How Acoustic Guitars Work

By: Marshall Brain

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The Gibson J45 Rosewood is a classic acoustic guitar.
Photo courtesy Gibson Guitars
The Gibson J45 Rosewood is a classic acoustic guitar.

­The guitar is one of the most popular musical instruments in use today, and it spans a huge range of musical styles -- rock music, country music and flamenco music all use the same instrument to create wildly different sounds. The guitar is an instrument that has been around since the 1500s, but it has undergone several big transformations during its history. The development of the electric guitar is the most obvious recent mutation, and it had a huge effect on the popularity of the guitar.

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Whether you're a musician or you simply enjoy listening to music, have you ever stopped to think about how a guitar works? What are frets for? What does the big hole in the front do? How does an electric guitar's pick-up work? In this article, we'll explore exactly how guitars make music! You will also learn a good bit about notes and scales in the process.

A guitar is a musical instrument with a distinctive shape and a distinctive sound. The best way to learn how a guitar produces its sound is to start by understanding all of the different parts that make up the instrument. We'll start here with the acoustic guitar and then look at the electric­ guitar later
in the article.


Guitar Parts

A guitar can be divided into three main parts:

  • The hollow body

Photo courtesy Gibson Guitars
The body of a Gibson SJ200 Vine acoustic guitar

  • The neck, which holds the frets

Photo courtesy Gibson Guitars
The neck of a Gibson SJ200 Vine acoustic guitar

  • The head, which contains the tuning pegs

Photo courtesy Gibson Guitars
The head of a Gibson SJ200 Vine acoustic guitar

The most important piece of the body is the soundboard. This is the wooden piece mounted on the front of the guitar's body, and its job is to make the guitar's sound loud enough for us to hear.

Photo courtesy Gibson Guitars
The body of a Gibson SJ200 Vine acoustic guitar

In the soundboard is a large hole called the sound hole. The hole is normally round and centered, but F-shaped pairs of holes, as in a violin, are sometimes seen. Attached to the soundboard is a piece called the bridge, which acts as the anchor for one end of the six strings. The bridge has a thin, hard piece embedded in it called the saddle, which is the part that the strings rest against.

When the strings vibrate, the vibrations travel through the saddle to the bridge to the soundboard. The entire soundboard is now vibrating. The body of the guitar forms a hollow soundbox that amplifies the vibrations of the soundboard. If you touch a tuning fork to the bridge of a guitar you can prove that the vibrations of the soundboard are what produce the sound in an acoustic guitar. (The process in an electric guitar is completely different, as described later in this article.)


Are you skeptical that the soundboard is really amplifying the sound?

Try this experiment:

  1. Tightly seal a largish bowl with plastic wrap as shown. (Tape the plastic wrap to the sides of the bowl to hold it in place if it is not clinging very well.)
  2. Tape a rubber band to the center of the taut plastic wrap and twang the rubber band.
  3. Compare how loud the sound is to a plain rubber band that is not taped to plastic wrap.
It's a big difference! The plastic wrap greatly increases the amount of surface area that is vibrating, so the sound is much louder.

The body of most acoustic guitars has a "waist," or a narrowing. This narrowing happens to make it easy to rest the guitar on your knee. The two widenings are called bouts. The upper bout is where the neck connects, and the lower bout is where the bridge attaches.

The size and shape of the body and the bouts has a lot to do with the tone that a given guitar produces. Two guitars that have different body shapes and sizes will sound a bit different. The two bouts also affect the sound: If you drop a pick into the body of a guitar and rattle it back and forth in the lower bout and then the upper bout, you will be able to hear a difference. The lower bout accentuates lower tones and the upper bout accentuates higher tones.

The face of the neck, containing the frets, is called the fingerboard. The frets are metal pieces cut into the fingerboard at specific intervals. By pressing a string down onto a fret, you change the length of the string and therefore the tone it produces when it vibrates. We'll talk a lot more about frets and specific fret spacings later on.

Between the neck and the head is a piece called the nut, which is grooved to accept the strings. From a musical standpoint, the saddle and the nut act as the two ends of the string. The distance between these two points is called the scale length of the guitar.

The strings pass over the nut and attach to tuning heads, which allow the player to increase or decrease the tension on the strings to tune them.

