The sound waves in a guitar are produced by the vibration of the strings. The sound is transmitted via the pressure waves in the instrument.
Guitars are some of the most common string instruments. The strings are fixes at both ends. They are elastic, which means they can vibrate when touched. The vibrations of a guitar are called standing waves. Different wave speeds lead to different frequencies or pitches of the strings. The guitar sound consists of fundamental standing waves and overtones. The overtones vary according to the way one is playing the guitar, such as with a finger versus a pick or in the middle versus near the bridge.