Cajon Sound Hole on the Back vs Downward Facing Sound Hole

Facebook Cajon Forum member Peter Wolf had some good observations about the downward facing sound hole: “… the sound depends on the floor, where you play. If there is concrete, the sound will be hard and loud, if there is wood, it will be softer and not so loud, an if you play on a thick carpet, there will not be very much sound. And if you rock with your cajon, the sound changes also.” Here is my edited response:

I agree that the design is not perfect. The floor does affect the sound. But a regular cajon has a sound hole facing backwards, away from the audience. If the situation is perfect the wall behind reflects the bass back to the audience and player. But if there is no wall? Or if it is metal or sheet-rock or brick, it will affect the sound. So both styles have sound hole placement issues. I play the bottom facing cajon mostly on a carpeted floor and prefer the sound vs on a wooden floor. I haven’t tried these on a cement floor or on a super-thick carpet. The thin carpet I use them on does not effect the sound much. I consider the floor issue as just a different variation of the same issue that a rear-facing sound hole has.

The other thing you mention, the effect of the player tilting backwards, I now consider to be a desirable feature. You can ‘tune’ these cajons by selecting the feet height that gives you the desired amount of bass. If you ‘tune’ it so they have just a little less bass when they are flat on the floor, you can add a little punch by angling the cajon backward as you play.

A big advantage of this design is the easy placement of the bass mic, it simply goes on the floor in front of the cajon, if the floor is wood I put a small piece of foam under it. Tilting the cajon has minimal effect on mic pickup and slight movements of the cajon have minimal effect on the mic. Whereas a cajon with a sound hole on the back has to have a mic stand of some sort and small movements in the cajon can effect or even hit the microphone.

With a mic on a backwards facing sound hole the player is mostly unaware of the effect of their movement on the mic placement. Dealing with rear sound hole issues has spawned a number of pricey, cajon specific mic’ing systems. With the downward facing sound hole you can always quickly see where the microphone is sitting. Any microphone will do and heavy bass microphones sit stable on a small piece of foam.

Perhaps the biggest issue I have with the downward facing sound hole is that it works too well, much of the sound is channeled forward. The cajon player doesn’t hear the full richness of the cajon. It’s easy to test this by simply leaning forward a little as you play, the tone and bass becomes more obvious. Sometimes I’ll sit directly in front of a low window or a solid door to reflect the sound back as I practice. This is a common issue with instruments that project their sound forward, trumpets, trombones etc. When evaluating the sound of any cajon it is best to also listen to someone else play the cajon.

 

Cajon Plans – Downward Facing Sound Hole and Inclined Tapa

How to Build A Cajon

This article describes the latest construction design and details of the Firebox Cajon ‘Open Hearth’ design.  It uses technical drawings, photos and text to help guide you through the process of building a cajon. Please see the related blog for a discussion on the features, pros and cons of this design.

This video has good audio recordings of two cajons with similar design. The main difference in sound seems to be due to the type of woods used.

Download the complete set of plans here

The plans are full of construction tips. Use the photos below to help understand the drawings and construction tips. The plans present two variations in the design: Option A uses long ‘bottom support’ pieces, Option B uses short ‘bass bar support’ pieces. The plans are free for your use but not for redistribution, re-purpose and not to be used commercially.

Assembled Parts

This design has an inclined front which makes construction a bit more difficult than a typical rectangular cajon. The side panels are based on ‘right trapezoids’ not rectangles.  However, because the pieces create a large rectangular sound hole, there is no need to cut and shape a large circular sound hole as with traditional cajon designs.

View of downward facing sound hole on a firebox cajon
Option A: cajon with downward facing sound hole using ‘bottom supports’. This cajon is a mix of Western red cedar and Sitka spruce
Option B: Short ‘bass bar support’ in place. This cajon is made from solid Sitka spruce.

