The large sounding wooden surface in the piano under the strings is the most susceptible to cracking. Soundboards are made of glued spruce planks and shrink if the humidity is below 50%, which can lead to cracks.
Cracks in the soundboard can cause the manufacturer's intended curvature of the soundboard and thus the bridge pressure to diminish. The sound volume and purity of sound are reduced and this can lead to background noise (clanking, etc.).
Older pianos often have cracks in the soundboard due to phases of excessively dry room air and can still sound very beautiful. However, this must be taken into account in the current value of the instrument. Through cracks are generally more critical than openings on one side only or hairline cracks with dust deposits that run in the grain of the wood. Cracks under bridges are generally more critical.
In the course of renovating the acoustics, a soundboard that already has cracks is usually intentionally dried out to provoke further cracking and repaired with shavings. The soundboard is then sanded, bleached if necessary and resealed with shellac or similar. A soundboard renovated in this way should also be able to withstand shorter phases of slightly dry room air. If the air humidity is too low for a longer period of time, even a renovated soundboard will not remain undamaged.
