Tuning the plates

Measure thickness
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.

Start tuning
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

Top unbraced mode 1
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.

Top unbraced mode 3
This is mode 3 at 287Hz

Top unbraced mode 4
This is mode 4 at 365Hz.  This is the most important mode.  The positions of the modes are marked in pencil.

Mark briidge position
Next the position of the bridge posts (if it was a Gibson style of bridge) are marked on the inside of the top.

Mark brace position
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 is marked out
X brace position marked.

Here is my stash of Red Spruce brace wood.

Slice brace in bandsaw
Cutting the brace wood to size in the bandsaw.

Thickness the brace
Thickness to 7mm thick on the linisher.

Mark the length
Mark the length of the brace.

Cut to length
Cut to length on the bandsaw.

Mark brace shape
A compass is used to mark the shaped of the brace.

Mark the end position
Mark the positions of the end of the brace

Shape the brace
Shape the brace on the linisher.

Fit the braces
Dry fitting the brace to the top.  Easier said than done because a close fit is required.

Glue first brace
Gluing the first brace.

Mark the X cut
Marking the position of the X join.

First cut
Cut on the marks

Chisel out
and then chisel out the groove.

Mark the other brace
Dry fit the other brace and mark.

Cut second brace
Cut the groove on the other brace and chisel it out.

Dry fit
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 second brace
Glue the second brace and wait 24hrs for the glue to dry.

Shave the braces
Shape the braces down to nothing on the ends and scallop in front of the soundhole.

Round off the braces
Round off the tops of the braces with a finger plane

Sand smooth
and then sand smooth.

Braces done
All finished.  Now measure the Chladni patterns.

Top braced mode 1
Mode 1 at 192 Hz.

Top mode 3 braced
Mode 3 at 308 Hz.  I don't like the look of this.

Top braced mode 4
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.

Graduate the back

and sanding the back.

Mode 1 back
Mode 1 at 160Hz.  Very nice.

Back mode 3
Mode 3 at 318Hz.  Also very nice.

Back mode 4

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.

Final top mode 1
Mode 1 at 169Hz

Top final mode 3
Mode 3 at 301Hz.

Final top mode 4
Mode 4 at 400Hz, nice sharp lines, strong amplitude and very close to the back.  Excellent.