Well – if not finished at least it is working. I’ve got the sound post in (somewhat unconventional – see below) and strung it up and it sounds lovely!
Before I go any further these are the things that still need doing:
- Get the neck to the right shape;
- Apply finish (boiled linseed oil probably);
- Notch the bridge for the strings.
Ok – I’ve not built anything like this before so I freely admit that I don’t know what I’m talking about! This is just the musing that lead to the design of the thing I built. I’m not pretending that any of this is correct.
The basic problem is how to produce decent volume from an acoustic bass. The full-size double bass does this with a large sound board. On a small instrument we only have a small soundboard, so, just as with loudspeakers, if the size is small then the amount of movement needs to get bigger.
This approach is worth a look. The soundboard is suspended from the body of the guitar with steel wires, allowing the top to flex without interference from the sides. I don’t have anything like the ability to copy this.
Another approach is just to make a bass banjo. Here the edge and the rest of the soundboard are very flexible. Banjos tend to be loud. It is possible to make a ukulele bass banjo from a drum.
I went for a fairly traditional approach based on the awesome Making The Violin website. Thus the bass has ‘f’ holes, floating bridge, tailpiece, soundpost and bass-bar.
In light of the comments above about making the soundboard suspended round the edges I used long ‘f’ holes to separate the bridge part of the soundboard from the edges of the soundbox. The bridge is then supported by a lightweight bass-bar and a soundpost.
The soundboard itself is made from birch plywood. This is apparently used in harps so is more than good enough for my first project! I’ve also seen comments that bass ukuleles sound exactly the same with solid wood and laminate soundboards so not worth stressing over at this stage. The key design parameter is that it should be flexible without danger of splitting. Plus, of course, cheap and fast to build.
Notes on construction
In no particular order…
The starting point was a Port Wine Box I was given by someone clearing out their house. It appears to be made of cheap Spruce with big knots, nailed together. The key points were:
- It was about the right size;
- It had a clear rather than dull sound when tapped;
- It looked nice.
The through-neck design was based on banjo design. Given my bit of wood was long enough to allow this I couldn’t see why it wouldn’t work. In retrospect I’m happy about this design because:
- I’m not confident about the structural integrity of the wine box;
- There is no string tension stress at all in the soundboard. In many designs the soundboard is crucial to providing structural integrity. If I was worried about this then the soundboard would need to be stronger and more rigid.
The neck under the soundboard is cut away to allow the soundboard to vibrate. I did need to add extra wood glued to the through-neck under the cut-away to stop the neck flexing.
The width of the bridge is required to reach outside of the through-neck and onto the bass-bar and sound-post.
The neck is currently a bit thick but works. I probably should make it a bit thinner.
I like the design of the headstock – it is reminiscent of a double bass – and overall works well. There is a slight problem with the angle of the centre two strings over the nut to the tuners – the angle isn’t big enough to prevent a slight buzz if the strings are plucked vertically i.e. down towards the soundboard. I could add something to hold the strings down if this becomes a problem, but the buzz is so slight I’m not worried for now. A better fix would be to increase the headstock angle by scarfing the headstock to the neck.
The tuners grip the strings well – this is the advantage of drilling each tuner specifically for each string rather than have a space large enough for the largest string. It is important that the hole goes right through the headstock as this allows the stretchy bass strings to be pretensioned.
The match of string to tuner was dictated by the length of the strings. The D string was much longer than the others so that goes to the furthest tuner.
The action is crazy high by normal standards. However the bass ukulele strings don’t operate to normal standards. Firstly the movement of the string is huge. Second the strings are stretchy, so pushing them down isn’t hard.
Ergonomics aren’t great due to the shape of the wine box – it has sharp corners that dig into knees! But it is ok – I play it upright sitting on my knee. It probably needs a spike as it is too big for the kids in this orientation.
As described above the bridge needs to be wide enough to reach outside the through-neck and line up with the bass-bar & sound-post. The bridge is positioned halfway down the soundboard for maximum resonance. I’m not sure of the wood used for the bridge – it is from a reclaimed parquet floor tile so is hard and strong. I’ve tried to make the bridge thin for minimum weight.
I still need to cut out the slots for the strings. It works ok as-is but plucking sideways moves the string sideways on the bridge which isn’t ideal. Cutting the slots will only take a few minutes.
The tailpiece has the string holes at an angle to try to reduce contact between the ends of the strings and the soundboard. The ends do need trimming.
As discussed above, 3mm birch ply works very well. Tone is good at least in the lower registers. Moving up the fingerboard on the G string loses tone somewhat but that really doesn’t worry me. I did need to glue spruce strips to the top and bottom of the soundboard. These are not for any acoustic reason – they are there because the ply I bought was slightly too small!
The ‘f’ hole shape was chosen based on:
- Aesthetics – I wanted a shape that matched the angular shape of the rest of the instrument
- Maximum flexibility of the maximum area of the soundboard i.e. the holes needed to be close to the edge and fairly long – just over 1/3 of the length of the soundboard
- Not too much unsupported board – I didn’t want a big flange with a C-shaped hole as I worried that the unsupported bit would vibrate in an uncontrolled manner.
- 20mm holes at either end of the slot to reduce any stress concentration and allow easier flexing.
The bass-bar is also spruce from a DIY-store plank. Because tiny amounts of wood are needed a suitable bit can be found in most planks. To reduce weight the bass-bar is sort of triangular – the tip that contacts the soundboard is around 2mm wide – it needs to be wide enough to glue ok – and the other side (the side away from the soundboard) is around 4-5mm. The ends are reduced down to allow more flexibility near the ends where the stress of the string load is lower.
Note that the edge of the bass-bar in contact with the soundboard is curved. This forces a slight curve (couple of mm) in the soundboard and pre-loads the soundboard against the force of the strings.
The soundpost is vital to provide ‘life’ to the soundboard. The soundpost is a bit of bamboo – I think it was a skewer or pot-plant support – chosen as it has very straight grain. It is about 5.5mm in diameter. Since the wine box is very deep and the ‘f’ holes are a long way from the soundpost location I didn’t think it was feasible to use the normal approach of wedging the post in by feeding it through the f-hole attached to a spike. Instead I drilled a hole in the back of the wine box and pushed it through. The hole is covered with a screwed-on plate to take the load. I cut the post to put the top under tension – a few mm longer than the zero-tension length.
The soundpost hits the soundboard about 10mm from the bridge towards the tailpiece.
As discussed the soundboard is birch ply bought on eBay.
The wine box appears to be made of spruce with lots of knots. It was nailed so I ran glue along all the joints to prevent buzz.
The neck is an old piece of Shorea left over from making a cupboard 15 years ago.
The fingerboard and tailpiece are mahogany from an old cupboard door. This is nice wood – straight grained and easy to work.
As discussed the bridge is from a parquet floor tile so is pretty hard.
The bass-bar and soundboard extension strips are DIY-store spruce, carefully chosen for straight grain.
The soundpost is bamboo – very straight grained.
Strings are Aquila Thundergut bought on eBay.
Tuners are bass guitar tuners, again bought on eBay. Bass guitar tuners are much cheaper than bass ukulele tuners but the slots are too narrow for the bass ukulele strings and need to be drilled out.
Glues are Titebond 3 for general stuff and Titebond Hide Glue for stuff that might need to be taken apart sometime. Both are nice to work with.
I’ve never played a bass before so I’m starting with the beginner double-bass tutorials on YouTube. It will take a while but is great fun.