The plans use 1/2 plywood for the cajon box. Many commercially produced cajons use plywood from hardwood trees, either birch or tropical hardwoods.  Many cajon makers use solid hardwood. Cajon builders usually recommend plywood with more plys and fewer in-sheet gaps. Marine plywoods can be used. Cabinet grade plywood produces a nice looking cajon. Specialty plywood stores can supply multi-ply hardwood plywood. I have also used 3/4 inch softwood plywood with good results, although the cajons are heavy.

Side Panels

The plans produce a cajon which is about 17 inches tall, which is short for some folks. Modify the plans to suit your height. I never cut all pieces at once. I start by cutting the right trapezoid shape to the height I want, then fit everything else to suit the sides. Cut the small notch at the bottom front after the bottom support/bass bar support is glued in place.

Left and right side panels with bottom support glued on.

It is important to get the cut on the front as continuous and clean as possible. You don’t want to have to sand the front surfaces. The tapa must seal well to maximize the bass, tone and volume. I typically make the top and bottom cuts, then the front cut, and finally the back cut.

Once  the sides are cut, measure the front angle.

Top Panel

Typically I’ll cut the top to the desired width, then cut the angled front.  When the front cut is perfect, I’ll make the back cut on the top such that the top fits the top of the side panels perfectly.  The front angle of the top panel must match the angle of the front of the side panels.

Bottom Supports

I prefer Option A, long bottom supports. I think they are faster to make and add strength. I glue them on before cutting the angled front surface.  This gives a cleaner joint. Option B, bass bar supports produces a bigger sound hole. I’ve not made enough cajons with these options to have an opinion on which produces ‘better’ sound.

Cutting the glued side panel and bottom support.
The dowels are added after the bottom support is glued on. The screw for a rubber foot will go between them.
Bass bar support with an additional thin strip of wood to reinforce the end grain. Not necessary if you are using plywood sides.

Bass Bar

The bass bar holds the bottom of the tapa and is the principle support element spanning the front. I use a 2.25 inch wide piece but it is worth experimenting with wider or even narrower pieces.  This piece is often used to carry the cajon so I typically round the edges in the back which is not shown in the drawing.  The front of the bass bar must be cut at the same angle as the front of the sides to ensure a tight fit of the tapa.

Bass bar with rounded back edges in place on bottom supports.

Bottom Panel

The simplest bottom panel is a rectangle.  The plans include a bottom panel with a cut out but I’ve not yet made one this way. I’ll update this when I can.  I round out the edges of where appropriate which is not shown in the plans.

The bottom panel is the lowest wooden piece on the cajon. View is from the back to the front. The bass bar spans the front of the cajon and sits on top of the bottom supports.

Rubber Feet

The back feet and front feet are different sizes. The back feet should be low. The front must be high enough to level the cajon. There seems to be some complex interactions between the width of the bottom panel, the width of the sound hole and the height of the feet. A small difference in the height of the feet can make a big difference in the sound of the cajon. In general too high is better than too low. I try different feet to get the best sound.  Although the plans suggest 0.75 to 1 inch I find myself using 1 inch most of the time. Rubber feet can be screwed to a scrap piece of wood and sanded to desired height on a bench sander.

Glue-up

The bottom supports are glued to the sides first and the dowels put in. When this is dry all pieces are glued. First the bass bar support is glued and clamped to the two sides and immediately after the top and bottom are glued and clamped to the sides.

Gluing the bottom supports (Option A) to the side panels.
Gluing the bass bar. This cajon uses bass bar supports, Option B.
I believe in at least 4 clamps on each side. But not too tight!

Corner braces are glued in after the box is dry.

Gluing triangular shaped corner braces between the top panel and the side.
Gluing corner brace between the bottom panel and the side. Only necessary if bass supports are used.

The Back

Here is an earlier blog about attaching the back.

I use 0.093 inch thick Lexan for most of my cajon back panels which I attach with 2 part epoxy and screws. If I use wood I am now inclined to use thicker dimensions for the back. The original plans suggest 4mm but I’ve updated them to 0.5 inch thick. I  think that for many types of plywoods 4mm is too thin.  Commercial cajons use thin, dense hardwood plywood  for the back.  The awesome cajons made in Peru use thick solid wood on the back and have a great sound. But they are heavy. If I was making a cajon out of hardware store plywood made from softwood trees, I’d use 1/2 plywood for the back panel as well. If I had dense hardwood plywood I’d use 4 mm or thicker.

