An acoustic guitar is a guitar that uses only an acoustic soundboard (the top of an acoustic guitar) to help transmit the strings’ energy into the air in order to produce its sound. The soundboard will add various tonal qualities due to its own mix of tonewoods and bracing, and the soundboard also has a strong effect on the loudness of the guitar. Without a soundboard, the string would just cut through the air without actually moving it much—because it is large, the soundboard can push the air, creating a much louder sound. In addition, the acoustic guitar has a hollow body that resonates, increasing the efficiency of its lower frequencies.
All of the complex air-coupling interactions, along with the resonant properties of the tonewoods themselves, are a key reason that different acoustic guitars will have different tonal qualities. The sound is a complex mixture of harmonics that give this type of guitar its distinctive sound. Some of the most important varieties are the classical guitar (nylon-stringed) and steel-string acoustic guitar.
Steel-string Acoustic Guitar The steel-string acoustic guitar is a modern form of guitar that descends from the classical guitar, but is strung with steel strings for a brighter, louder sound. The most common shape is the dreadnought (originally developed by C.F. Martin & Company), which incorporates a deep soundbox and a smaller and less pronounced upper bout, giving it a wedge-shaped appearance. Other popular body styles include the larger jumbo body shape, as well as the smaller auditorium (000) body style and even smaller grand concert (00) body style. Check out the Martin Ed Sheeran X Signature Acoustic Electric for a smaller body type with a punchy sound.
Classical Guitar The classical guitar is a variation of the Spanish Guitar, from its construction, size, weight, wood and the sound it produces. Classical guitars have six nylon strings, rather than metal strings used in other acoustic guitars. The shape, construction, and material of classical guitars vary, but typically they have a modern classical guitar shape, or historic classical guitar shape (e.g., early romantic guitars from France and Italy). Classical guitars are also typically played with the fingers rather than a pick (as steel-string acoustic guitars are often played).