Cylindrical Speaker Cabinet for Guitar Amplifier (Wooden Cylinder)

C = πd

Θ = π(n-2)/2n

s = C/n = πd/n

Pi comes in handy for building things that involve circles. The equations above use pi to determine how we can cut a bunch of boards and glue them into a cylinder. Although this instructable is for building a speaker cabinet, a wooden cylinder can be used for many other applications: a planter, a stool, an end table, or whatever. Just tweak the dimensions and follow the relevant steps.

The main difference between this speaker cabinet and a traditional rectangular cabinet is that it projects sound in all directions.

Before starting, eat a slab of this savory pie; it's brain food!

This cylinder will be a regular polygon with 20 sides, which is called an icosagon. In fact, the more sides the polygon has, the closer it gets to being a true circle. Check out this excellent tutorial on estimating pi using polygons. I'd like to give a special thanks to Python Everyday because I used their code to generate my cylinder graphic.

Speaker cabinet design is a complicated science and I'm only marginally qualified to talk about it. Since the cylinder may seem like an acoustically difficult shape for a speaker cabinet, I explain my strategy and risk mitigations in a dedicated step a little later. That section is optional, but if you plan to change the dimensions or use the speaker cabinet for an instrument with a different frequency range, you might find it interesting or helpful. I included a sound clip of my Bugera T5 playing through the speaker cabinet.

Check out the materials list and let's get started.

TOOLS:

A router that can plunge

Router bits:

• 1/4" straight bit
• Inlay bit kit like this for making the inset top
• Flush trim plunge bearing bit like this for making decorative holes around the top

Table Saw

Miter Saw

Orbital Sander

Drill

Soldering Iron (or some way to crimp or connect wires from the connector to the speaker)

Parchment paper, freezer paper, or wax paper (to protect the working surface from excess glue)

Nylon rope, around 50 feet

Cable ties, 6 inches or longer

Diagonal cutters (for removing the cable ties)

Scrap wood, including a dozen or so smallish sticks, 12-18 inches long, to use for various construction tasks

Painter's tape

2'x2' - 1/4" thick MDF for making the circular inset router template

MATERIALS:

• 4 boards: 6 foot 1x6 pine
• 12" square of plywood for speaker baffle, 1/2" thick
• 12" square of pretty wood for the top. I used a 1/4" thick piece of bird's eye maple (1/2" might be better)
• 1/4" TS mono connector like shown (has the long body)
• 8" speaker (search for a guitar amp speaker that supports the right frequencies and power)
• 4 screws and matching T-nuts for mounting the speaker to the baffle

It's easy to design and build a beautiful speaker cabinet that sounds terrible. I've done it myself multiple times. As the speaker vibrates, sound waves bounce around and resonate within the cabinet. And since different frequencies have different wave lengths, sounds will bounce and resonate in ways that are difficult to predict.

The bouncing and resonating of sound waves within the cabinet become a problem if the resonance at a particular frequency is (1) too powerful and (2) within the audible frequency range.

The perfect speaker cabinet would reproduce all frequencies with even power. However, every speaker cabinet is susceptible to resonance at some frequency or frequencies, which are called harmonic frequencies. A thoughtful design will avoid having harmonic frequencies in the target frequency range of the cabinet, or limit how powerful the harmonic resonance points are.

Cylinders, or tubes, are used in physics classes to perform standing wave experiments. A standing wave occurs when a frequency is projected into the tube and tuned until the wave length is a multiple of the tube length. The relationship between the sound wave length and the tube length is well known and easy math can be used to determine what the harmonic frequencies are. This relationship can be used to avoid or identify bad harmonics in the cylindrical speaker cabinet.

Whether the tube has an open or closed end determines whether it is more susceptible to even or odd harmonics. This cylindrical cabinet design is closed, so only the odd harmonics are considered.

