(Reprinted from an article by Larry Fine)
Solid spruce soundboards swell and shrink with seasonal changes in humidity and, over time, can develop cracks. One of the problems that comes up most frequently in buying a used piano is judging the significance of a cracked soundboard.
Contrary to popular belief, cracks in the soundboard, while often unattractive, are not necessarily important, as long as the tone is acceptable. Very extensive cracking, however, can indicate that the piano has suffered great climatic extremes, and that its life expectancy may be short. In such a case, other symptoms of this will usually be evident elsewhere in the piano. If the cracks have been filled with wooden shims, this means that, at some point, the piano was rebuilt and the cracks repaired.
The ribs run perpendicular to the grain of the soundboard, and therefore perpendicular to any cracks. Any separation of a rib from the soundboard at a crack is a potential source of buzzing noises. A piano with a cracked soundboard should be carefully checked for rib separations before purchase. Repair of rib separations can usually be done at reasonable cost without rebuilding the piano.
When manufactured, the soundboard has built into it a curvature or crown. In a traditionally made, solid spruce soundboard, the crown is maintained by the compression of the wood fibers, whose elasticity causes the crowned soundboard to push back against the downbearing pressure of the strings on the bridges. Together, these two opposing forces enhance the tone of the piano. Over many years, because of the drying out of the wood and the loss of the wood's elasticity, the soundboard loses some or all of its crown, a condition that can be accompanied by the appearance of cracks.
A related condition is that of compression ridges. When a soundboard's compression exceeds the elastic limit of the wood fibers, those fibers may become crushed, producing slightly raised ridges in the soundboard's surface. This can happen, for example, in humid climates, or due to conditions related to the soundboard's manufacture. Compression ridges are quite common, and do not necessarily affect the piano's tone. However, when crushed, wood fibers lose their elastic properties, so the compression ridges are likely to turn into cracks as the soundboard's crown diminishes over time.
Although, in theory, cracks and a loss of crown should result in a deterioration of tonal quality, the actual results vary greatly from piano to piano; therefore, the tone quality of each such instrument must be evaluated on its own merits. In addition, your tolerance for such imperfections will depend on how expensive the piano is, and on your use of and expectations for it.
For more information on this subject, see The Piano Book.