How Does a Piano Work? Hammers, Strings and the Trick of the Escapement

The keyboard instruments before the piano had a problem each. The harpsichord plucked its strings with a quill, which gave a bright, even sound at one volume only, however the key was struck. The clavichord pressed a small brass blade against the string, which responded to touch but was so quiet it could barely be heard across a room. The organ had volume but no touch at all. A keyboard that could play a note louder or softer according to how hard the key was pressed, and could be heard in a hall, did not exist, and Cristofori's solution was to hit the string with a hammer.

A piano key is a lever. Pressing the front end raises the back, which through a series of smaller levers called the action throws a felt-covered wooden hammer upward at the string. The faster the key is pressed, the faster the hammer flies and the louder the note, which is where the touch comes from. The critical part is the escapement, Cristofori's invention: just before the hammer reaches the string, the lever driving it slips free, so that the hammer completes its journey on its own momentum, strikes, and bounces off. Without escapement the hammer would be held against the string and stop it sounding, as the clavichord's blade did.

At the same time the key lifts a damper, a felt pad that normally rests on the string, so the string is free to ring. Release the key and the damper drops back and the note stops. A repetition mechanism, added in the 1820s, lets the hammer be thrown again before the key has fully returned, which is what allows fast repeated notes. The sequence for a single note:

• •The key goes down and the damper lifts off the string
• •The action accelerates the hammer towards the string
• •The escapement releases the hammer just before contact
• •The hammer strikes, bounces off in about a thousandth of a second, and is caught
• •The string vibrates and passes its vibration through the bridge to the soundboard
• •The key is released and the damper falls back to silence the string

A modern piano has around 230 strings for its 88 keys: the lowest notes have one thick string wound with copper, the middle two, and the upper three, tuned in unison to give more volume and a richer tone. Together they pull on the frame with a force of around twenty tonnes, which is why the frame has been cast iron since the 1820s; earlier pianos were built of wood and could not stand the tension that a concert hall demanded. The strings run over a wooden bridge glued to the soundboard, a thin sheet of spruce nearly the size of the whole case, and it is the soundboard, not the string, that moves the air. A string on its own is almost silent; the board turns its vibration into sound the way a guitar's body does.

The hammer strikes each string at about one seventh of its length, a position chosen to suppress the harsh seventh harmonic, and the felt is graded from hard at the bass to soft at the treble and worked over by a technician with needles to shape the tone. A piano's voice is mostly in its felt.

The right pedal lifts all the dampers at once, so that every string keeps ringing after the key is released and strings not struck vibrate in sympathy with those that are, which gives the washed, resonant sound of Romantic piano music. The left pedal, on a grand, shifts the whole action sideways so the hammers strike two strings instead of three and hit them with a softer, less-used part of the felt; on an upright it moves the hammers closer to the strings so they strike with less force. The middle pedal, where there is one, holds up only the dampers of notes already being played, letting a bass note ring under a passage played staccato above it.

A grand piano lies flat, its strings horizontal and its hammers striking upward and falling back by gravity, which gives the most responsive action; an upright stands the strings vertically to save space and returns the hammers with springs, which is why an upright's repetition is a little slower. Digital pianos replace all of it with a weighted keyboard and recordings of a real instrument played back at the velocity sensed, and the best of them are now hard to tell from the real thing through headphones. What they cannot do is move a soundboard, and a concert grand at full volume is still one of the few instruments that can be heard unamplified against a symphony orchestra.

A piano's twelve notes per octave are tuned in equal temperament, which divides each octave into twelve identical steps. Each semitone is higher than the last by a ratio of about 1.0595, the twelfth root of two, so that twelve steps exactly double the frequency. The advantage is that every key sounds equally in tune, so music can move freely between keys. The cost is that almost no interval is perfectly pure: a major third on a piano is noticeably wider than the simple ratio a choir or string quartet might naturally sing, a compromise most listeners never notice.

There is a second, subtler adjustment. Piano strings, especially the short, thick ones at the extremes, are stiff, and stiffness makes their overtones run slightly sharp of the ideal whole-number multiples. If a tuner set each octave to an exact doubling, the upper note would clash with those sharpened overtones of the lower one. So tuners stretch the octaves, tuning the treble a little high and the bass a little low, by amounts that depend on the individual instrument. That is one reason short upright pianos, with stiffer strings, sound different from long concert grands, and why tuning remains a skilled craft.

The piano's mechanism was built to keep the player's hands away from the strings, and twentieth-century composers took pleasure in getting round it. The American composer Henry Cowell, in the 1910s and 1920s, wrote tone clusters to be played with the flat of the hand or the whole forearm, pressing down a whole block of neighbouring keys at once. In The Banshee, from 1925, the pianist does not touch the keys at all. An assistant holds down the right pedal to lift every damper, while the performer reaches into the case and sweeps, scrapes and plucks the bare strings, producing wailing sounds no hammer could make.

Cowell's student John Cage went further. In 1940, asked to write music for a dance performance in a space too small for a percussion group, he pushed screws, bolts, pieces of rubber and other objects between the strings of the piano, so that each key produced its own thud, rattle or muted gong-like tone. He called the result a prepared piano and wrote precise instructions for where each object should go, measured in inches from the dampers. His Sonatas and Interludes of the late 1940s remains the best-known work for it, and preparing an instrument to play it can take hours.

Cristofori's pianos had a range of about four octaves, and for much of the eighteenth century five octaves, some 61 keys, was normal; it is the range Mozart wrote for. As makers in Vienna and London built stronger instruments, the range kept extending at both ends. Beethoven's career tracks the growth. In 1818 the London firm of Broadwood presented him with a six-octave grand, heavier and louder than his Viennese pianos, and his late works use a deeper bass and higher treble than his early ones could. By the later nineteenth century makers had settled on 88 keys, just over seven octaves.

The 88 keys run from a low A vibrating about 27.5 times a second to a high C at about 4,186 times a second, a range that covers the fundamental pitch of every note in a symphony orchestra, from the bottom of the double bassoon to the top of the piccolo. Below the lowest A the ear hears pitch less clearly and more as a rumble, and above the top C notes sound thin and lose their sense of pitch. A few makers still go beyond: the Austrian firm Bösendorfer builds a concert grand with 97 keys, extending the bass down to a low C.

A piano key throws a felt hammer at a string through a chain of levers, and the escapement releases the hammer just before impact so it strikes freely and bounces off, which is what lets the instrument play loud or soft according to touch. Dampers silence the string when the key is released, an iron frame holds twenty tonnes of string tension, a spruce soundboard turns the vibration into sound, and the pedals lift the dampers, soften the strike or sustain chosen notes.