Why a piano's strings need a soundboard, per Yamaha's guide

A key sends a hammer up into the strings, and Yamaha notes that this alone produces only a soft sound. One end of each string rests on a bridge attached to the soundboard, and the vibrating board is what moves the air.

Open a grand and the strings and metal frame draw the eye first, but the soundboard is the wooden board beneath them, not the frame. Yamaha's manufacturing guide describes it as spruce planks glued edge to edge, with ribs on the underside running perpendicular to the grain so that vibration spreads evenly and the board is stiffened, and with a slight dome arching up toward the strings.

Yamaha also ran an informal hammer test, replacing the felt with five other materials. Leather sounded closest to felt, but with a leather hammer the tone barely changed however hard the key was pressed. With felt, harder playing changed the character of the note as well as its loudness.

The listening link is Hélène Grimaud playing Liszt's piano transcription of Bach's Prelude and Fugue in A minor. It is not a demonstration of Yamaha's test; use it to listen for colour as well as volume.