Kumiko Cell Phone Speaker

Recently I finished up a collaborative project here in Tokyo with a design company called Consentable. The project is a purely acoustic speaker designed to amplify your cell phone’s speaker, and features the smallest kumiko pattern that i have ever made.

After some discussion about the design and fabrications details I agreed to fabricate four prototypes, 3 that were identical and one that was a bit longer than the others. The designer is still working out certain details related to sound transmission and the length of the speaker is one of the details he’s experimenting with. Below are a series of shots I took throughout the making of the four prototypes.

Since I’m essentially working out of my apartment until I find a real shop space here in Tokyo, I did most of the work by hand, including final planing of all the speaker’s parts to size. I did do general milling at Somacousha, the shop where I have been doing traditional carpentry work, but since they are several hours away by train I had to finish the bulk of the work with hand tools.

The material for each speaker is straight grain hinoki cypress. It’s beautiful wood to work with, though I’ve discovered that I’m pretty allergic to it. Planing it is no problem but as soon as I make some hinoki dust, I start sneezing like mad… time to start wearing my dust mask!

All the parts were milled to 5mm thick in the shop and here in the apartment I cut the parts to length and width, and finish planed all the surfaces.

The real challenge for this project is the kumiko, which the designer requested be small. In this case the kumiko dimensions are 1mm thick by 5mm deep. I had to do some experimenting to figure out how to work with material that thin, and the biggest challenge was coming up with an efficient way to cut the half-lap notches for the kumiko grid. I ended up going around to different tool stores in search for a saw with a kerf that was at or just under 1mm, that way with one cut I could have each kerf made exactly the right size. What’s amazing is that in Japan the majority of mass produced saws have the saw kerf size listed on the the packaging! (Along with the tooth size, pitch and blade thickess). So after some searching I found a saw with a kerf of 0.8mm. It’s a bit of a course tooth saw, but with care and after a couple of practice runs, it seemed to cut the perfect size notch for the 1mm kumiko.

Below is the saw along with a set of parts for the kumiko panels. The notches are 5mm apart. I ended up using a 90 degree guide block to help start the saw and keep things true and square.

Here I’ve finished cutting the kumiko parts and I’m now doing the first assembly. Each speaker has 4 kumiko panels, so in total I made 16 panels.

The saw kerf was nearly perfect for these tiny kumiko. Though at 0.8 mm, and with a tiny bit of variation in the thickness of the kumiko themselves I did end up with some tight joints and a bit of distortion, but overall the panels stayed pretty flat once they were all assembled. If the joints are too tight the whole panel will turn into a potato chip shape as each consecutive piece slightly bows the piece it is mated to.

Next I need to trim the ends of the kumiko to length and also cut a 45 degree angle so they could be attached to the side panels.

At 1 mm x 5 mm the hinoki was so delicate that I ended up having to be extremely careful about breaking the kumiko when cutting the ends to length. I ended up using a marking knife with a really acute bevel to slowly slice the ends.

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Below is the first test assembly of the kumiko panels to the surrounding frame. Since all parts are 5 mm in depth I had to come up with a solution for attaching the kumiko, that would be strong and somewhat efficient. I ended up cutting 45 degree kerfs to accept the 45 degree ends of the kumiko. That way from the front side the kumiko all appears flush and no joinery is visible, but from the backside you can see the joinery.

After doing this first test assembly I realized that I didn’t need to attach every single kumiko to the surrounding frame. Attaching every kumiko end was just overkill and really took too much time to cut all the notches in the surrounding frame, and it also made assembly really hard. So I opted to attach every 5th kumiko instead on the next three speakers, leaving the majority to just butt up to the surrounding frame. (You can see this change in the above photo where I’m trimming the kumiko ends, leaving every 5th end angled.)

After attaching the kumiko panels to the surrounding frames it was glue-up time. Pictured below the front and back kumiko assemblies are being glued to the base ends. No matter what scale glue-ups are always slightly stressful.

One more challenge for this project was to come up with a way of holding a cell phone power cord. The designer wanted the power cord to be fixed so that a phone could be plugged in and held rigidly in position. The cord also needed to be removable, since the speaker is intended to be usable with a range of phone makes/models. So we came up with a design where a couple of wedge shape blocks clamp onto the power cord plug, and get pressed into a fixed block, with mating wedge shaped mortise, glued to the base of the speaker. The wedge shape allows the cord to be held rigidly when it’s inserted into the base but can also be easily removed when need to be.

Here is the first trial of a set of blocks cut for an iphone plug.

And here the blocks and plug are pressed tightly into the base.

The concept worked pretty well but it became apparent that it would be crazy to try and make a set of specially shaped wedge blocks for every different plug out there. Even the differences between a standard iphone plug and a generic one are huge, so we came up with a slightly different solution. Here is the second version, this time using some firm foam that can conform to a variety of plug shapes.

Here you can see the revised plug in place. In the end I think this turned out to be a pretty reasonable solution.

Here is the underside of the speaker where the narrow end of the wedge shape blocks can be seen as well as a dovetail-shaped groove for holding the power cord.

The last few shots here show the finished product, and some of the interior details. The round cutouts will eventually hold a sound tube that will project the sound from the phone’s speaker out through the kumiko grills.

Overall this was a fun project though it turned out to be way more complicated than I thought. Bidding projects is definitely a skill of it’s own, and a lot of woodworkers that I talk to seem to have the same problem as me when it comes to underbidding jobs. It’s easy to see a project in broad strokes only, but it’s all the little details that really slow things down and really eat up time. Anyhow I feel really fortunate to have been asked to do the fabrication. It was quite a contrast in terms of scale to the traditional carpentry work I’ve been doing lately, and it also opened up a whole new world of kumiko design possibilities. I never would have thought it possible to use kumiko so small but surprisingly these little panels became pretty strong once they were all assemble.

And now my mind is full of miniature scale kumiko project ideas…..