Why Are Piano Hammers Covered with Felt?

When a key is pressed, the action throws a wooden hammer towards the string and then lets it escape so the string can vibrate freely. Bare wood striking steel would make an extremely brief, hard contact, bringing out a sharp attack and more mechanical noise. Covering the hammer with felt does not merely make the piano quieter. It creates a contact surface that yields on impact, springs back quickly and can be controlled by the player.

That controllable variation is the point. Yamaha compared ordinary felt with urethane foam, corrugated cardboard, rubber, leather and papier-mâché. Changing the material altered loudness, brightness, rebound and the ease of repeating notes. Leather came closest to felt, yet it did not change tonal character with touch as distinctly. Felt therefore acts as a finely judged interface: a gentler blow can remain rounded, while a faster blow becomes brighter and more penetrating without preventing the hammer from returning promptly.

When listening to a piano, it is worth noticing more than whether one note is loud or soft. Hear how its texture changes at different dynamics. A pianist is not controlling decibels alone; through the resilience of hammer felt, finger speed becomes tonal shading. Felt gradually compacts and develops string grooves, so a technician's voicing work helps preserve this responsive boundary between mechanism and sound.

https://www.yamaha.com/en/musical_instrument_guide/piano/mechanism/mechanism002.html
https://www.yamaha.com/en/musical_instrument_guide/piano/mechanism/mechanism005.html