How Acoustic Guitars Work

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.)

Experiment 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.) Tape a rubber band to the center of the taut plastic wrap and twang the rubber band. 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.