The Tapa

The tapa is the front of the cajon, the striking surface. It is the ‘drum head’ of a cajon, it needs to flex so 2.5 to 3 mm is typical. I use 3 mm, 3 ply ‘birch’ plywood from the local hardwood store for my tapas. I believe the inner ply is not actually birch, only the thin outer skin.  I’ve also used recycled hollow door skins successfully. Older hollow doors skins were made from tropical hardwood and produced excellent tapas.  Commercially made cajons often use 2.5 to 3 mm, 5 ply birch hardwood plywood for the tapa.  5 ply birch plywood can be found on the internet from specialty plywood suppliers.

Screws hold the tapa in place. I’m inclined to use a lot of screws. My current thinking is the tighter the box, the better the sound.  The gaps between screws leaves possible air escape passages. Some commercial cajons now glue the lower portion of the tapa and use screws on the upper parts. Screws should be countersunk.

Round the Edges

The plans don’t show it but it is important the round the edges and corners, especially the top. A cajon with hard edges is difficult to sit on for long.

Snare Wires

Many cajons include snare wires pressed against the inside of the tapa to produce a rattle. There are many possible snare wire configurations. I don’t include any in these plans. Check out the photos in this Pinterest board for ideas. I think any of these snare wire designs would work in this cajon design.

The Finish

I use 100% pure tung oil on the tapa. It is all natural and the players hands will have direct contact with the tapa.  The box can be finished with just about any finish you’d like. It’s a drum so it will need to take some abuse.

Completed cajon with a 100% pure tung oil finish.

Time to Completion

Folks often ask me how much time it takes to make one.  That’s a tough question for me because I usually have several cajons in the works at one time and life in general often creates work gaps. And plywood vs solid wood will also effect the time it takes to make one.  Plus there are four steps, 1) gluing the bottom support to the sides, 2) gluing the box together, 3) gluing the corner braces and 4) attaching the back, where the adhesive should dry overnight. Depending on the finish you use it will need to dry days to weeks. I think a skilled woodworker with a good shop could cut and shape the pieces from plywood in an hour or two.  Gluing and clamping after that should take a couple more hours. Sanding and finishing a couple more hours.  Add to this time the overnight glue drying and a week is about right. I often take two weeks start to finish.

Cajon Design – Downward Facing, Offset Sound Hole

The sound hole on this cajon is on the bottom. And most significantly, the front of the sound hole is offset from the back. In theory, because the bottom of the cajon is lower in the back, escaping sound will be focused out the front, and that is what seems to happen.

Firebox Cajon Open Hearth Model showing the large, downward facing, offset sound hole.
Looking into the Firebox Cajon Open Hearth 'bass reflex' port. This is the only sound hole on the cajon.
Looking into the sound hole from the front. This is the only sound hole on the cajon.
Interior of the Open Hearth model showing the bass port bar in front and the lower panel on the bottom towards the back of the cajon.
Interior of the Open Hearth model looking from the front to back showing the bass port bar in front and the lower panel on the bottom in the back of the cajon.

Pros:

  • As with bass reflex port cajons and ‘busking’ cajons, sound is directed forward, toward the audience.
  • There is greater tapa surface area than a bass reflex cajon of the same height because the bass port bar is lower.
  • Mic’ing the cajon is simple because the bass mic can rest on the floor on a simple foam pad or on the carpet.
  • The large opening means the interior of the cajon is readily available. You could easily pack a small suitcase’s volume of clothing in the cajon.
  • On-off snare assemblies that require access via the sound hole are much easier to access.
  • The cajon may be lighter. The large sound hole uses less wood than a conventional sound hole although the bottom bracket adds weight. Designs with less bottom bracket material are possible.

