Official Luthiers Forum!

Andy Birko asked:
"I also wonder if the no-change thing will be different on a tailpiece type of instrument like the bandura I normally build."

That depends on whether the top is arched or not. Just as stretching a diaphragm raises the pitch, so compressing it will drop the pitch. On a flat top the plate is compressed in front of the bridge, and streched behind it, and the overall effect seems to be a washout. The down load of the bridge on an archtop drops the pitch of the 'main top' mode, sometimes by a fair amount. I don't know enough about banduras to be able to make a guess about that.

Very interesting. I've tried putting a dome on the tops using a 'solera' type of method (i.e. the rim was flat but the top was domed) but by the time I was finished stringing it up the tops have been flat. The next instrument I'm building I'm planning on using a 25' spherical dome with some additional bracing to help hold the dome. I'll also be profiling the sides to match the sphere and profiling the bridge with a 25' dome. I'm hoping that will do the trick.

Do you think that would be enough 'arching' that it would drop in pitch when strung up?

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Hum?? that is interesting as I have built several domed tops and glued to flat planed rims. The only thing that happened to them was, because waist is lower in elivation on a flat plane rim than it would be on a dome sanded rim is there was a pull in effect causing a secondary arch between the the waist valleys. I have never had one go flat on me.

Hmm - I'm thinking a domed top would be higher in pitch than a flat top, all things being equal, because a domed top would be a bit stiffer.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

I think mine have collapsed for a number of reasons:

My banduras have a 3-ply laminated rim about 3/8" thick which I've been planing flat.

Banduras are a tailpiece type of instrument with 49 strings on the main bridge, 24 of which are chromatics that start at top level, go up to the bridge and then back down to top level creating a huge downforce.

My 'solera' had a max depth of only about 1/8"

Because the dome was not a sphere, I could never get my bridges to fit well to the top so I epoxy them on using a go bar deck.

The grain of the top runs 45o to the strings and the braces run parallel to the strings. With the braces in place but not glued to the rim, the top is not domed but rather cylindrical...sort of.

To try and fix this, on my next instrument I'm trying the following:

Using a 25' dish to shape and glue the braces.

Profile the rim using the 25' dish.

Adding 1 to 3 secondary CF topped braces 90o to the grain of the top (45o to the main bracing).

Using a 25' dome to fit the bridge

Using a caul and cam clamps to glue the bridge.

Hopefully that will do it.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Rob,
Let me try to demystify it for you. I think I can make it simple. Just like the artist carving on a block of granite, just remove stuff that doesn't look like a face - Just remove wood that doesn't sound right. See - isn't this easy?
If you have a good ear, trust it. Carve and tune the top so that when you tap it and it sounds right to you, stop. Finish the guitar. Remember that tap sound. If you are happy with the sound of the guitar, then you did well. Repeat this process.
If you are not happy, repeat on the next guitar, this time make it sound pleasing to you being careful to note what the last guitar lacked. If it was too bright, make this one tap to a lower tone by making the top thinner (less stiff) and the braces shorter. If it has too much bass, make this one tap to a higher tone by making the top a bit thicker (stiffer) and the braces a little taller.
Give that a try and see how it works.
If you like it, I can send you Paypal information to help out with my kids college fund.
If you don't like it, then you learned a method that does not work for you. Congratulations.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Watching the Somogyi vids that Chris here mentions, Erving Somogyi uses three main criteria when describing the movement a top makes, then describes what sound each action provides these are

Monopole, this is where a top moves up and down like a piston and this provides the bass

Next is what he calls Cross Dipole, this is how the top rocks sides to side around the central axis of the guitar ie either side of the neck, and this motion promotes the presence of the guitar how loud it sounds in close proximity

Finally he talks about long dipole where the top moves in a see-saw motion about the bridge and this is about projection, the carrying power of the guitar how it sounds at a distance.

He shows a standard braced, but uncarved top and proceeds to carve it and show how the sound alters when decoupling the braces. Now all I've got to do is re-watch it a few times to understand it then keep building to live it!

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Formerly JJH

I learn more from my mistakes than my successes