Making and gluing ribs

Stringed instruments of all sorts—not just harpsichords but also pianos, guitars, violins and viols—have one or more ribs under the soundboard. These keep the soundboard from buckling under the pressure of the strings and have an effect on the tone of the instrument. Italianate ribbing often spans the full width of the soundboard and helps to keep the board fairly flat, which makes this type of harpsichord less susceptible than Franco-Flemish designs to tuning fluctuations caused by changing humidity.

The Trasuntino's extensive ribbing scheme is a mishmash of wooden bars that reflect all the modifications the instrument has undergone during its existence. Some of them clearly pertain to the now-missing 4-foot register, while the logic behind others is elusive. What likely happened is that changes were made at various times without necessarily removing items that no longer contributed to the instrument in its then-current state.

After consulting with my friendly harpsichord maker, we nixed a couple of ribs that had to do with the 4-foot and reduced the remaining ribs to 3 or 4, in line with more traditional Italianate designs. With a little study and further consultation, I decided to use 4 ribs, placed about 14 inches apart and more or less perpendicular to the bridge curvature.

The rib stock is of hemlock, 1/2" wide and 3/4" high, with sides tapered inwards about 7 degrees:


These were cut to an appropriate length: the full width of the soundboard at each of the 4 locations minus about 3/4" from each end, to make sure the soundboard edge could be glued to the liners without interference.

Humidity control is important when gluing the ribs, so the workshop was allowed to dry out to 39-40% before gluing. By doing so, the soundboard crowns upward slightly when the humidity returns to more typical levels, since the board is constrained on the underside by the ribs. The crown provides additional support against string pressure, along with tonal benefits.

To glue the ribs, each end was clamped down, then I clamped boards directly onto the end clamps and used them to press the ribs down with wedges:


One important feature of an intelligent Italianate ribbing scheme is that the ribs should be slightly cut out where they pass under the bridge; if they aren't, the strings crossing the bridge at that point have a "pinched" tone. These cutouts are about 1.5" wide and 3/16" deep:


Note the brass tack which holds the bridge from underneath.

Finally, to keep the soundboard edges flexible, the rib ends were cut out in a scallop about 1.5" long:


With both bridge and ribs glued on, the soundboard has become much more rigid. Before, it behaved like a big piece of cardboard and I always had to move it around rather gingerly, but now it feels stronger and safer to handle.