Piano sound boards are curved to get a projected robust sound. Here’s a small demo of the tone difference between non-curved, and then bent or curved piece of sound board giving you an audible example.
Old pianos nearing 80 to 100 years old can sound dull or seem to miss that depth of tone (and lack of sustain), because over the years the pianos lose their crown with the 1000 pounds (about) down bearing force the strings produce. Even though there are ribs under the sound board trying to hold it up, it still gives out over time.
When you lose sustain, you need to play a piano faster to sound musically correct, because if you played it slower the sound would die out quickly causing there to be gaps or a decayed sound to your piano piece. It would be really hard to romance a song if you are constantly playing fast.
This video lets you hear the difference between an old Knuckle (clicking sound) and a new Knuckle (quiet). The reason to change out this particular part on a piano is to get rid of an annoying click heard on every note you play. When I install a player system it seems to amplify those bad parts due to the way the player resets the key quickly, as it’s preparing to play the next key. This one part alone (if bad and changed out) can actually restore the beautiful sound that a piano once had. An interesting side note is that brand new (inexpensive) baby grand pianos can have a loud knuckle right out of the box (I guess they had to cheapen up somewhere). Our goal is to locate the part that’s substandard and change it out so you can get the most out of your investment. We’ve had pianists want to hug us when we’ve done this upgrade for them, as they can romance a piano piece when action noise is at a minimum.