The physics of playing guitar – Oscar Fernando Perez


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 intune. 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.