The odd harmonic frequencies are given by:

f = nv/4L, where n = 1, 3, 5, ..., and v = speed of sound (1125 feet per sec), and L = length of the tube from the bottom to the speaker baffle (1.666 feet).

n=1; f1 = 169 Hz ---> close to E3 and F3

n=3; f3 = 506 Hz ---> close to B4

n=5; f5 = 844 Hz ---> close to Ab5

n=7, f7 = 1182 Hz ---> close to D6 (the highest note on my electric guitar)

Typically, the first harmonic given is the most powerful, with each subsequent harmonic decreasing in power. The frequency range of a typical electric guitar is roughly 82 Hz to 1300 Hz.

So, the first four harmonic frequencies are within the range of a typical electric guitar.

Since this design does not avoid the target frequency range, the design aims to minimize the impact of these harmonics.

I planned several design mitigations to reduce the impact of the harmonic frequencies:

1. Install the speaker off center with the baffle angled so that it would not be perfectly centered and aligned for generating standing waves.
2. If the previous trick didn't work, I would try filling the tube with some type of acoustic foam.
3. If the previous trick didn't work, I would try inserting a 2nd baffle in the cylinder to partition the cabinet into two separate volumes with a strategic open area joining the two internal volumes.
4. If the previous trick didn't work, I would remove the closed bottom and cut semicircles around the bottom of the cylinder so that the cylinder would function as an open tube rather than a closed tube.

I only needed the first trick: the angled and off-center speaker baffle. The increased power levels at the harmonic frequencies are detectable if you are trying find them, but they aren't noticeable during normal playing. I believe that if the cabinet had a larger air volume the harmonic resonance would be stronger. So, if the cabinet had been taller or had a larger diameter, I might have needed to employ some of the additional tricks.

Also worth noting is that, in contrast to a general-purpose stereo speaker, it is acceptable for an electric guitar speaker cabinet to color the sound. In fact, many digital sound processors for instruments provide different types of cabinet simulations to shape the sound.

If you made it this far, reward yourself with a piece of this delicious Key Lime pie!

I appreciate any feedback, including corrections and suggestions to improve my understanding of speaker cabinet design.

20 boards must be cut with the profile shown in the pictures. Before cutting, we need to know the side length, s, and the interior angles, Θ.

C is the circumference of the cylinder, which will be about 20 times the side length, s, of each board. The diameter, d, of the cylinder is 11.5 inches.

The side length, s, is π times 11.5 divided by 20, which is 1.8 inches.

The interior angle, Θ, is π times 18/40, which is 1.414 radians. Converting that to degrees we get

Θ = 81 degrees

s = 1.8 inches

Board length is 27 inches

Use the table saw to cut both edges of the 20 boards to 81 degrees such that the widest part of each board is 1.8 inches. Chop these boards to a length of 27 inches.

The next few steps will be rather physical, so consider getting some protein with this hearty Sheppard's Pie.

There is no easy way to glue up a few boards at a time and have the resulting cylinder be truly circular. The strategy employed here is to have everything in position, all supplies ready, and a few helping hands on standby. Then, in the next step, the glue will be spread and the boards will be rolled up and "clamped" in one fluid monumental motion.

So do the following preparation steps:

1. Cover a table or working space with wax paper or equivalent.
2. Cut five (5) pieces of rope, each 5 feet long, and tie each one into a loop.

• These loops will be used as clamps by being placed over the cylinder, then a stick will be inserted in the loop and twisted to squeeze the boards together, and a cable tie will be used to secure the sticks after twisting the ropes tight. The pictures will show the technique.

1. Unroll several strips of painter's tape STICKY SIDE UP. The tape will help hold the board together during roll up.
2. Have the following items open and within reach:

• Glue, preferably multiple containers so that two or three people can be spreading glue at once
• The rope loops
• The sticks for twisting the rope
• The cable ties
• The diagonal cutters (because once the cylinder is set in place, you may remove a cable tie to perform additional tightening if gaps are found)
• Paper towels or damp cloths for removing excess glue

1. Lay the 20 boards along the painter's tape.

Before jumping into the roll up, prepare your mind by meditating on this acrostic poem which was written using the Pi/Pie Poetry instructable:

Collect your tools and parts and wood

You're about to build something really good

Live in the moment, which is about to get busy

In the next step, you'll probably get dizzy

Nice and neat and perfectly round

Dream of the cabinet and how it will sound

Enjoy each step, eating pie along the way

Rock all night, and party every day

Review the pictures to prepare yourself for this step. Once the glue has been spread on the boards you have to keep going until the cylinder is well clamped.

