You tune your guitar every day. You pluck a string, it vibrates, it sounds. But do you know what PHYSICALLY happens when you play an A? Why that note and not another? In the next few minutes, you’ll understand. And you’ll look at your guitar differently after.
Direct answer
Sound is a vibration. When you pluck a guitar string, it vibrates and pushes the air around it. This vibration travels to your ears. The number of vibrations per second (the frequency, measured in Hertz) determines whether the sound is low or high. An A string vibrates at 110 Hz. At the 12th fret, it vibrates twice as fast (220 Hz), that’s the octave. And harmonics, these additional vibrations at double, triple, quadruple the base frequency, give your instrument its unique color: that’s the timbre.
1. Vibration: when you pluck a string, you create motion
Grab your guitar. Play your low E string, the thickest one, open. And watch it. You can see it vibrate. That back-and-forth movement, very fast, is vibration. It pushes the air around it. The air transmits this movement to your ears. And your ears turn it into sound.
That’s what sound is: air in motion. Not magic, pure physics.
By the way, guitar is one of the few instruments where you can SEE the sound being produced. That blur on the string when you play? That’s vibration in action. Literally, you see sound being born under your fingers.
2. Frequency: why an A sounds higher than a low E
Now, THE question: why does your A string sound higher than your low E string?
Because it vibrates faster. It’s that simple.
The number of vibrations per second has a name: frequency. And we measure it in Hertz (Hz). Your open A string vibrates 110 times per second. 110 Hz. Your low E string, 82 Hz. The higher the frequency, the higher the pitch.
And that’s why your bass strings are thicker and longer. More mass means slower vibration. Makes sense, right?
Common self-taught mistake
Many guitarists think “low” means “louder.” No. Low means slower vibration (lower frequency). Volume is something else: it’s amplitude, the width of the string’s movement. You can play a low note quietly, or a high note loudly. These are two completely independent dimensions.
When your tuner displays “A 440 Hz,” it means your string vibrates 440 times per second. It’s physics, it’s precise. Not feeling. Now you know what that value means.
3. The octave: why the 12th fret is exactly double
Ever noticed? Your open A string and your A string at the 12th fret sound… the same, but higher.
That’s the octave. And it’s mathematical.
When you play your string open, the entire length of the string vibrates. 110 Hz for an A. Now, when you press down at the 12th fret, you divide the vibrating length exactly in half. A string twice as short vibrates twice as fast. 110 Hz becomes 220 Hz.
The octave is exactly the doubling of frequency. Same note, doubled frequency, higher pitch.
And here’s something cool: the natural harmonic at the 12th fret, you might already play it. That crystalline sound you get by lightly touching the string right above the 12th fret, without pressing down? That’s exactly this first harmonic: double the frequency of your open string. The 110 Hz A becomes a 220 Hz A.
4. Harmonics: why your guitar doesn’t sound like a piano
Last key, and it’s the coolest.
When you play an A on your guitar, your string doesn’t ONLY vibrate at 110 Hz. Actually, it also vibrates at double (220 Hz), triple (330 Hz), quadruple (440 Hz), and so on.
These additional vibrations are quieter, but they’re there. We call them harmonics.
And it’s precisely THESE harmonics that explain why your guitar and a piano don’t sound the same, even when they play the exact same note. The note is the same (the base frequency, the fundamental, is identical), but the harmonic mix is different.
That’s what gives sound its color. In music, we call it timbre. The unique sonic signature of your instrument.
Now, play an A on your G string, 2nd fret. Then play the open A string. Listen to the difference. Same note, but doesn’t sound exactly the same.
Why? The fundamental frequency is the same, it’s an A. But the string is shorter, thinner, the contact point is different. So the harmonic mix changes. And that’s what makes the guitar rich: two positions of the same note don’t sound identical. Timbre varies.
On your guitar: find your strings
Play the lowest string (the thickest one, closest to your chin): it’s an E. Then its neighbor just below: an A. Compare with the sounds below.
On your guitar: experiment now
Exercise 1: See the vibration
Play your open low E string. Watch it vibrate. Really. That blur is 82 vibrations per second. Now play your high E string. It vibrates so fast (330 Hz) you can barely see the movement. The higher the frequency, the faster it is.
Exercise 2: Hear the octave
Play your open A string. Then play the harmonic at the 12th fret: place your finger lightly above the fret, pluck, release. Listen to the difference. Same note, octave above. Do it on each string. Six strings, six octaves.
Exercise 3: Compare timbres
Play an A on your open A string. Then play the same A on your G string, 2nd fret. Alternate between the two. Same note, different timbre. It’s the harmonic mix that changes.
Challenge of the week
Next time you tune, watch your strings vibrate. Compare the low E and the high E. Bass strings vibrate more slowly, and it’s visible to the naked eye. Now you know why.
Common questions
Why does my A string vibrate at 110 Hz?
Sound is air vibration. Your A string vibrates 110 times per second (110 Hz). This precise frequency is what defines the note A. The higher the frequency, the higher the pitch. It’s pure physics, not an arbitrary convention.
Why does the 12th fret sound the same note but higher?
At the 12th fret, the vibrating length of the string is divided by two. A string twice as short vibrates twice as fast: 110 Hz becomes 220 Hz. This mathematical relationship (frequency doubling) defines the octave. Same note, doubled frequency, higher pitch.
Why doesn’t my guitar sound like a piano on the same note?
When you play an A on your guitar, your string doesn’t only vibrate at 110 Hz. It also vibrates at double (220 Hz), triple (330 Hz), etc. These additional vibrations are harmonics. An instrument’s timbre is the unique mix of these harmonics. Same note = same fundamental, but different harmonics = different timbre.
What’s the difference between low and loud?
Low means slower vibration (lower frequency, in Hz). Loud means wider vibration (higher amplitude). They’re two independent dimensions. You can play a low note quietly, or a high note loudly. One concerns pitch, the other volume.
How does the tuner know a string is in tune?
Your tuner measures the vibration frequency of your string in Hertz. If you tune an A and it displays 440 Hz, that means your string vibrates exactly 440 times per second. It’s not feeling, it’s a precise physical measurement. Each note has its reference frequency.
Summary
- Sound is vibration of the air. When you pluck a string, it vibrates and pushes air to your ears.
- Frequency (measured in Hertz) is the number of vibrations per second. The higher it is, the higher the pitch. Your A string vibrates at 110 Hz.
- The octave is exactly the doubling of frequency. At the 12th fret, your string vibrates twice as fast: 110 Hz becomes 220 Hz.
- Harmonics are additional vibrations (at double, triple, quadruple the fundamental) that give the instrument its color. That’s timbre.
- Low and loud aren’t the same: low = slow vibration, loud = wide vibration. Two independent dimensions.
Now that you know this, next time you tune, you know what’s happening. You’re adjusting the vibration frequency of your string. And when a chord sounds “full,” it’s because the strings’ harmonics align with each other. It’s not magic, it’s physics.
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Open the demoNext in series
You now know that sound is vibration. That each note has a precise frequency. But why did we choose THESE notes? Why are there 12 notes and not 15, not 8, not 20?
How did we go from something infinite (all possible frequencies) to an organized system that gives us C, D, E, F, G, A, B?
Article 2: Musical Notes → Back to music theory table of contents