Acoustic guitars get their name quite simply; they produce sound acoustically, that is, without help from an amplifier. The vibration of the strings to the air is what makes the notes and ultimately the music.
Because the main sound from an acoustic guitar comes from the strings, they are thought to be the most important part of the instrument, but the sound waves from these strings actually go through the guitar’s body to create the sound and this also involves a sound box that strengthens the vibrations of the strings in order to create those beautiful notes. Ultimately, the whole thing works together to create sound, so although the strings are what the musician plucks or strums in order to make music, the body of the guitar, the neck and also the sound hole are all important elements to the piece.
So how does it all work? The sound box, or sound board on a guitar is found at the top and it works to make the sounds louder and stronger. If someone just plucked the string without this sound box, the sound wouldn’t move the air much and thus the note wouldn’t be loud. The soundboard increases the area of the vibrations and can move the sound that much better than the string alone since it is larger and flat. This impacts the whole guitar’s energy transfer and the notes can be heard that much louder.
The guitar’s body is of course hollow, and this also works to increase the energy transmission of each note. The air that is in the body of the guitar resonates with the vibrations as each string is strummed. At low frequencies the body increases or decreases the volume of the sound depending on how the air in the body moves in or out of phase with the strings. In phase with the strings and you get an increase of 3 decibels, out of phase with the strings and it will decrease by 3 decibels.
This air inside the body of the guitar works with the outside air through that all important sound hole. This results in air pushing air which in turn makes those notes all the more resilient. Since the guitar has several sound coupling modes– string to soundboard, soundboard to air, inside air to outside air– you get different tones from different guitars.
When we think of strumming a guitar, we may not give much thought to the process being undertaken but there is much going on to make those sounds!