since 1972
since 1972
How Acoustic Guitars Work
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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 and they
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.
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? 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.
Guitar Parts 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. 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 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.
In almost all tuning heads, a tuning knob turns a worm
gear that turns a string post.
Worm gears are used when large gear
reductions are needed. It is common for worm gears to have reductions of 20:1,
and even up to 300:1 or greater.
Many worm gears have an interesting property that no
other gear set has: the worm can easily turn the gear, but the gear cannot turn
the worm. This is because the angle on the worm is so shallow that when the
gear tries to spin it, the friction between the gear and the worm holds the
worm in place.
Sound, Tones and Notes
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. A 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.
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:
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:
Why are these particular fractions chosen in the major
scale? Simply because they sound pleasing.
These particular tones have been given letter names,
and also word names, like this:
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:
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.
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 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:
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:
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:
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. 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).
The Guitar's Sound An acoustic guitar generates its sound in the following way:
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."
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