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An acoustic guitar is a guitar that uses only acoustic methods to project the sound produced by its strings. It is a retronym, coined after the advent of electric guitars, which depend on electronic amplification to make their sound audible.

Sound production

In all types of guitars the sound is produced by the vibration of the strings. However, because the strings can only displace a small amount of air, the volume of the sound needs to be increased in order to be heard. In an acoustic guitar, this is accomplished by using a soundboard and a resonant cavity, the sound box. The body of the guitar is hollow. The vibrating strings drive the soundboard through the bridge, making it vibrate. The soundboard has a larger surface area and thus displaces a larger volume of air, producing a much louder sound than the strings alone.

As the soundboard vibrates, sound waves are produced from both the front and back faces. The sound box provides both a support for the sound board and a resonant cavity and reflector for the sound waves produced on the back face of the soundboard. The air in this cavity resonates with the vibrational modes of the string (see Helmholtz resonance), increasing the volume of the sound again. The back of the guitar will also vibrate to a lesser extent, driven by the air in the cavity. Some sound is ultimately projected through the sound hole (some variants of the acoustic guitar omit this hole, or have f holes, like a violin family instrument). This sound mixes with the sound produced by the front face of the soundboard. The resultant sound is a complex mixture of harmonics that give the guitar its distinctive sound.

No amplification actually occurs in this process, in the sense that no energy is externally added to increase the loudness of the sound (as would be the case with an electronic amplifier). All the energy is provided by the plucking of the string. The function of the entire acoustic system is to maximize intensity of sound, but since total energy remains constant, this comes at the expense of decay time. An unamplified guitar (one with no soundboard at all) would have a low volume, but the strings would vibrate much longer, like a tuning fork. This is because a damped harmonic oscillator decays exponentially, with a mean life inversely proportional to the damping. When the strings are driving the larger soundboard and sound box, the damping is much higher.

Amplification

An acoustic guitar can be amplified by using various types of pickups or microphones. The most common type of pickups used for acoustic guitar amplification are piezo and magnetic pickups. Piezo pickups are generally mounted under the bridge saddle of the acoustic guitar and can be plugged into a mixer or amplifier. Magnetic pickups are generally mounted in the sound hole of the acoustic guitar and are very similar to those found in electric guitars.

Construction

Various shapes determine the way the soundboard vibrates. The thinner and lighter the soundboard (less mass), the louder the sound. However, there are practical limitations to how thin the soundboard can be made without breaking. Braces are used inside the guitar to provide strength and resilience. The mass and position of these braces have consequences for the range of frequencies reproduced. During the wave cycle, different regions of the soundboard may be moving in different directions, depending on the sound frequency. Different configurations of bracing and different shapes of soundboard produce different vibration patterns, giving subtle variations in the range of sounds produced.

The materials and shape of the guitar produce a complex series of damping, resonating, and phase cancelling or reinforcing effects. The range of factors determine the overall acoustic qualities or timbre of the instrument. Artisan luthiers tap potential pieces of wood to determine their acoustic resonance, but this is usually not done for mass-produced instruments. Different timber species have different tones and careful selection of timber is required when designing and making an instrument. Guitars have been made with steel soundboards and resonators, and some experiments have been conducted with novel materials including aluminium and plastics. Even the hardness or viscoelasticity of the glues and varnishes can have a dramatic effect on the sound, damping or resonating some or all frequencies. Quality instruments are made with hard glues and lacquers which have less damping on the transmission of vibrations around the structure of the instrument. Most people prefer the sound of wooden instruments, although the steel resonator guitar has found favor in some genres, like blues. Sitka spruce is traditionally the favored material for the soundboard because of its high strength-to-weight ratio and stiffness. In recent years King William pine has been found to produce very good results.

How Acoustic Guitars Work

The acoustic guitar is one of the most successful instruments of all time, and also one of the simplest to learn and play. And its ingenious, straight-forward design allows for a huge variety of sounds. Learn more about acoustic guitars.

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.

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
The neck, which holds the frets
The head, which contains the tuning pegs
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.

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.

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
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 transmits its vibrations to the soundboard.
The soundboard and body amplify the sound.
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."

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