With the help of your volunteer(s), spread glue on all the angled edges of the boards.

Next, with you at one end of the boards and a helper at the other end, begin rolling up the boards from both ends, trying to meet in the middle. Then, set the emerging cylinder up on its end.

Glue will be everywhere and some boards might fall out. Don't panic. Once the cylinder is on its end, just reinsert the boards and reposition as necessary to form the cylinder.

Once the cylinder is stable, place a rope loop around the center of the cylinder, put a stick in it and twist until there is some tension on the rope, but not too much. Use a cable tie to secure the stick so that it won't unwind. Get all five loops in position, spread evenly along the length of the cylinder, and tightened a little.

Once all five ropes are holding the cylinder together, look over the cylinder and make any adjustment to boards to improve alignment.

Finally, processing one rope at a time, cut the cable tie while holding the stick, then continue to tighten the rope until all gaps in the boards are closed. Do this for all five ropes.

Wipe up any excess glue inside and around the cylinder. It is easier to remove now before it has dried.

Take a well-deserved break and thank your helpers by treating them to some delicious Frangipani Apricot Blueberry Pie!

After the glue has dried, probably 24 hours at least, remove the ropes. Use an orbital sander to remove the excess glue and smooth over the edges.

Inlaying a piece of wood into another piece of wood used to seem like a time consuming nightmare. However, using a router bit kit that is made for that application takes the magic out of it.

The instructions that come with this kit explain how to do it better than I can. Plus, there are videos that explain it pretty well. The big idea is that you make one template, and then use the SAME template to cut the piece that will be inlaid AND to route out the wood where the inlay will occur.

First, I glued up pieces for the top. I used 1/4" bird's eye maple. In retrospect, 1/2" might have been a better choice. Once I made the cylinder, it occurred to me that it is very natural to want to sit on it. I feel like the 1/4" top is too thin to sit on. However, I added some supports to it to make it stronger. If I had used 1/2" wood for the top, it would not need additional support.

Next, I made a template for the recessed route: a circle with a diameter of 11.25 inches so that top will rest inside the top edge of the cylinder.

Next, I used the template to cut the bird's eye maple top.

In order to use the template to route around the top edge of the cylinder, I needed a way to support the template so that it wouldn't move and would support the router as I moved it around the edge.

To provide support for the template, I used rope loops to "clamp" six sticks around the cylinder with their tops flush to the top of the cylinder. Then, I put a drop of glue on the top of each stick (not on the cylinder), aligned the template on the cylinder and placed hand weights on it to glue it down. This secured the template enough to support the router.

Don't glue the top in the cylinder yet. First, the speaker baffle must be installed.

Now you are ready to go to the next step. Or, if you complete this awesome instructable, you can ask J.A.R.V.I.S. to complete the project for you!

The top 5-6 inches of the cylinder need holes through which the sound can escape. The speaker will be mounted near the top of the cylinder facing up. The sound will emanate through the sound holes in all directions.

Feel free to take some artistic liberty with the sound holes. I used an alternating 'T' shape, where I circled the cylinder once with the template right-side-up, and once with the template upside-down. But there are many hole shapes that would look nice:

ꓕTꓕTꓕT

•••  

Δ Δ Δ Δ  

׀׀׀׀׀

⃝

\\\\\\

////

<<<<<

Once you pick the shape, cut your template out of a 12-14 inch piece of 2x4. Then, cut a semi-circle chunk out of the template so that it can be tightened flush against the curve of the cylinder. Finally, route two 1/4" channels along the top and bottom edges of the template (and drill holes at the end of each channel for the rope to pass through) so that the ropes will be recessed when they are tightened. This allows the router base to move smoothly against the flat face of the template.