Cons:

  • The composition of your floor will effect the tone and loudness of the cajon.  I prefer it on carpet. But on a wooden floor it is louder. In some perspectives this could be listed as a ‘Pro’ – it gives tone and loudness options.
  • As with any bass reflex cajon, the tapa surface area is lower than a standard design.
  • The design is more complicated to make.
  • The design is not traditional.

Construction Tips

There are several ways to build the bottom structure, some of which are much simpler than my preferred method. I like the look of this design, there is a bar of nice wood across the bottom of the side panels.  Also, I’m using western red cedar which is not a strong wood, so I’m reinforcing it.  I’ll show some of the simpler designs in later posts.

Note that the sides of my cajons are ‘right trapezoids’, not rectangles – the box has an inclined front. So while the top of the cajon is 12×12 inch square, the bottom is a 12×14 inch rectangle. The principles would be the same for a square bottom, only the proportions would be different.

View from the back towards the front showing the left side, the bottom bracket and the bottom back panel. The bottom back panel will be glued to the lower portion of the bottom bracket.
Bottom bracket from the back. These channels are easy to rip on a table saw. Or the bottom bracket could be made by gluing 3 separate pieces to produce the same structure.
Back plate on the left, bass port bar on the right. I’ve glued a long block in the front portion of the bracket’s lower channel. This reinforces the bracket and give width for the large rubber feet.
Back to front view with the bass port bar in place on top of the bracket.
Bottom assembly from above. Back is on the bottom of the photo.

I first glue the sides to the bottom bracket,  then glue the rest of the box.

Gluing the top, sides and bottom pieces.

There are a lot of possible ways to build the offset sound hole design.  Please go wild and post some photos in the comments area.

You can hear 2 cajons with these types of sound holes in the YouTube video below. Note that I use polycarbonate for the backs of all my cajons. So the tone and resonance of these cajons will be quite different than most plywood backed boxes.

Please add your comments below.

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Building Cajons – Attaching the Back Panel Without Clamps

This method is a fast, easy way to get a tight, fully sealed back attached  to your cajon build.  It works well with any back material. No clamps are used. Here’s what you need:

  • camping pad foam material, must be compressible foam of some type, about 1/2 inch (1.3cm) thick
  • a small amount of hot glue or similar to attach the foam to the back plate
  • 12 small blocks of plywood or wood,  (about 4x1x1 inch {10×2.5.2.5 cm} depending on the thickness of your back material)
  • a drill and screws for screwing down the blocks
  • some heavy objects to weigh down the box, total about 100 lb (45kg)

This method works well even with surfaces that are not quite perfectly flat.  Clean-up is minimal. Glancing at the photos below might be enough information to get you going.

In these photos I use polycarbonate for the cajon back but the method is the same for plywood. There’s some polycarbonate specific info at the end of the blog for those of you who want to walk on the wild side.

I cut and sand the back slightly larger than the box by about 1/16 in (1.5mm), then, after it is attached to the box, sand the edges flush.

Hot glue secures foam to edges of polycarbonte used for cajon back.
Use hot glue to attach narrow strips of camping pad foam along the edge of the OUTSIDE surface of the back.

Cut the foam strips to be an 1/8 inch larger than the box side thickness.  Usually about 1/2 inch to 1 inch (1.3 cm to 2.5 cm). The camping pad foam strips serves two purposes. It prevents the back from moving and it provides a compressible surface to distribute pressure evenly throughout the attachment zone. Do not use a full sheet of camping pad foam, it will cause the back to bow in the center and you won’t have a ‘stretched’ back.

Cajon construction using a polycarbonate sheet as the back. Blocks in place to hold the cajon box.
Turn the back over, the back goes face down with 6 small blocks offset from the edges. The foam supports the back. It is the only thing supporting the back.

The first tier of blocks must not touch the edges of the back. Leave them back about 3/8ths inch (1 cm). If you use wood blocks you’ll need to pre-drill holes for the screws or the blocks will split. I prefer plywood blocks. The first tier must be high enough so the second tier will not touch the adhesive oozing from the joint.

