into a box. It's really just a matter of the forward and reverse sound waves not mixing very much.
In the system here, the 6" cabinet was carefully sealed (and is air tight), while the bottom 8" has a port. Both the port and enclosed volumes are important considerations. I will not endorse a volume calculator, but a search for "volume of sealed speaker cabinet" and "ported speaker cabinet" will give many results. Also, in my case the information was available from the manufacturer.
Recall that you can calculate the volume of any regular solid by multiplying the area by the length.
The area of a circle is A=pi*r^2, where pi=3.14,and r is the radius. For example, the top enclosure has an 8" diameter (r=4"). So the Area=3.14*16=50.25 sq.in.
The inside length is 14". So AxL=50.25x14=703 cubic inches.=0.41 cubic feet. This was specified as the upper number for this particular speaker. The bottom enclosure was designed the same way.
The bottom enclosure also contains a port. It's possible in ported systems for the port to generate noise--an annoying "wooshing" sound as air is driven out and sucked in. This can be removed by using a port that widens as it goes to the exit. The sketch shows the one used here--a trapazoid shaped port that tapers from 2.75" to 3.5" at the opening. It was made from thin card stock, held together with glue (paper reinforced corners) and finally covered with fiberglass cloth and epoxy.
Step 2: Materials
The main body of the enclosures are made from cardboard concrete forming tubes. Being cylindrical, they are surprisingly strong and really cheap. They come in a large number of sizes, from 4" up to 24". I selected 8" Sonotube for the 6" speaker, and 10" Sonotube for the 8" driver.
The tubes were fiberglassed for strength. I used West system epoxy (available from several places online) which is a great product. It's easy to sand, and best of all, it has very little odor (I was working indoors in the winter). There is no reason not to use standard auto-store polyester resin. It has an intense smell (the "amine blush"), but is really strong as well. I used regular hardware store cloth.
The end caps were made of 1/2" plywood. You will probably notice that the Sonotubes have been warped a little bit in shipping. Because of this, you can't just trace the inside of the tube onto the plywood. I used an actual compass set to the radius of the tubes, and first cut the piece from cardboard to make sure it would fit. Be careful with the cutting (I used a jigsaw--a circle cutting router jig would be better). You want it to be a snug fit.
The feet were made of 1x2 pine which were screwed to "cradles" made out of small sections of sonotube. It seems to be holding up fine.
Being sealed, the speaker are not accessible after the enclosures are assembled. To couple the speakers to the outside, I used 3/16 brass bolts. I soldered heavy copper wires to these, which could be soldered on the speaker wires inside the enclosure, and to the speakers themselves. Watch to polarity of the speakers--if you install one of them backwards, they will be out of phase.
Finally, the enclosures here were covered in cotton fabric. I used a product called Modpodge. It's a kind of water based glue/varnish used for decoupage. It is a really fun product, and perfect for something like this. Finish however you'd like. Fiberglass can be topcoated with more resin, and sanded out for painting, or covered in fabric or anything you're like. For some reason, tie dye seemed like a cool choice for this project!
Step 3: The Build
I started with the dirtiest step--glassing the tubes. I measured back from one end the required length (determined by the required volume), adding about 2" to account for the thickness of the caps, and the volume taken up by the cone itself. Mark around the tube measuring every couple inches. I left the tube full length at this point. To make glassing easier, I inserted a 2x4 through the tube, and lifted both ends on chairs. This lets you rotate the tube as you work, without the tube touching the ground. I worked by mixing some resin (the slowest you can find--this, if I remember right, was 30 minute). I would pour some on the top (on the cardboard), the lay the cloth on the resin, and with a squeegee, I would work out the bubbles and slack. You have enough resin if the glass is basically clear. You want it looking "wet" but are trying to avoid drips. By rotating the tube, and applying glass, and squeegeeing out the excess, you can do a pretty nice job. As I mentioned, I used West system epoxy (the stuff they use on boats and aircraft), but regular polyester resin would be fine. It does smell and should not be used indoors. Let the resin harden for a couple of days if possible. In theory, 24 hours should be enough, but it really takes a bit of time to reach full strength and all the gumminess is gone. If it's not fully hardened, you will be gunking up sandpaper and making a mess. After curing for a couple days, I cut the tubes off using a hand miter saw. I wrapped the tubes in masking tape, following the marks that were placed on the tube before glassing. it's a bit tedious, but I carefully went around with the saw, just cutting through the glass, then cutting all the way through the cardboard. The front can be cleaned up by using a sanding block and sanding through the glass until you reach the cardboard. With this method, the front endcaps go in last. To make sure they don't fall into the tube, and to ensure a solid fit for the front, some stops were installed in the tubes 1.5" behind the opening. these were made by cutting a 1" slice off the the Sonotubes, and removing about 1/4" from it (so the slice can fit inside of the tube). these were glued in place with epoxy, using spring clamps to secure them. If a port is to be installed, now is the time to cut the hole for it. The port can be installed with more glass and resin. I glassed mine from the inside, sanded the outside flush, and then glass the outside too.
