A piano is a string instrument played with hammers. Each of its 88 keys throws a felt hammer at one, two or three steel strings. About 230 strings pull with nearly 20 tonnes in total, so they are stretched across a cast-iron plate, above a thin spruce soundboard.
Press a key and a chain of levers flings a felt hammer at steel strings pulling with nearly 20 tonnes. Take a grand piano apart in 3D, slow the hammer down 50 times, and play it yourself.
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Hammers, strings and a wooden board
A piano is a string instrument played with hammers. Each of its 88 keys throws a felt hammer at one, two or three steel strings. About 230 strings pull with nearly 20 tonnes in total, so they are stretched across a cast-iron plate, above a thin spruce soundboard.
A chain of levers that lets go
The key lifts a lever called the wippen, which pushes the hammer up through a jack. Just before the hammer reaches the string, the jack trips on a button and slips away, so the hammer flies free, hits and bounces back. Érard's repetition lever of 1821 lets a note repeat before the key has fully risen.
Length, tension and weight set the note
A string's frequency is f = (1/2L)·√(T/μ): halve the length for one octave up, or pull four times as hard. It also vibrates in halves, thirds and quarters at once, and those overtones shape the sound. Stiff steel makes them slightly sharp.
The soundboard makes it loud
A string is too thin to push much air, so on its own it is nearly silent. It presses on a bridge glued to the soundboard, and the board, over a square metre of spruce, pumps the air. The trade-off: a louder note also fades sooner.
88 keys, each 1.0595 times the last
From A0 at 27.5 Hz to C8 at 4,186 Hz, with A4 tuned to 440 Hz. In equal temperament every semitone is the twelfth root of 2, so twelve steps make exactly one octave. Tuners stretch the octaves a little to match the strings' sharp overtones.
Soft, loud, and three pedals
Press faster and the hammer flies faster: louder, and brighter because the felt touches the string for less time. The right pedal lifts every damper, the left shifts the hammers onto fewer strings, and the middle one holds up only the dampers already raised.
Keys, hammers, dampers, strings, an iron frame and a big wooden board.
A piano is a string instrument that you play with hammers. Under the lid are about 230 steel strings. Each of the 88 keys (52 white, 36 black) is a lever that throws a felt hammer up at its strings.
A felt damper rests on top of each note's strings to keep them quiet. Press a key and its damper lifts, so the string can ring. Let go and the damper drops back.
Each string pulls with the weight of a grown-up, about 80 kg. All together that is close to 20 tonnes, so the strings are stretched across a heavy cast-iron plate. Under the strings is the soundboard, a thin sheet of spruce that turns their shaking into sound. The longest bass strings are wrapped in copper and cross over the others. The pedals at your feet move the dampers and the hammers.
A chain of levers throws the hammer, then lets it go.
Each key is a lever on a pivot, the balance rail. Press the front down 10 mm and the back rises. A little post on the key, the capstan, lifts the wippen, which carries the jack. The jack pushes the hammer shank up by its knuckle, and the felt hammer swings up about 45 mm towards the string. All together the levers turn 1 mm of key into about 5 mm of hammer.
If the hammer stayed pushed against the string, it would stop it ringing. So just before it gets there, the jack's toe hits the let-off button and the jack tips out from under the knuckle. The hammer flies free for the last 2 mm, hits, and bounces straight back. This trick is called escapement.
On the way down, the backcheck catches the hammer so it can't bounce up again. Then the repetition lever, invented by Sébastien Érard in 1821, holds the hammer up while the jack slips back underneath. You can play the note again after letting the key rise only halfway. Meanwhile the back of the key has lifted the damper off the string, and it drops back when you let go.
Length, tension and weight: Mersenne’s three laws.
Hit a string and it bends, then springs back, overshoots and swings to and fro hundreds of times a second. That swinging is its frequency, measured in hertz (Hz): swings per second. The higher the frequency, the higher the note.
Three things set it. A shorter string swings faster: half the length, double the frequency, one octave up. A tighter string swings faster too, but only with the square root: four times the tension for one octave. A heavier string swings slower. Marin Mersenne wrote these rules down in 1636: f = (1/2L) × √(T/μ), where μ is the mass of one metre of string.
A string doesn't just swing as a whole. It also swings in halves, thirds and quarters at the same time. These overtones (harmonics) are 2, 3, 4 times the main note, and their mix gives the piano its sound. Steel is stiff, so on a piano they come out a little sharp. Low notes would need impossibly long strings, so the bass strings are wound with copper: heavier, but still bendy.
A thin string can’t push air. A big wooden board can.
A string on its own makes very little sound. It is so thin that the air just slips around it as it swings. Stretch a piano string on a heavy steel frame and you can barely hear it.
So the strings press down on a bridge, a long strip of hard wood glued to the soundboard. The soundboard is a big, thin sheet of spruce, about 9 mm thick and over a square metre in area. The strings shake the bridge, the bridge shakes the board, and the board pushes a lot of air. That is the sound you hear.
Wooden ribs are glued across the grain underneath to make it stiff, and the board is crowned: arched up a few millimetres so it pushes back against the strings. There is a trade-off. A board that gives the string's energy to the air quickly makes the note loud, but it also fades sooner. With no soundboard, a note is quiet but rings for much longer.
From 27.5 Hz to 4,186 Hz, each key 6% higher than the last.
A piano has 88 keys, from A0 at 27.5 Hz to C8 at 4,186 Hz. That's seven octaves and a bit. The A above middle C, A4, is tuned to 440 Hz, and every other note is worked out from it.
