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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 amazing 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.
The guitar is truly an incredible instrument!
Whether you are a musician or you simply enjoy listening to music, have you ever stopped to think about how a guitar works? If you have ever looked at a guitar and wondered things like, "What are frets for?", "What does the big hole in the front do?" and "How does an electric guitar's pick-up work?", then this edition of How Stuff Works will be unbelievably interesting to you as we explore how guitars make music! You will also learn a good bit about notes and scales in the process.
Guitar Parts
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.
A guitar can be divided into three main parts:
Are you skeptical that the soundboard is really amplifying the sound? Try this
experiment. 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). Now tape a rubberband
to the center of the taut plastic wrap and twang the rubber band.
Compare how loud the sound is to a plain rubberband 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.
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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. Any two guitars that have different body shapes and sizes will sound subtlely 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 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 distances. 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 in a later section.
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. 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. Let's start at the beginning with the question, "What is sound?" in order to better understand music.
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 (oscillations per second) and 15,000 hertz (oscillations per second). We cannot hear sounds below 20 hertz and above 15,000 hertz.
A tone is a sound that repeats at a certain, specific frequency.
A tone is made up of one frequency or 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 non-existent station, you are hearing noise.
A musical note is a tone. However, a musical note is a tone that 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:
This particular collection of tones is known as the major scale. If you multiply 264 by the fraction 9/8 you get
297. If you multiply 297 by the fraction 10/9 you get 330. If you multiply 330 by the fraction 16/15 you get 352.
If you multiply 352 by the fraction 9/8 you get 396. If you multiply 396 by the fraction 10/9 you get 440.
If you multiply 440 by the fraction 9/8 you get 495. If you multiply 495 by the fraction 16/15 you get 528.
Why are these particular fractions chosen in the major scale? Simply because they sound pleasing. Listen:
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:
The notes at 297 hertz (D), 396 hertz (G) and 495 hertz (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 hertz) is pretty close to the E note in the key of C (330 hertz). 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 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..., or you could..., or.... It can get pretty messy.
Therefore, over time, most
of the musical world came to agree on a scale called the tempered scale, with the A note set at 440
hertz 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:
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 3 octaves as shown above.
The open 6th 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 hertz. Then it is a fact that E at the 5th fret on the second string is the same as the open first string, so you you match those two notes up by tuning the second string. Similarly:
[Fun fact: a piano has 88 keys stretching through more than 7 octaves. The lowest note on a piano vibrates at 27.5 hertz and the highest vibrates at 4,186 hertz.]
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 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. And so on. The 12th fret should be exactly half-way 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.
| Note | Fret | Scale length | Frequency (1st string) | Fret position from saddle |
| E | open | 26 | 329.6 | 26.00 |
| F | 1 | 349.2 | 24.54 | |
| F# | 2 | 370.0 | 23.16 | |
| G | 3 | 392.0 | 21.86 | |
| G# | 4 | 415.3 | 20.64 | |
| A | 5 | 440.0 | 19.48 | |
| A# | 6 | 466.1 | 18.38 | |
| B | 7 | 493.8 | 17.35 | |
| C | 8 | 523.2 | 16.38 | |
| C# | 9 | 554.3 | 15.46 | |
| D | 10 | 587.3 | 14.59 | |
| D# | 11 | 622.2 | 13.77 | |
| E | 12 | 659.2 | 13.00 |
The Guitar's Sound
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. When the strings on a guitar vibrate, they transmit their vibrations to the saddle. The saddle, in turn, transmits its vibrations to the sound board. The sound board and body amplify the sound from the sound board. 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 "sound".
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 hertz A note would sound like this:
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 2-times, 3-times and 4-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 hertz tone with an 880 hertz tone (at half the amplitude) added to it:
A guitar also adds an envelope to any note it plays. The note just doesn't 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:
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 6 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. Electric guitars have solid bodies, magnetic pickups and several knobs instead of the hollow
resonating cavity found in an acoustic guitar. If you pluck a string on an electric guitar that is
not plugged in, the sound is barely audible. Without a soundboard and the hollow body, there is
nothing to amplify the string's vibrations.
To produce sound, an electric guitar senses the vibrations of the strings electronically and routes that electronic signal to an amplifier and speaker. The sensing occurs in a magnetic pickup mounted under the strings on the guitar's body. A simple magnetic pickup looks like this:
This pickup consists of a bar magnet wrapped with perhaps 7,000 turns of fine wire. If you have read the HSW article on electromagnets, then you know that coils and magnets can turn electrical energy into motion. In the same way, they can turn motion into electrical energy. In the case of an electric guitar, the vibrating steel strings produce a corresponding vibration in the magnet's magnetic field and therefore a vibrating current in the coil.
There are many different types of pickups. For example, many pickups use a separate magnet for each string, like this:
Separate magnets allow the guitarist to adjust the heights of the magnets independently.
The coil sends its signals through a very simple circuit on most guitars. The circuit looks something like this:
The upper variable resistor adjusts the tone. The resistor (500K ohms max) and capacitor (0.02uF) form a simple low-pass filter. The filter cuts out higher frequencies. By adjusting the resistor you control the frequencies that are cut out. The second resistor (500K ohms max) controls the amplitude (volume) of the signal that reaches the jack. From the jack the signal runs to an amplifier, which drives a speaker.
Many electric guitars have two or three different pickups located at different points on the body. Each pickup will have a distinctive sound, and multiple pickups can be paired, either in-phase or out, to produce additional variations.
Links
Guitar Makers
Special thanks to the Webb family for a very nice breakfast and a wonderful conversation that acted as the motivation for writing this article!
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How Stuff Works (http://www.howstuffworks.com) is a production of BYG Publishing, Inc. http://www.bygpub.com - questions@bygpub.com P.O. Box 40492 - Raleigh, NC 27629 |
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