Peter Coombe Mandolins and Guitars
Tuning the plates
Firstly the top is graduated. This involves thinning the top in little steps with a finger plane until it starts to feel right in the hands, and sounds right when tapped. Sorry, no pictures because this is a 2 handed job. This is where experience comes in. By "right" I mean from experience I think it is likely to sound good. I feel the stiffness of the top in my hands and tap it. I do not tune the top to a particular frequency because I think that is a complete waste of time. Final tuning is done by measuring the Chladni patterns. Usually what happens is I stop graduating and just measure, there is no more adjustments made to the thickness of the top. The tops end up 6-7mm thick in the centre and about 3mm on the recurve. With this top, the stiffness feels about right, but it feels a bit heavy in the hand so I am a bit suspicious it might not be the perfect top. no matter, plough on.
This is my measuring equipment. 140watt power amp on the right and digital signal generator (I built these). On the left is a high powered 8 inch speaker. The method is to sprinkle sawdust on the plate and scan the frequencies until a pattern emerges. There are 4 main modes, but mode 2 is usually not visible. For more information on Chladni plate tuning read my paper
This is mode 1 at 164Hz. Not very distinct, but that is unusual because with unbraced tops often mode 1 and mode 2 are very close together in frequency so you don't get a strong pattern.
This is mode 3 at 287Hz
This is mode 4 at 365Hz. This is the most important mode. The positions of the modes are marked in pencil.
Next the position of the bridge posts (if it was a Gibson style of bridge) are marked on the inside of the top.
I mark the position of the X braces according to what is measured in the Chladni patterns. I try to position the X so it falls close to the position of Mode1 and goes directly under the bridge posts. In this case mode 1 is indistinct so I have to guess.
X brace position marked.
Here is my stash of Red Spruce brace wood.
Cutting the brace wood to size in the bandsaw.
Thickness to 7mm thick on the linisher.
Mark the length of the brace.
Cut to length on the bandsaw.
A compass is used to mark the shaped of the brace.
Mark the positions of the end of the brace
Shape the brace on the linisher.
Dry fitting the brace to the top. Easier said than done because a close fit is required.
Gluing the first brace.
Marking the position of the X join.
Cut on the marks
and then chisel out the groove.
Dry fit the other brace and mark.
Cut the groove on the other brace and chisel it out.
Dry fit the second brace. Should be a snug fit. If too tight, a bit of fine cutting with a sharp blade should get it fitting snug.
Glue the second brace and wait 24hrs for the glue to dry.
Shape the braces down to nothing on the ends and scallop in front of the soundhole.
Round off the tops of the braces with a finger plane
and then sand smooth.
All finished. Now measure the Chladni patterns.
Mode 1 at 192 Hz.
Mode 3 at 308 Hz. I don't like the look of this.
Mode 4 at 393Hz. Mmm I don't like the look of this either. The pattern is not very sharp and mode 4 is the most important mode and the sawdust should jump around more than what I am observing. I am now seriously considering not using this top for this mandolin. It is a special order for a local musician who is very particular about sound. No matter, let's continue on and see how the back turns out.
and sanding the back.
Mode 1 at 160Hz. Very nice.
Mode 3 at 318Hz. Also very nice.
Mode 4 at 402Hz. Excellent. Absolutely nothing wrong with this back. Sounds good when tapped, nice and stiff and light in the hands.
Ok, now need to re-consider the top. Top is so so, and mode 4 is at 393Hz, a bit low to match the back. Big breath, this top is really not good enough for this customer, I will use it on another mandolin, probably not a special order. .... 3 months later I have another top ready. This is why the mandolins take so long to make and why I never get paid enough! However, they do sound exceptional so the extra care is worth it.
In the end, this turned out to be a really good decision. This instrument turned out to be exceptionally nice, whereas the mandolin I eventually made with the original top sounded disappointing.
Here are the Chladni patterns for the new top. This top is excellent.
Mode 1 at 169Hz
Mode 3 at 301Hz.
Mode 4 at 400Hz, nice sharp lines, strong amplitude and very close to the back. Excellent.