From key to sound: what happens inside the piano

How does a piano work? Every press of a key sets off a short, fast chain: a lever passes the movement to a felt-covered hammer, the hammer strikes steel strings, and their vibration passes to a large wooden board that turns it into a full sound. The instrument's original name, pianoforte, joins the Italian for "soft" and "loud", because ever since it was built in Florence in the early 18th century it has responded to every strength of touch.

Inside an acoustic piano thousands of parts are at work, and each key has dozens of parts of its own. We will go through them in the order in which they awaken, from silence to the sound that reaches you.

The silence before the sound

When no one is playing yet, the piano is silent thanks to the dampers. On the strings of almost every key sits a small block of felt that keeps them still. When you press the key, the action lifts the damper in the same movement that drives the hammer, and the string is free to sing. When you release, even only part of the way, the damper returns to its place and the sound stops at once.

In the very highest notes the strings are so short that the sound fades by itself, so there the piano manages without dampers. The right pedal lifts them all at once, and we wrote about it and the other two pedals in the piano pedals guide.

From the key to the soundboard

The key itself is a wooden lever that rocks on a pivot. You press the front, the back rises, and a small screw on it pushes the assembly of levers above. At the heart of the assembly is the jack, a small L-shaped arm that pushes the hammer toward the string. The levers multiply the movement, so a few millimetres of key become a long, fast swing of the hammer. The speed of the press sets the speed of the hammer, and from it comes the volume of the sound.

Just before the strike, the jack meets an adjusting button and slips aside. The hammer continues on its own momentum, strikes the string and bounces straight back, so the string keeps vibrating even while the finger is still on the key. On the way back a small check catches it, and it waits for the next strike. This assembly is called the action.

A string on its own makes a weak sound. It passes over a wooden bridge glued to the soundboard, a thin, wide board of spruce, and through it the vibration spreads to the whole board and to the air around it. The wood amplifies the low frequencies and mutes the metallic ring of the upper ones, which is why an acoustic piano's sound comes out round and warm. The longer and thicker the string, the lower the sound, and the long bass strings are wound with copper wire. Behind the 88 keys about 230 strings are stretched: three per note across most of the keyboard, and in the bass they reduce to two and finally to one.

A frame that holds tonnes

Every string is held under tension, and together the strings pull with a force of about 20 tonnes. This force is taken by a cast iron frame, the gold-coloured one revealed when you lift the lid. In 18th-century pianos the frame was wooden and the strings were stretched gently, and the move to metal in the 19th century opened the way to today's powerful sound.

One end of each string is wound around a tuning pin, set into a multi-layered block of wood and held in it by friction. Tuning a piano returns each string to its exact pitch. The wood in a piano responds to humidity, so stable humidity helps the whole system work precisely, as we detailed in the care and humidity guide.

Grand, upright and digital piano

In a grand the strings and soundboard lie horizontally, and the hammer rises to the string from below and returns by gravity. A special repetition lever holds it halfway, so you can play the same note again even before the key has fully risen, up to about 15 times a second. A grand's keys feel slightly heavier, because its rotating parts carry more momentum. More on it: how to choose a grand piano.

In an upright the whole system stands vertically, the hammer moves toward the string from the side, and small springs and cloth tapes help it return. That way the whole instrument stands flush against the wall, and there is more about it in the how to choose an upright piano guide. On a digital piano, the hammer-action keyboard imitates the feel with moving weights, heavier in the bass and lighter in the treble, and we wrote about it in the digital piano guide.

Seeing it up close

The most beautiful way to understand a piano is to see it in motion. At the store and gallery at 28 Ben Atar we will gladly lift the lid of an upright and a grand, play a few notes and show you the hammers and dampers in action. And when your piano needs attention to a hammer, a damper or a key, the repair and restoration are done in your home. We are here for you.

Links: Piano pedals · Piano tuning · Care and humidity · How to choose a grand piano · How to choose an upright piano · Digital piano

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