Guitars, like all musical instruments, produce sound by vibrating. When a guitar string is plucked, it begins to vibrate up and down. These vibrations cause the air molecules surrounding the string to be pushed together and pulled apart rhythmically, producing high- and low-pressure waves of sound. If you pluck a guitar string that is stretched between two chairs, the sound that is produced will not be very loud. However, when the string is attached to the sound box of the guitar, its vibrations cause the sound box to vibrate at the same natural frequency. The sound box in turn causes the air molecules inside the box to vibrate, also at the same frequency. Thus, the string, guitar, and enclosed air molecules are all vibrating at the same natural frequency and are all causing surrounding air molecules to vibrate at this frequency. This phenomenon, called resonance, results in an increase in the amplitude and thus the loudness of the sound.

Many factors influence the type of sound a guitar makes. The faster a string vibrates, that is, the higher its frequency of vibration, the shorter the wavelength and the higher the frequency of the sound waves produced. The higher the frequency of the sound waves, the higher the pitch. A tight, or high-tension, string therefore produces higher-pitched sounds, while a lower-tension string produces lower-pitched sounds. The frequency of a vibrating string also decreases with that string's weight; a heavier nylon or wire produces lower pitches. Another factor influencing frequency is string length. Guitar players play different notes primarily by manipulating string length. They do this by pressing a string against the neck of the guitar and, thus, shortening its length and causing it to vibrate at a higher pitch.

While a guitar string vibrates with one fundamental frequency, which is determined by its length, weight, and tension, it and the guitar body also vibrate at many other frequencies, each with its own amplitude or loudness, based on the materials used and the shape and size of the instrument. This combination of frequencies determines the quality of sound that a particular instrument produces.