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  • Hendrix, Cobain and Page.

  • They can all shred,

  • but how exactly do the iconic contraptions in their hands

  • produce notes, rhythm, melody and music.

  • When you pluck a guitar string, you create a vibration called a standing wave.

  • Some points on the string, called nodes, don't move at all,

  • while other points, anti-nodes, oscillate back and forth.

  • The vibration translates through the neck and bridge to the guitar's body,

  • where the thin and flexible wood vibrates,

  • jostling the surrounding air molecules together and apart.

  • These sequential compressions create sound waves,

  • and the ones inside the guitar mostly escape through the hole.

  • They eventually propagate to your ear,

  • which translates them into electrical impulses

  • that your brain interprets as sound.

  • The pitch of that sound depends on the frequency of the compressions.

  • A quickly vibrating string will cause a lot of compressions close together,

  • making a high-pitched sound,

  • and a slow vibration produces a low-pitched sound.

  • Four things affect the frequency of a vibrating string:

  • the length, the tension, the density and the thickness.

  • Typical guitar strings are all the same length,

  • and have similar tension, but vary in thickness and density.

  • Thicker strings vibrate more slowly, producing lower notes.

  • Each time you pluck a string,

  • you actually create several standing waves.

  • There's the first fundamental wave, which determines the pitch of the note,

  • but there are also waves called overtones,

  • whose frequencies are multiples of the first one.

  • All these standing waves combine to form a complex wave with a rich sound.

  • Changing the way you pluck the string affects which overtones you get.

  • If you pluck it near the middle,

  • you get mainly the fundamental and the odd multiple overtones,

  • which have anti-nodes in the middle of the string.

  • If you pluck it near the bridge, you get mainly even multiple overtones

  • and a twangier sound.

  • The familiar Western scale is based on the overtone series of a vibrating string.

  • When we hear one note played with another that has exactly twice its frequency,

  • its first overtone,

  • they sound so harmonious that we assign them the same letter,

  • and define the difference between them as an octave.

  • The rest of the scale is squeezed into that octave

  • divided into twelve half steps

  • whose frequency is each 2^(1/12) higher than the one before.

  • That factor determines the fret spacing.

  • Each fret divides the string's remaining length by 2^(1/12),

  • making the frequencies increase by half steps.

  • Fretless instruments, like violins,

  • make it easier to produce the infinite frequencies between each note,

  • but add to the challenge of playing in tune.

  • The number of strings and their tuning

  • are custom tailored to the chords we like to play

  • and the physiology of our hands.

  • Guitar shapes and materials can also vary,

  • and both change the nature and sound of the vibrations.

  • Playing two or more strings at the same time

  • allows you to create new wave patterns like chords and other sound effects.

  • For example, when you play two notes whose frequencies are close together,

  • they add together to create a sound wave whose amplitude rises and falls,

  • producing a throbbing effect, which guitarists call the beats.

  • And electric guitars give you even more to play with.

  • The vibrations still start in the strings,

  • but then they're translated into electrical signals by pickups

  • and transmitted to speakers that create the sound waves.

  • Between the pickups and speakers,

  • it's possible to process the wave in various ways,

  • to create effects like distortion, overdrive, wah-wah, delay and flanger.

  • And lest you think that the physics of music is only useful for entertainment,

  • consider this.

  • Some physicists think that everything in the universe

  • is created by the harmonic series of very tiny, very tense strings.

  • So might our entire reality

  • be the extended solo of some cosmic Jimi Hendrix?

  • Clearly, there's a lot more to strings than meets the ear.

Hendrix, Cobain and Page.

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B2 US TED-Ed string sound pluck guitar wave

【TED-Ed】The physics of playing guitar - Oscar Fernando Perez

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    羅紹桀 posted on 2015/09/21
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