PIANO
The soundboard is crucial
Piano strings pass over a bridge, and energy is coupled from the strings into the soundboard, a crucial element in the piano's sound.
Follow the vibration
THE PHYSICS OF SOUND
The string sings,
the board speaks.
A string on its own hardly makes a sound.
It's thin and slips through the air without disturbing much of it. In a piano or an acoustic guitar, the vibration passes through a bridge into a large, light wooden board, and that board moves enough air for us to hear.
2.5 mm of spruce does the talking
Follow the vibration- String to bridge
- Bridge to board
- Board to air
- No power socket required
THE SHORT ANSWER
Big surfaces
push air.
A sound wave is a disturbance of the air.
The soundboard gives the string's energy a big surface to push with.
GUITAR
A thin, springy top
A guitar's top plate is usually spruce or another light, springy wood about 2.5 mm thick, braced inside to keep it flat.
See the numbers
PATTERNS
Sand shows the shape
Sprinkle sand on a vibrating plate and it gathers at the nodes, the lines that stay still, drawing a Chladni pattern.
Read the patterns
FOUR THINGS TO TRY
Hear the board
at work.
A few simple experiments from the acoustics pages show how much the body and the air inside it matter.
Feel the top
Rest a fingertip lightly on a guitar's top while you play a low note and feel it vibrate.
Sing into the hole
Sing a low note near the sound hole: at one pitch the air inside resonates and you'll hear it bloom.
Play unplugged
Strum an electric guitar with no amplifier and notice how little sound a body that barely moves makes.
Try the soft pedal
On a piano, the left pedal shifts the keyboard so the hammers strike fewer strings, giving a softer sound.
TWO NUMBERS WORTH KNOWING
Huge forces,
delicate wood.
The pull of all the strings in a grand piano,
and the thickness of a typical guitar top.
≈ 30 tons
Total string tension in a grand piano
A grand piano has about 230 strings for its 88 keys, held in a cast iron frame strong enough to take the tension. Iron frames arrived in 1820.
PIANO · HYPERPHYSICS
≈ 2.5 mm
Typical thickness of a guitar top plate
Usually spruce or another light, springy wood, braced underneath so it stays flat while still free to vibrate up and down.
GUITAR · UNSW
The piano's 88 keys span 27.5 Hz to 4186 Hz. Hammers strike about a seventh of the way along the string, to discriminate against an out-of-tune 7th harmonic.
FOLLOW THE VIBRATION
Which part
makes the sound?
Pick a part of a stringed instrument to see what it does. The bar shows how much of the sound it radiates into the air.
Based on the University of New South Wales music acoustics pages and HyperPhysics (Georgia State University).
CHLADNI PATTERNS
Drawing sound
with sand.
When a plate vibrates at a resonance, parts move up while others move down. The dividing lines, called nodes, stay still, so sand shaken off the moving regions collects there.
A guitar top's patterns are more complicated than a simple rectangle's because of the sound hole and the bracing. The patterns show the geometry of the different ways a top plate vibrates.
Questions
Questions people ask
What does a piano soundboard do?
Piano strings pass over a bridge, and energy is coupled from the strings into the soundboard, which HyperPhysics calls a crucial element in the piano's sound.
Does a guitar body amplify the sound?
Not in the technical sense. All the sound energy comes from the player's finger; the body makes the conversion of string vibration into sound more efficient.
Why is an electric guitar so quiet unplugged?
Its strings make hardly any sound on their own, and very little of their vibration is turned into sound by the body.
What wood is a guitar top made from?
Usually spruce or another light, springy wood, about 2.5 mm thick, with braces inside to strengthen it and keep it flat.
What is a Chladni pattern?
The pattern sand makes on a vibrating plate. It collects along the nodes, the lines that stay still, revealing the shape of the vibration.
Sources
- HyperPhysics (Georgia State University): The piano
- UNSW Music Acoustics: Introduction to guitar acoustics
- UNSW Music Acoustics: Guitar plates and Chladni patterns
- UNSW Music Acoustics: Strings, standing waves and harmonics
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