Photo courtesy Gibson Guitars
The tuning pegs of a Gibson SJ200 Vine

In almost all tuning heads, a tuning knob turns a worm gear that turns a string post.


Sound, Tones, and Notes

The guitar is a musical instrument, so its goal in life is to make music. Music is the arrangement of tones into patterns that the human brain finds pleasing (or if not pleasing, then at least intriguing). In order to better understand music, let's start at the beginning: "What is sound?"

Sound is any change in air pressure that our ears are able to detect and process. For our ears to detect it, a change in pressure has to be strong enough to move the eardrums in our ears. The more strongly the pressure changes, the "louder" we perceive the sound to be.

For our ears to be able to perceive a sound, the sound has to occur in a certain frequency range. For most people, the range of perceivable sounds falls between 20 Hertz (Hz, oscillations per second) and 15,000 Hz. We cannot hear sounds below 20 Hertz or above 15,000 Hertz.

A tone is a sound that repeats at a certain specific frequency.

  • Click here to hear a 440-Hz tone. (At the dialog select, click "Open.")
This 440-Hz tone can be pictured as a sine wave, like this:

A tone is made up of one frequency or a very small number of related frequencies. The alternative to a tone is a combination of hundreds or thousands of random frequencies. We refer to these random-combination sounds as noise. When you hear the sound of a river, or the sound of wind rustling through leaves, or the sound of paper tearing or the sound made when you tune your TV to a nonexistent station, you are hearing noise.

  • Click here to hear noise. (At the dialog select, click "Open")
    Note: This is an unpleasant sound -- turn down your speakers before playing it.
Noise not only sounds random but also presents itself graphically as randomness:

A musical note is a tone. However, a musical-note tone comes from a small collection of tones that are pleasing to the human brain when used together. For example, you might pick a set of tones at the following frequencies:

  • 264 Hz
  • 297 Hz
  • 330 Hz
  • 352 Hz
  • 396 Hz
  • 440 Hz
  • 495 Hz
  • 528 Hz

This particular collection of tones is known as the major scale. Each tone in the scale is multiplied by a certain fraction to come up with the next tone in the scale. Here's how the major scale works:

  • 264 Hz * 9/8 = 297 Hz
  • 297 Hz * 10/9 = 330 Hz
  • 330 Hz * 16/15 = 352 Hz
  • 352 Hz * 9/8 = 396 Hz
  • 396 Hz * 10/9 = 440 Hz
  • 440 Hz * 9/8 = 495 Hz
  • 495 Hz * 16/15 = 528 Hz

Why are these particular fractions chosen in the major scale? Simply because they sound pleasing. Listen:

  • Click here to hear the major scale. (At the dialog select, click "Open.")

These particular tones have been given letter names, and also word names, like this:

  • 264 Hz - C, do (multiply by 9/8 to get:)
  • 297 Hz - D, re (multiply by 10/9 to get:)
  • 330 Hz - E, mi (multiply by 16/15 to get:)
  • 352 Hz - F, fa (multiply by 9/8 to get:)
  • 396 Hz - G, so (multiply by 10/9 to get:)
  • 440 Hz - A, la (multiply by 9/8 to get:)
  • 495 Hz - B, ti (multiply by 16/15 to get:)
  • 528 Hz - C, do (multiply by 9/8 to get:)

And the sequence repeats.

The names are totally arbitrary, as with the fractions. It just turns out that they have a pleasing sound to human ears.

One thing to notice is that the two C notes are separated by exactly a factor of two -- 264 is one half of 528. This is the basis of octaves. Any note's frequency can be doubled to "go up an octave," and any note's frequency can be halved to "go down an octave."

You may have heard of "sharps" and "flats." Where do they come from? The scale of tones shown above is "in the key of C" because the fractions were applied with C as the starting note. If we were to start the fractions at D, with a frequency of 297, then we would be "tuned to the key of D" and the frequencies would look like this:

  • 297 Hz, D, do (multiply by 9/8 to get:)
  • 334.1 Hz, E, re (multiply by 10/9 to get:)
  • 371.3 Hz, F, mi (multiply by 16/15 to get:)
  • 396 Hz, G, fa (multiply by 9/8 to get:)
  • 445.5 Hz, A, so (multiply by 10/9 to get:)
  • 495 Hz, B, la (multiply by 9/8 to get:)
  • 556.9 Hz, C, ti (multiply by 16/15 to get:)
  • 594 Hz, D, do (multiply by 9/8 to get:)

And the sequence repeats.

