Tapering the soundboard

Now comes the part I've dreaded a little bit since the beginning of the project last summer: the soundboard has to be selectively hand-planed in spots on its underside. This will definitely have an impact on the tone quality and sustain of the completed instrument.

The process worries me because of my lack of experience: a professional would know how to adjust the planing to suit the specific pieces of wood he was working with at the moment. Experience and judgment would tell him whether to remove more or less material, given the peculiarities of the wood he had at hand: no two soundboards would be planed exactly the same way, even though the resulting instruments might very well turn out similarly.

There is no substitute for experience, so all I can do is follow the general principles and humbly accept the whole experience as a learning exercise.

Some of the general principles in thinning a soundboard are:

• Italian soundboard thinning takes place along the outside edges, and some sort of taper towards the edge is aimed for. Having said this, it is possible to find historical Italian instruments in which the soundboard doesn't seem to be tapered at all.
• Mass = inharmonicity. Inharmonicity is desirable in the bass, less so in the treble, so the treble areas will be thinned out more than the bass. This concept was already made use of back when the soundboard was assembled, by choosing "ringier" boards for the bass and "duller" boards for the treble.
• There is a correlation between soundboard mass and the resulting instrument's volume and sustain.

I've taken a preliminary stab at the tapering process with a block plane, first tracing out the bridge curve on the underside in pencil and then planing outward from there with a block plane:

You can see a bit of a shadowy area along the bentside curve, which is the planed zone.

The grain has torn out here and there on one section, which probably means I inadvertently turned that board around at some point before it got glued into the completed soundboard. Luckily this is the underside of the board and a cosmetically even appearance is not important: you can see I didn't work hard at cleaning off dried glue spots either. In historical instruments the underside of the soundboard often shows plane marks and scratches. Many makers feel some irregularity to the board is actually a good thing: apparently this roughness keeps the board relatively neutral and prevents it from overly favouring any one particular vibrating frequency.

I might sand along the very edges of the board later, so that a smooth surface will be present when the soundboard is glued into the case.

Note that I also cut out a hole for the rose (a parchment or metal ornament glued to the underside of the soundboard). I have a charming parchment rose that consists of two layers: a disc glued underneath the soundboard and a ring around the hole at top. The hole was easily made with a drill and a 2.5" hole saw, which gives a hole slightly larger than the top ring. This way the hole edges won't be seen. I decided to locate the hole halfway between the spine and bridge, and twice this distance forward of the bellyrail; clearly there is one exact location that satisfies these criteria simultaneously, and that is where I drilled the hole.