To understand why a piano needs a soundboard, wave one finger in front of your face.  Now wave your entire hand.  Which one moved more air?

Sound is vibration in air, so whatever moves more air will transmit more sound.  A string is like your finger waving in front of your face:  if you’re close enough, you can tell that something is happening.

But when you want to move enough air to fill a room or an auditorium with sound, then you need a soundboard.  That very thin, extremely sensitive sheet of wood can transmit an amazing range of pitches and intensities from the strings, via the bridges.

The vibrations of a soundboard are similar to the ripples made by a pebble tossed into a pond.  Each string being played is a separate pebble, however, and the patterns of ripples multiply, cross, and affect each other to create the sound of an acoustic piano.  There’s nothing else like it!

A soundboard does more than simply move air, however.  It also creates vibrations in strings that are not being played.  If vibrations move from the strings through the bridges to the soundboard, then they can also travel back the other way, as long as the dampers are not touching the strings and impeding their vibration.

To hear that, play and hold one well-tuned note in the middle of your piano.  Listen to the sound.  Now press down the damper pedal while you play and hold that same note.  The sound will be fuller, richer, and more textured the second time (just how much depends on the soundboard’s condition). 

Although you played only one note, vibrations in the soundboard caused all the strings on the piano to vibrate in response - and you could hear them because their dampers were raised.  Only an acoustic instrument can give you that kind of sound and control.

What about cracks in the soundboard?  A cracked soundboard often indicates that the piano’s environment is too dry, which can lead to many other problems.  But the crack itself is usually only a problem if it causes a buzz.  And that can be fixed by your technician.

So ask you technician about your piano’s soundboard and its condition.  Your technician can measure the soundboard’s “crown” (the curve that makes it so sensitive), clean and inspect the soundboard and its supporting ribs, and tell you much more about your piano.