William Cumpiano

Repairing a Bulging Soundboard

by Harry Becker, Becker & Cumpiano Stringed Instruments


This old Martin is in excellent condition except for a serious bulging in the soundboard. A mild gradual swelling of a soundboard is generally not a cause for alarm. However, an abrupt deformation as seen here is an indication of major structural failure.



The deformation of the soundboard is most pronounced at the corners of the bridge. An inspection with a probe confirms that the X braces, located directly below this area, have separated from the soundboard.


Further inspection of the soundboard from the side reveals another element of concern: the plane of the soundboard behind the bridge does not line up with the plane of the soundboard in front of the bridge. There is also evidence that the bridge was at one time removed, suggesting the soundboard may have been damaged during the procedure.


The double-wound portion at the end of the low E string extends beyond the surface of the bridge and touches the saddle. This is generally an indication that the bridge-plate is worn and the ball ends of the strings have work their way through the bridge-plate--causing the double-wound portion of the string to emerge over the bridge itself and mount the saddle. In this case, the problem is compounded because the bridge-plate is also cracked and collapsed upward into the area of the soundboard.


A visual inspection of the bridge-plate prior to removal reveals it has warped and also cracked along the line of the bridge-pin holes. The ball ends of the strings have worn deep pockets in front of the holes. This explains why the ends of the strings are so exposed over the bridge. But why is the bridge plate warped so severely?



Once the bridge is safely removed, an explanation becomes evident. The soundboard was severed and splintered during a previously botched bridge removal. This set the stage for a series of unfortunate and cascading events.





A small light source inside the guitar within a darkened room reveals the ugly truth: large, splintered portions of the soundboard are actually missing. Prior to replacing the bridge, the previous repairperson made no attempt to reconstruct the damaged soundboard. Instead, the previous  repairperson filled the gaps with glue and forced the bridge plate to comply with the cavity created by the lost splinters.



The soundboard had been so badly damaged on it's top surface as to cause large voids. So when the bridge was then re-glued, the remaining fibers were forced upward towards with the bridge--along with portions of the bridge-plate. This explains why the bridge-plate was deformed so severely.




After the separated portions of the X braces were re-glued, reconstruction of the soundboard begins. First, the splintered and bent fibers under the soundboard must be restored to their original flat position. This is achieved by fashioning a convex caul carefully shaped to conform to the mangled areas. The object is to force the fibers to return to a flat plane determined by a new rosewood bridge plate.



Above the bridge location, to the left, we see the padded convex exterior bridge caul that will pop the fibers back flat on the underside of the soundboard. Below the bridge location--to the right--mounted on a similarly-shaped clamping caul, a slightly oversized rosewood bridge-plate. It will provide a firm, level platform upon which the soundboard can be reconstructed. Blue tape delineates the area that the bridge-plate will occupy beneath the soundboard. It will help locate the convex exterior caul when gluing the bridge-plate.


A straight-edged object is shown placed on the soundboard after the bridge-patch installation is completed. The undersurface of the soundboard in the bridge region has now been returned to its original, flat configuration. The original void in the soundboard that was caused by the wood lost during the previous, abusive repair-- is now clearly evident. 




With a router and a carefully constructed jig, the damaged, hollowed soundboard surface is leveled, taking care to remove no more material then was necessary to insure a good gluing surface. Since there was more damage to the bass side, the jig was tilted so as not to needlessly remove undamaged material from the treble side. A matching spruce insert (seen here on the right, below the bridge location) is then fashioned to conform to the beveled cavity.


A precise fit is essential for success. There must be wood-to-wood contact only, and no gaps and pockets of glue when the bridge is re-installed. To insure this, the two surfaces must be perfectly flat and free of irregularities. The angles of the insert are adjusted to achieve a perfect fit. During this shaping process the top surface of the insert must not fall below the level of the soundboard.



After being glued in place, the slightly-taller insert is chiseled and sanded to match the surface-level of the soundboard. The remaining damaged surfaces at either side of the new graft will be cleaned of old glue.




With its loose splinters and old glue removed, the remaining damaged surfaces are prepared to replaced with well-fitting spruce veneers. The veneer patches end up slightly higher than the level of the soundboard, to allow for leveling after the glue has dried. The successfully reconstructed soundboard is now ready to re-bridge.


The bridge is reinstalled, the bridge-pin holes are drilled and the instrument is re-strung and set up. With its integrity now restored, the soundboard no longer bulges under tension.




The new rosewood bridge-plate provides a sound footing for the ball ends of the strings. Now the double-wound portion of the string is now contained below the surface of the bridge and away from the saddle.



With the soundboard plane restored, the bridge no longer tips upward and forward. Low action with proper saddle height is once again possible. The guitar's voice is noticeably louder and clearer. The fine tone and long sustain that old Martins are known for has been restored.


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