Antique pianos sound old because the soundboard, bridges, hammers and strings are no longer working properly. Often in the art of rebuilding, hammers and strings are replaced with new ones, but the soundboard and bridges just seem to be treated with a band-aid mentality.
There is more that meets the eye when it comes to acoustic piano soundboards. Visit- Voicy
First the Shape.
Most people are surprised to learn a piano soundboard is arched in shape. The arch is so small however, that the soundboard may look flat, but they are made so that the center is roughly 1/8″ higher than the outer edges. After the strings are installed, and the downward pressure is exerted on the soundboard, the arch can be as small as 1/16″.
The balancing act.
When the soundboard is doing its job correctly, it is moving freely while holding up a thousand pounds in the form of downward pressure. Now here is an amazing fact- Soundboards are purposefully under engineered. If it was engineered correctly to hold that thousand pounds throughout its existence, the support beams (ribs) would be much more massive, and the soundboard would lose it freedom of movement. Translation – It would sound awful. So what trickery did engineers use to ensure that a soundboard will be strong enough and have freedom of movement?
An invisible force.
Piano makers used wood’s natural characteristic of shrinking and expanding to advantage. When a new soundboard is made the moisture in the wood is removed, and the soundboard is in this shrunken state when it is glued into the piano. The board will then grow as it takes on moisture. But since it is confined in the piano as it grows, it will not grow physically, but internally as a hygroscopic force. This force adds the remaining strength the board needs to hold up the thousand pounds.
But there was a price to pay for freedom.
Hygroscopic force is temporary. I like to compare the piano soundboard in this regard to a tennis ball. A new ball is full of bounce and energy, but after being struck repeatedly during the course of the game, the energy dissipates and the ball becomes “old” with less energy. The piano soundboard behaves exactly like that. As the soundboard goes through time, it expands and contracts and fatigues.
How Long do they last?
No one can say for sure how long in years a soundboard can last. History shows that the sounds character changes around 75 years ( or even before) and most are toast at 100 years. The signs are flatness, cracks, and a short sustain. That’s the “old” and tired sound an antique piano has. Simply repairing a crack is cosmetic, and doesn’t address the real problem of fatigue.