Having managed to get to the initial graduations I am finally able to start the plate tuning.
First I took stock by weighing the plates and measuring mode frequencies.
I used the same setup I used for the cello:
- Android tablet to generate the signal tone
- 20W class D amp
- 4″ driver unit (used an 8″ on the cello)
- Tea-bag for tea leaves
Starting point with initial graduations
The top weighed in at 88g and had perfectly matched x-mode and ring mode (an octave apart) but still a bit high as expected at this stage.
The back weighed in at 119g and is still quite a bit too stiff with the ring mode going off both ends and not closing. The x-mode frequency needs bringing down quite a bit to get to an octave below the ring mode.
After a bit of work I managed to match the back and top ring modes an octave above the x-modes at about 180Hz. The plates are still a bit heavy but this gives me something to work with when tuning the bass bar after cutting the f-holes.
I marked out the outline of the f-holes using the printed paper template I had designed and pricking through with a pin to mark the outline on the top. I then drilled the top and bottom holes with a forstner bit and opened them up to correct dimensions with the reamers I use for peg holes.
With the top carefully clamped to the cork faced form, I used a fine toothed coping saw to cut a rough outline and then carved off the excess with a scalpel.
First results are shown here but still needs a bit of fettling to get the two holes balanced.
Next I sliced up a nice piece of quarter sawn spruce on the bandsaw and sanded down to 5.5mm wide on the drum sander. I cut this to length and marked the underside profile with a pencil using a block of wood while the blank bar was gently clamped in place on the belly. Then I planed the underside profile down to the pencil lines and chalk fitted to the inside of the top aiming to get the outside edge of the bass bar 1mm inside the outside edge of the bridge foot with a 2mm slant outwards from top to bottom.
After gluing in the bass bar I tuned the bar to match the top and then gradually worked both the top and back down in frequency to match each other and get as low a weight as possible (my client wants a light easy to play instrument) with the ring tones just below F.