Go up one key, white or black, and the frequency goes up by the same small step: × 1.0595, the twelfth root of 2. After 12 steps you have multiplied by exactly 2, which is one octave. This way of tuning is called equal temperament. Every key is equally in tune (and equally slightly out), so music sounds the same in any key.
Behind the keys you can see each note's strings at their real lengths. Low notes get one thick copper-wound string, the middle two, and most notes three, tuned together as a unison so they sound louder and ring longer. Tuners also stretch the octaves a little, low notes slightly flat and high notes slightly sharp, because stiff steel strings have overtones that are a touch sharp.
Hit harder for louder and brighter. The pedals do the rest.
Cristofori called his invention the gravicembalo col piano e forte: a harpsichord that plays soft and loud. On a harpsichord, a quill plucks the string the same way however you press. On a piano, pressing faster throws the hammer faster. A faster hammer makes a louder note, and it's brighter too: the felt squashes harder and touches the string for a shorter time, which brings out the high overtones.
The right pedal (sustain) lifts every damper. Notes keep ringing after you let go, and the other strings are free to join in: a string whose overtones match the note starts to hum along. This is sympathetic resonance, and it gives the pedalled sound its glow.
The left pedal (una corda, "one string") slides the whole keyboard and action a few millimetres to the right. Each hammer now hits only two of its three strings, with a softer part of its felt. The middle pedal (sostenuto) is clever: it keeps up only the dampers that were already up when you pressed it, so you can hold a bass note while playing crisp notes above.
What makes a piano string sound when you press a key?
A felt hammer hits it. The key is a lever that throws a felt-covered hammer at the string. That is why a piano counts as a string instrument and a percussion instrument.
What is the damper for?
It rests on the strings to stop them ringing when you let go. The damper lifts when you press the key and drops back when you let go, which silences the string.
Why does a modern piano need a cast-iron plate?
To hold the pull of all the strings, close to 20 tonnes. Around 230 strings each pull with about 80 kg. A wooden frame alone would slowly bend and the piano would go out of tune.
Why does the jack slip out from under the hammer just before it hits the string?
So the hammer can bounce off and let the string ring. A hammer still pushed against the string would mute it at once. Letting it fly free for the last 2 mm means it hits and rebounds.
What did Érard’s repetition lever (1821) make possible?
Playing the same note again quickly, before the key has fully risen. It holds the hammer up so the jack can reset while the key is only halfway up, so a pianist can repeat a note very fast.
You press the front of a key down 10 mm. About how far does the hammer head travel?
About 45–50 mm. The key, wippen and hammer shank are three levers that together multiply the movement about five times.
You halve the length of a string and keep everything else the same. What happens to the note?
It goes up an octave (double the frequency). Frequency is proportional to 1/L. Half the length, twice the frequency: one octave higher.
How much tighter must a string be pulled to sound one octave higher?
Four times as tight. Frequency grows with the square root of tension: √4 = 2.
Why are a piano’s bass strings wrapped in copper wire?
To add weight without making them stiff, so they can be low but not too long. A heavier string vibrates more slowly. Winding adds mass while the thin steel core keeps it flexible.
Why is a string on its own so quiet?
It is so thin that air slips around it instead of being pushed. A string only pushes a tiny sliver of air. A soundboard has thousands of times more area.
What carries the vibration from the strings into the soundboard?
The bridge. The strings press down on the bridge, which is glued to the soundboard.
Take the soundboard away and strike a note. What happens?
It is much quieter but rings for longer. The soundboard is how the string loses energy to the air. Without it the energy stays in the string: quiet, but long.
A4 is 440 Hz. What is A5, one octave higher?
880 Hz. Each octave doubles the frequency: 2 × 440 = 880 Hz.
In equal temperament, how many semitone steps make one octave?
12. Twelve equal steps of × 1.0595 multiply up to exactly × 2.
Why do most notes on a piano have three strings?
Three strings tuned together are louder and ring longer. The hammer hits all three at once. Together they give more sound, and they pass energy between them so the note lasts.
What does the right (sustain) pedal do?
Lifts all the dampers so the strings keep ringing. With every damper up, notes ring on after you let go, and other strings can resonate in sympathy.
Why does a note played harder sound brighter as well as louder?
The hammer felt squashes harder and touches for a shorter time, bringing out high overtones. A shorter, harder blow puts more energy into the high overtones. Felt gets stiffer the more it is squashed.
On a grand, how does the left (una corda) pedal make notes softer?
It slides the action so each hammer hits fewer strings with a softer part of its felt. Una corda means "one string". Modern grands shift so the hammer strikes two of the three strings.
Words worth knowing
Action
The set of levers between a key and its hammer and damper. A grand piano has 88 of them.
Escapement
The jack slipping out from under the hammer just before it hits, so the hammer flies free and bounces off the string.
Damper
A felt pad resting on a note's strings that stops them ringing when the key is let go.
Overtone
A higher vibration of a string in 2, 3, 4 or more parts, sounding along with the main note.
Soundboard
A thin, wide sheet of spruce under the strings that turns their vibration into sound.
Equal temperament
Tuning in which every semitone is the same ratio, the twelfth root of 2, so twelve make an octave.
Unison
Two or three strings tuned to the same note and struck together by one hammer.
Sustain pedal
The right pedal, which lifts every damper so notes keep ringing and other strings resonate.
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