Fig 8Bridge and soundboard← → tension · space to pluckstrings slack · board at rest
The bridge, carrying the strings' downbearing into the boardCrowned spruce board and its four ribsSix strings, 1.2° down over the bridge each sideThe strings don't run straight over the bridge: they come down to it and rise again, about 1.2° on each side, so each string presses the bridge into the board. At 80 kgf a string, that is 3.35 kgf per string and 20.1 kgf for these six; over a whole piano it comes to a few hundred kilograms. The board is built to carry it: its spruce is crowned (bowed up 2.8 mm here, a 16 m radius) and held in that arch by ribs across its grain. Brought up to pitch, this section gives 0.25 mm. A plucked string then rocks the bridge with its changing slope, a few micrometres (about 3 µm for a 1 mm pluck), and the whole board moves with it. A board's area moves far more air than a string's thin wire, and that is the sound you hear. The motion is drawn 100× for the sag and 1500× for the vibration; the dial reads the true value, 0.001 mm a division.