Cardboard doesn't sand to a nice finish. You can get a good, smooth edge by sanding it fairly smooth. Then, using some cheap bulk superglue, go around the edge. Give it an hour to dry, and sand it again. the hardened up fibers sand out great.
Then, 2 8" diameter and 2 12" diameter plywood end plugs were cut. The front caps should be built by cutting the inside diameter first (make a template to dial in the exact dimension of the speaker), then the outer dimension. The other end cap is simply cut out. Go around them with an orbital sander or sanding block to remove splinters.
I used 1.25" 3/16" brass machine screws to act as the electrical lugs. I soldered some heavy copper wire to them (20A wire, about 2mm). Holes were drilled to accept the bolts. The rear end caps were painted (here painted, then sanded). They were installed using epoxy glue. To ensure an airtight fit, I ran a bead of epoxy glue along the entire inside corner. This strengthens the joint and seals it against air leaks.
Sound Deadening: One apparent problem with cylindrical speaker enclosures are standing waves. It is a good idea to line the inside with some kind sound scattering/absorbing materials. On the inside of the rear end caps, I glued a 2x2 section of paper egg cartons. the paper is acoustically dead, and the shape scatters sound waves. The insides of the tubes were lined with those paper drink holders from coffee shops (again, irregular sound scattering shape). I also glued strips of cotton fabric across the diameter of the tubes.
Covering: Tie Dyes were made using Walmart variety dye on thin cotton. They were cut to length and width, and applied to the tubes using Modpodge. The Modpodge was applied to the tube with a sponge brush. The fabric was then layed down, and the bubbles and wrinkles worked out (iron the cotton first or it will be a mess). Then a second coat of Modpodge was laid down. After drying, it was sanded with 120 grit, wiped clean, and covered a 3rd coat. The third coat was allowed to dry, and sanded out with 120, and 220. Finally, it was topcoated with 2 coats of poly. After drying, the fabric is cut square to the tube by sanding through it at an angle. It really comes out clean if you are careful.
The top and bottom tubes were bolted together. Careful measurement and drilling will let you line up2 or 3 holes. I made mine with the port sticking out one side, and was careful to align the seam on the fabric so it would point down (and not be seen). To strengthen the assembly, 2 mending plates were screwed into the rear end caps.
Feet were build by fiberglassing 3x6" piece of the 12" sonotube, and cutting it out. this formed a kind of cradle that the tube would sit in. The cradle was painted, and then glued/screwed onto 1' lengths of 1x2. 3 holes were drilled through the bottom Sonotube. The feet were installed onto the bottom tube using epoxy glue, and then the holes were filled with epoxy glue from the inside.
The front flanges are drilled to the proper bolt pattern for the speaker flanges. I then epoxy glued 3/16"x1" bolts into the holes. Make sure there is no glue on the threads. Then the front flanges are glues into place. To ensure they were very strong, I filled any gaps with small drops of super glue.
Wiring: Mark the outside end caps with the polarity of the wires. Then, cut thick speaker wire and solder it to the lugs on the brass bolts. It should be the length of the tube, plus 8". now, install the bolt and seal them off. I used epoxy on the inside. I bolted them from the outside, and used wingnuts to hold the external wires in place. Finally, solder the other end of the wire to the speakers.
Make sure to use either a gasket, or silicone caulk when you mount the speakers. I didn't do this the first time, and was amazed at the amount of buzzing coming from the sealed cabinet! a thin bead is all you need here.
That about covers the basic construction. I'm very happy with the setup. As a test rig for components, and for use with my 22W amp pedal, it's perfect.
Step 4: Parting Thoughs
For this build, I needed 8ohms impedance (what my amp pedal needs). Some of my other electronic gizmos are built for 4ohm. So I decided to do a 2-speaker system. Both drivers are 4ohm, so wired in series, I have 8ohms. The speakers are rated at 15W and 25W. This should allow 40W of power handling without damage.
The project was fun. Some of the things could be improved. Next time, I think I would build a kind of double cradle to hold the two tubes together. The bolts to mount the speakers could maybe be thought out better (threaded rods glued into place? Tee nuts?). Other than that, fiberglassed Sonotube is surprisingly strong. With the end caps in place, I stood on them without caving them in. It's a very strong shape, and with the end caps in place, they are really rigid.
Have fun--if someone builds it or has an upgrade idea or different application, I would love to hear it!