The notes at 297 Hz (D), 396 Hz (G) and 495 Hz (B) in the key of D match the same notes in the key of C exactly. The E note in the key of D (at 334.1 Hz) is pretty close to the E note in the key of C (330 Hz). The same applies for the A note. F and C, however, are distinct in the two keys. F and C in the key of D are therefore referred to as F# (F sharp) and C# (C sharp) in the key of C. (Note that F sharp is also known as G flat, and C sharp is also known as D flat.) If you apply the fractions to several different keys, merge together all the identical and pretty-close notes and then look at the unique sharps that fall out, you realize that you need A#, C#, D#, F# and G# to handle all the keys.

You can see that, with all of these mergings of keys, the major scale can leave you with some pretty arbitrary decisions to make when you tune an instrument. For example, you can tune the major notes to the key of C, and then the sharps for F and C to the key of D, and the sharps for D and G to... It can get pretty messy.

Read on to learn how this problem was solved.


Tempered Scale

Over time, most of the musical world came to agree on a scale called the tempered scale, with the A note set at 440 Hz and all of the other notes tuned off of that. In the tempered scale, all of the notes are offset by the 12th root of 2 (roughly 1.0595) instead of the fractions we saw above. That is, if you take any note's frequency and multiply it by 1.0595, you get the frequency for the next note. Here are three octaves of the tempered scale:

  • 82.4 E - open 6th string
  • 87.3 F
  • 92.5 F#
  • 98.0 G
  • 103.8 G#
  • 110.0 A - open 5th string
  • 116.5 A#
  • 123.5 B
  • 130.8 C
  • 138.6 C#
  • 146.8 D - open 4th string
  • 155.6 D#
  • 164.8 E
  • 174.6 F
  • 185.0 F#
  • 196.0 G - open 3rd string
  • 207.6 G#
  • 220.0 A
  • 233.1 A#
  • 246.9 B - open 2nd string
  • 261.6 C - "middle C"
  • 277.2 C#
  • 293.6 D
  • 311.1 D#
  • 329.6 E - open 1st string
  • 349.2 F
  • 370.0 F#
  • 392.0 G
  • 415.3 G#
  • 440.0 A - 5th fret on 1st string
  • 466.1 A#
  • 493.8 B
  • 523.2 C
  • 554.3 C#
  • 587.3 D
  • 622.2 D#
  • 659.2 E - 12th fret on 1st string

As you can see in this table, we have finally been able to get the discussion back to guitars! This is how a guitar is tuned. A guitar with 12 clear frets has a range of three octaves, as shown above. The open sixth string is the lowest note, and the 12th fret on the first string is the highest. Here is the actual layout of all of the notes on a guitar.

You can see in this diagram that there are 72 fret positions, but the table above shows only 37 unique notes. Therefore you have multiple ways to finger identical notes on a guitar. This fact is frequently used to get all of a guitar's strings tuned. For example, you can tune A on the first string (5th fret) to 440 Hz. Then you know that E at the 5th fret on the second string is the same as the open first string, so you match those two notes up by tuning the second string. Similarly:

  • The 4th fret on the 3rd string (B) is the same as the B on the open 2nd string.
  • The 5th fret on the 4th string (G) is the same as the G on the open 3rd string.
  • The 5th fret on the 5th string (D) is the same as the D on the open 4th string.
  • The 5th fret on the 6th string (A) is the same as the A on the open 5th string.

Once you have all of the strings on a guitar perfectly tuned, using 440 Hz for A as the primary note, then the guitar will have notes with the frequencies shown in the table above, and it is said to be tuned to "concert pitch."


Strings and Frets

Now the question becomes: How does a guitar generate the frequencies shown above? A guitar uses vibrating strings to generate tones. Any string under tension will vibrate at a specific frequency that is controlled by:

  • The length of the string
  • The amount of tension on the string
  • The weight of the string
  • The "springiness" of the string's material (a rubber band is a lot "springier" than kite string)

On a guitar, you can see that the different strings have different weights. The first string is like a thread, and the sixth string is wound so that it is much thicker and heavier. The tension on the strings is controlled by the tuning pegs. The length of the open strings, also known as the scale length, is the distance from the nut to the saddle. On most guitars, the scale length ranges from 24 inches to 26 inches. When you press down on a string at a fret you change the length of the string, and therefore its frequency when vibrating.

