Explainer #5
How does a piano key make a sound?
When you press a key, it tips like a seesaw and pushes up a part called the jack, which lifts the hammer. Just before the hammer reaches the string, the jack slips out from under it. The hammer flies the last bit on its own, hits the string and bounces straight back, so the string can ring. At the same time, the far end of the key lifts the damper off the string. When you let go, the damper drops back and the sound stops.
The principle
The hammer is thrown, not pushed. The key is a lever resting on a balance point. Pressing the front lifts the back, and through a chain of small levers the hammer moves several times faster than your finger, so a short key movement swings it a long way.
The clever part is the escapement. The hammer is lifted by the jack, a small sprung lever under the hammer. Shortly before the hammer reaches the string, the jack’s toe touches the let-off button and the jack tips away from under the hammer. From then on nothing holds the hammer: it travels the last few millimetres on its own momentum, strikes the string and bounces back immediately. If it stayed against the string, it would stop the vibration at once and you would hear a dull thud instead of a note. On the way down it is caught by the backcheck, so it cannot bounce up and hit the string twice.
Every string also has a damper, a felt pad resting on it. While the key is down, the back of the key holds the damper up through a damper lever, so the string can ring for as long as you hold the key. When you let go, the damper falls back onto the string and silences it. (The right pedal lifts all the dampers at once.) How hard you press only changes how fast the hammer is thrown, and that is what makes the note louder or softer.
Common myth
The hammer is pressed against the string
It feels as if you are pushing the hammer into the string, but you are not. The escapement disconnects the hammer from the key just before it hits, so at the moment of impact the hammer is flying freely. That is why holding a key down harder after it has sounded does nothing to the note: the hammer has already bounced back and is resting on the backcheck. Only the speed of the hammer at impact sets the loudness and the tone.How this video was made: Rendered 3D side view of a simplified grand piano action for one key. The parts are not to scale, the number of levers is reduced, and the first press is shown in slow motion.
