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Fixing a soundpost crack

by Bows for Strings on

Recently our workshop completed a soundpost crack repair on a fine German cello, which prompted us to put together the following article discussing the process of a repairing a soundpost crack.

The sound post is a key component that enhances a cellos sound. It's inserted through an f-hole and stands near the treble bridge foot, held between the front and back plates solely by friction. The sound post's vibrational patterns produce powerful sound projection, granting the cello its distinct loudness. However, it can also be a weak point prone to cracks, impacting the cello's structure and sound quality, especially if the instrument is dropped.

When a sound post crack happens, fixing it like a regular crack isn't enough. When a cracked instrument is played again, the forces from the sound post can cause the crack to reopen if it's only glued with hide glue and internal studs.

Therefore, fixing a sound post crack isn't straightforward. The conventional method involves fitting a patch in the area of the soundpost crack - this is a process that's well-established but requires a great deal of expertise.

After the top of the cello is carefully removed, the repair starts with making a counter form to support the cello's exterior using plaster (see above). The cello top is placed on this form, and a hollow is carved out in the plate around the crack. Almost the entire thickness of the plate at its deepest point at the centre of the post is removed. A plug made of carefully selected matching wood is inserted into this hollow, its fit perfected using chalk. Once the fit is ideal, glue secures the patch in place, and excess wood is carved away to match the thickness of the plate.

Repairing a sound post crack involves meticulous and skilled work, however when done well it serves to extend the instrument's life without impacting its tonal qualities. 

See the finished repair below.

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