On the opposite side of the cylinder from the template, where the ropes will be tightened, I added a piece of scrap wood to protect the cylinder. Since pine is soft, tightening the rope can create indentations in the wood right where the rope is being wound.

Now, count how many times your pattern will be repeated around the cylinder and make evenly distributed marks around the cylinder so that you'll know exactly where to tighten the template for each route.

To perform these routes, I sat with the cylinder between my legs, leaning away from me against a step stool. I placed hand weights on the step stool to prevent it from sliding away from me during the route. Then, I plunge routed one hole at a time, loosened the template, slid it over to the next mark, tightened the template, routed the next hole, etc.

If you're a little dizzy after all that routing, use this wild looking Crazy Vegetable Pie to re-calibrate your eyes and your stomach!

The speaker baffle is a 1/2" or thicker piece of wood mounted inside the cylinder that will have the 8" speaker mounted to it.

Since the speaker baffle will be mounted at an angle, it will have an elliptical shape. If I were better at using TinkerCAD (which I'm working on), I could model the elliptical cross section of the cylinder and get the dimensions of it. However, what I did instead is cut a circle with a 12.5" diameter, and began trimming and sanding it until it fit well inside the cylinder at an angle.

On the lower side of the baffle, cut the 8" hole for the speaker. Check your speaker's mounting holes. Sometimes you need a slightly smaller hole than the speaker size. Once the speaker hole is cut, mark and cut the mounting holes for the speaker.

Once the baffle is ready to be permanently mounted, use a small wood screw to secure the bottom (lower part) of the baffle, apply glue around the baffle, insert the baffle, and use another small wood screw at the top of the baffle to squeeze it in snugly to the inside of the cylinder. Make sure that the highest point of the baffle is below the decorative sound holes.

Perform a test mount of the speaker to make sure you can install it and remove from the open bottom end of the cylinder, as the top will be inaccessible after the top is glued in. The test mount will also pull the T-nuts into the baffle where they should remain permanently.

To mount the connector, drill the hole to get the main body of the connector through the side of the cylinder. Then, drill a slightly larger inset so the the connector will be flush with the outside of the cylinder.

If you feel baffled by this step, then check out this wild derivation of pi, which is also rather baffling.

With the baffle in place and the mounting holes for the speaker all ready, paint the top of the baffle black, as well as the inside of the upper cylinder that will be visible through the sound holes. Also paint the underside of the top black. This makes the inside of the speaker cabinet look like a black abyss. It makes the inside of the cylinder and the speaker baffle pretty much invisible, while highlighting the shape of the sound holes.

Next apply glue around the recessed upper edge of the cylinder and the edge of the top. Insert the top into the cylinder. Set some hand weights on it to secure it while the glue dries. Wipe away the excess glue with a damp cloth.

After the glue has dried, use the orbital sander to make the top and the edge of the cylinder truly flush.

I was disappointed that my inlaid top had some gaps, so I got some saw dust from my bird's eye maple, mixed it with hide glue, then pressed the mixture into the gaps with a rubber spatula. After it dried, I used the orbital sander to smooth it out. It looked much better.

With the money you'll save making this cabinet, you'll need this fried pie purse to store your extra cash!

Remove the speaker and connector. Inspect the cylinder for rough spots and sand it all smooth.

I built a fixture to hold the cylinder off the ground so that I could access the entire cylinder during the finishing process.

I applied a few coats of tinted shellac.

If you make it this far before realizing that you don't need a speaker cabinet, you still have something well suited for holding a pie. It's similar to, but less attractive than, this beautiful pie stand!

Install the speaker. Attach a pair of wires to the connector. Mount the connector in its hole.

The connector has two terminals: Tip and Sleeve. The Tip terminal should be connected to the speaker's positive terminal. The Sleeve terminal should be connected to the speaker's ground or negative terminal.

Hook it up!

After wailing a few guitar solos, have some Whale Pie!

As always, feedback, suggestions, and corrections are welcome :)