The physical characteristics of an acoustic guitar are straightforward to identify and quantify. Body shape, dimensions, tone woods, top wood, bracing, and decorative elements can be specified with precision.
On the other hand, the sound qualities you hear are much harder to discern and evaluate - and will be subjective from one person to the next. Besides being subjective, the terms used to describe tonal qualities can be confusing. Knowing tonal terminology and using it consistently is essential to reduce both ambiguity and subjectivity. We follow the terminology used by Larry Sandberg in his classic book The Acoustic Guitar Guide and list these terms below. (Unfortunately, this book is out of print, but is still available in limited quantities new or used. Follow the link above to check its availability on Amazon.com)
Tone is the quality that makes one guitar sound different from another. When you set a guitar string in motion, it vibrates along it's entire length. This big vibration is called the fundamental. The string also vibrates in smaller portions, which are called overtones. All guitars share some combinations of the fundamental tone and overtone in common. However, each individual guitar will have its own personal combination of fundamental and overtones. That's what makes guitars sound unique.
The main factor in a guitar's tone color is the woods used for the back and sides. These are called the "tonewoods." Rosewood gives a soulful, darker sound. Mahogany is sweeter and softer, as well as rounder and nicely balanced. Maple is louder, like rosewood, but has less bass resonance and a more brittle tone.
Volume is how loud your guitar is. At its simplest, you can objectively measure volume using a decibel meter. In real life, there's more than meets the meter because of subjective perception of other qualities such as balance and presence also play a part in how loud a guitar seems.
Volume is mainly a function of how freely the top of the guitar moves when driven by the strings. This movement is affected by the type of wood used, wood quality, and bracing. The back and sides also influence this because their rigidity influence how efficiently the air chamber of the body works.
All other things considered, bigger guitars sound louder. There's more top area and a larger sound chamber. Heavier gauge strings are louder, because they vibrate the top more than lighter gauge strings.
Presence is a mysterious, subjective quality. It can make a guitar seem louder even though a meter would not show it. It is a gauge of how full your guitars sound is. A good measure of presence is how satisfying your guitar sounds when you play it softly. A strong presence means that the tone quality does not deteriorate with less volume. The efficiency of the guitars top plays a large part in the perceived presence.
Balance is the relationship between the high and low notes in point of fullness and volume. In a balanced guitar, the notes have equal authority throughout the entire range of the instrument. Guitars that are over-balanced toward the bass are called boomy. Flatpickers and folk singers often prefer this type of balance. Fingerstyle guitarists might prefer a guitar that is balanced toward the high strings. Balance is usually directly related to the size of the guitar. Balance is also affected by the body woods (Rosewood is boomier than Mahogany), and the size of the soundhole (a larger soundhole usually balances the guitar toward the high strings).
Separation is the ability of an instrument to express simultaneously played nots so that they are perceived distinctly and individually rather than as a homogenous whole. In other words, when you play a chord, is what you hear individual strings or all the strings together in one glob? How much separation you want is a matter of taste. It's harder to build a guitar with good separation, and luthiers generally consider it an achievement.
Sustain is the measure of how long a note keeps sounding after you initiate it. If the sound decays too fast, you have poor sustain. Generally sustain is a quality of fine guitars and is something you want. It is one of the most important factors in creating the immediate impression of that a guitar sounds or does not sound good. It is the vibration of the top that gives you an honest, clean sustain that preserves all the components of the tone throughout its duration.