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.
|
Graduating
|
|
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. |