Have your second tier of 6 blocks ready to go. Once the adhesive is on you must work quickly. Use your favorite adhesive, in these pictures I’m using clear 2 part epoxy but for a wooden back I’d probably use Titebond III (not hot glue, that’s just for the foam to back temporary bond).

Apply a thin layer of adhesive around the outer edge of the cajon box first. This will give it time to soak in a little.
Apply a layer of epoxy to the edges of the polycarbonate.
Apply a thin layer of adhesive to the edges of the back.

Once your adhesive is on, turn the box over and carefully place it exactly on the back, aligning the edges.

For 15 or 20 seconds, slowly press the box down while slightly shifting its position to spread the adhesive evenly. Keep watching the edges, realigning the box as necessary and press down more. The box will slip around a lot at first and you must go slowly in the first few seconds to minimize slippage.

Place about 1/3rd of the weights on the box to keep it stationary and then remove tape if you taped the edges. Check to see if the box has moved and reposition if necessary.

Work your way around the box screwing in the second tier of blocks atop of the first. The second tier fits snug against the box. The second tier must be high enough that the adhesive oozing out of the joint never touches the upper block.

The lower tier of blocks is offset so as to not contact the adhesive. The upper tier of blocks hold cajon box in place.
The lower tier of blocks is offset so as to not contact the adhesive. The upper tier of blocks hold the cajon box in place.

I realign the box exactly on the back before securing each upper block. Once all the blocks are all in place, the box cannot move.

Turn the box over and carefully place it exactly on the poly, aligning the edges.
The box and back with the lower and upper tiers of blocks in place.

Once all the blocks are in place add the remaining weight. I use a lot of weight. Some of my early attempts with less weight were less successful. If you want, you could calculate the optimum amount for your adhesive because all the parameters are easy to measure.

Using big blocks to weighing down the cajon box while gluing the back on.
Weighing down the box. These are sitka spruce blocks which are fairly light. Total weight is about 110 lbs (50kg)

With the weights in place, reach inside the box, through the sound hole and scrape out the access adhesive that oozes from the joint. Then wipe the area with the appropriate solvent. The more excess adhesive you can remove the easier clean up will be after it has set.

Next day remove the weight and sand the edge to make the back flush.

The edge of the back sanded flush with the sides and top of the cajon.
The edge of the back sanded flush with the sides and top of the cajon.

A tight box is critical for good sound and durable construction. While building my first cajons I used clamps to attach the back. They worked but it was messy and a bit complicated. I’m sure there are other ways to do this. And plenty of improvements are possible to this method. Please comment below. Thanks, Bill

Firebox Cajon with polycarbonate back, red cedar and yellow cedar sides top and bottom.
Firebox Cajon with polycarbonate back, red cedar and yellow cedar sides, top and bottom. Made in Alaska, USA
Firebox Cajon with luaan tapa, red cedar and yellow cedar sides, top and bottom. Tung oil finish. Made in Alaska, USA
Firebox Cajon with luaan tapa, red cedar and yellow cedar sides, top and bottom. Tung oil finish. Made in Alaska, USA

Attaching polycarbonate as a cajon back:

Polycarbonate gives a cajon a lot of resonance. I cut the poly with a ‘ceramics’ blade on a jig saw which is much, much safer than other methods I’ve tried. Then I sand both sides to ‘frost’ the sheet with either a belt sander or an orbital sander. After cleaning the inside edges with alcohol the polycarbonate it is ready for ‘gluing’ with either 2  part epoxy or AquaSeal as the adhesive.  Critical is an epoxy that is not runny. It needs to be a bit thick to fill and adhere.  If you are using epoxy you must work fast to coat both the wood and the polycarbonate before the epoxy starts to set. Any end grain must be sealed with a thin layer of epoxy the day before glue up. Many epoxies are heat soluble, if you make a cajon with an epoxied back don’t let it get to hot. And it’s a good idea to put a few screws in to help ensure it stays put.