Technical Library

STRUCTURE V: Loose ribs

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Loose ribs…
A pattern of ribbing is glued to the underside of most harpsichord soundboards. Ribs not only support the board, but also serve to stiffen it, and divide it acoustically into different vibrating areas. If a rib runs directly under a bridge, it is usually undercut at that point to provide clearance, allowing the bridge to vibrate freely so the tone is not degraded. The ends of the ribs are usually scalloped to leave their extremities more flexible and able to breathe with the expansion and contraction of the board.

It’s not uncommon, though, for ribs to detach. This can happen with expansion or contraction of the soundboard with extremes of humidity, or the extraordinary forces an instrument is subjected to when dropped. With a detached rib, the soundboard can split and/or drop as it loses support against the downbearing of the strings. The loose rib can buzz or rattle. If there is extreme distortion of the soundboard, the instrument can be rendered unplayable. A loose rib can often be diagnozed by a distinct rattle when the soundboard is strongly rapped with your knuckle, although the resulting sound can be similar when a rose is loose. A definite drumming sound can often be generated when the board is repeatedly depressed over the offending rib, as the end of the rib alternately makes and loses contact with the board.

Access must be gained inside the harpsichord to effect repair. Some makers thoughtfully provide an inspection hatch or two in the bottom which can be undone with a few screws. Without a hatch, a hole must be cut clear of internal bracing. The loose rib can be glued and supported while drying with a short go-bar bearing on the harpsichord bottom or internal brace.

Detached Italian harpsichord soundboard rib 31K jpeg Reattached Italian harpsichord soundboard rib 29K jpeg
Italian soundboard rib detached at bentside Rib glued and its end supported while drying

Movie thumb 2K jpeg Movie 1K gif Loose soundboard rib
Carey Beebe diagnozing and reattaching a loose soundboard rib in the field.
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