Photo courtesy Gibson Guitars
The neck of a Gibson SJ200 Vine acoustic guitar

The frets are spaced out so that the proper frequencies are produced when the string is held down at each fret. The magic number to use in positioning frets is 17.817. Let's say that the scale length for a guitar is 26 inches. The first fret should be located (26 / 17.817) 1.46 inches down from the nut, or 24.54 inches from the saddle. The second fret should be (24.54 / 17.817) 1.38 inches down from the first fret, or 23.16 inches from the saddle. The 12th fret should be exactly halfway between the nut and the saddle. The following table shows all of the fret positions and the frequency of each note on the first string (assuming a scale length of 26 inches).

(1st string)
Fret position
from saddle
Table assumes scale length of 26 inches


The Guitar's Sound

Cool Fact
A piano has 88 keys stretching through more than 7 octaves. The lowest note on a piano vibrates at 27.5 Hz and the highest vibrates at 4,186 Hz.

Have you ever noticed that a piano, a harp, a mandolin, a banjo and a guitar all play the same notes (frequencies) using strings, but they all sound so different? If you hear the different instruments you can easily recognize each one by its sound. For example, anyone can hear the difference between a piano and a banjo!

An acoustic guitar generates its sound in the following way:

  1. When the strings on a guitar vibrate, they transmit their vibrations to the saddle.
  2. The saddle transmits its vibrations to the soundboard.
  3. The soundboard and body amplify the sound.
  4. The sound comes out through the sound hole.

The particular shape and material of the sound board, along with the shape of the body and the fact that a guitar uses strings, give a guitar its distinctive "sound."

Photo courtesy Gibson Guitars
The body of a Gibson guitar during construction

There are a number of different ways to modify sounds to get the particular voice of the instrument. For example, if a guitar produced a pure tone, a guitar's 440-Hz A note would sound like this:

  • Click here to hear a 440-Hz tone. (At the dialog select, click "Open.")

Here is what that tone looks like -- it is a pure 440-Hz sine wave:

One modification that a guitar makes to that tone is to add harmonics to it. For example, when you pluck one string it plays the pure note, but the string also rings at harmonics like two-times, three-times and four-times the pure tone. Other strings also pick up the vibrations from the saddle and add their own vibrations as well. Therefore, the sound you hear from a guitar for any given note is actually a blend of many related frequencies. To get an idea of the effect this sort of blending has, here is a 440-Hz tone with an 880-Hz tone (at half the amplitude) added to it:

  • Click here to hear 440-Hz and 880-Hz tones. (At the dialog select, click "Open.")

Here is what these two blended tones look like together:

A guitar also adds an envelope to any note it plays. The note doesn't just start and stop abruptly -- it builds and trails off. Over the course of the note, the amplitude (loudness) of the note changes. For example, a guitar's envelope might look like this:

Here is what this envelope sounds like when applied to the blended tone:

  • Click here to hear an envelope applied to the 440-Hz and 880-Hz tones. (At the dialog select, click "Open.")

This does not sound anything like a guitar, but it is much closer than a pure 440-Hz tone is. To make it sound exactly like a guitar you would have to:

  • Get the right set of harmonics at the right frequencies and amplitudes and blend them
  • Get the right envelope
  • Get the right tonal modifications: The body of a guitar favors some frequencies (amplifies them better) and discriminates against others (does not amplify them as well). You would need to apply that distortion to the tones.

If you did all of that, you could artificially create a note that would sound very guitar-like. That is what synthesizers do when they emulate the sounds of different instruments.


Electric Guitars

If you have ever compared an electric guitar to an acoustic guitar, you know that they have several important things in common. Both acoustic and electric guitars have six strings, they both tune those strings with tuning pegs and they both have frets on a long neck. Down at the body end is where the major differences are found.

Photo courtesy Gibson Guitars
The Gibson Flying V electric guitar

For a complete description, please see How Electric Guitars Work.

For more information on guitars and related topics, check out the links on the next page.


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