Inside the Piano: How It Works

When a pianist presses a key, it sets off a sequence of motions that lasts a fraction of a second and involves hundreds of parts made of wood, felt, metal, and leather. In less than the blink of an eye, a lever becomes a hammer, the hammer becomes a vibration, and the vibration becomes a sound that fills the room. It is worth following the path of that energy from the fingertip to the ear.

The key and the action

The key is a lever resting on a pivot point. When it is pressed, it pushes a set of parts called the action. In a standard modern piano, each of the 88 keys has its own action, made up of dozens of components, whose purpose is to throw the hammer at the string and then bring it back.

The most ingenious feature is the escapement: just before reaching the string, the hammer disengages from the mechanism and travels on its own momentum. If it kept being pushed, it would stay against the string and choke the sound. After rebounding, it falls back and is caught by a supporting piece, ready to act again. This principle comes from the piano's inventor, Bartolomeo Cristofori, and was refined by later makers; in the nineteenth century, the Frenchman Sebastien Erard patented the double escapement, which lets a note be repeated quickly without the key returning all the way up. This history is also covered in keyboard instruments: mechanisms and differences.

Hammers and strings

The hammer is a wooden piece covered with compressed felt. The hardness of the felt affects the tone: soft felt produces a darker, rounder sound, while hardened felt gives a brighter one. The technician adjusts the hardness with needles or special products in a procedure called voicing.

The strings are made of steel. In the bass, they are wound with copper wire to add mass without requiring excessive length. In the middle and treble, each hammer strikes two or three strings tuned in unison. In all, a concert piano has more than two hundred strings, and their combined tension is on the order of tons, which is why the structure has to be so robust.

The length, thickness, and tension of each string determine the pitch of the note. A long grand piano allows longer bass strings, which usually results in richer low notes.

Frame, bridge, and soundboard

The tension of the strings is carried by a cast-iron plate and a reinforced wooden frame. At one end, the strings wrap around tuning pins set in a block of dense wood; this is where the tuner turns the pins to adjust the tension.

At the other end, the strings pass over the bridge, which supports them and transmits their vibration to the soundboard, a large spruce panel beneath the strings. It is the soundboard, not the strings, that moves most of the air. Its area lets it radiate the frequencies efficiently, and wooden ribs glued underneath give it stiffness and shape. The design resembles what happens in other stringed instruments, such as the acoustic guitar, where the body amplifies sound the string could never project alone.

Dampers and pedals

Each string has a damper, a felt pad that rests against it to silence the note as soon as the key is released. While the pianist holds the key down, the damper stays lifted and the string vibrates freely.

The pedals change this behavior:

1. Sustain pedal (right): lifts all the dampers at once, so notes keep sounding and other strings vibrate in sympathy.
2. Una corda pedal (left): shifts the action so that the hammers strike fewer strings, or strike them with a different part of the felt, which softens the sound.
3. Middle pedal: on grand pianos, it keeps lifted only the dampers of notes already held down when the pedal is pressed. On uprights, its function varies by model.

Grand piano and upright piano

In the grand piano, the strings lie horizontally and the hammer falls back by gravity, which makes fast repetition easier and offers subtle control. In the upright piano, the strings run vertically and the hammer is pulled back by springs, which saves space. The principle of sound production is the same, but the feel of playing and the dynamic range can differ.

Tuning and maintenance

With so many strings under tension, a piano gradually goes out of tune, especially with changes in temperature and humidity. The tuner adjusts the strings with a special wrench, taking the note A, commonly tuned to 440 hertz, as the reference. There are also small corrections between the low and high registers so that the whole instrument sounds balanced to the ear. The subject is explored further in musical tuning and the role of reference pitches.

Beyond tuning, the technician regulates the action and voices the hammers. Avoiding direct sun, drafts, and damp places, and cleaning the instrument carefully, extends its life, as explained in basic care for musical instruments.

Why touch changes the timbre

On a piano, playing louder does not simply mean turning up the volume. The faster the hammer is moving when it hits the string, the more high harmonics are excited, and the brighter and sharper the sound becomes. Playing softly produces fewer harmonics and a darker tone. The pianist therefore controls the color of the sound with the same action that controls its loudness, and the response of the action is as important as the quality of the strings. This relationship among force, harmonics, and color is part of what defines the timbre of the instrument.

A note on regulation and the player's experience

Much of what a pianist feels under the fingers has little to do with the strings. It depends on regulation, the fine adjustment of the many small parts in the action so that every key has the same depth, the same weight, and the same point at which the hammer lets go. When regulation drifts, some notes feel heavy or sluggish, others fail to repeat, and the keyboard loses its evenness. That is why a well-maintained piano feels responsive even when it is not especially expensive, and why a neglected one can be frustrating no matter how good its materials are.

Where to Go From Here

If you have access to a piano, lift the lid (or watch a grand piano with the lid open) and see the hammers move while someone plays a slow scale; then try pressing the right pedal and notice the effect. When you read music, pay attention to how writing for the piano exploits these possibilities, as described in an introduction to reading sheet music. To check what you have learned, test your knowledge in the quiz.