Acoustic guitars have sound holes to allow for greater vibration of the soundboard and facilitate the release of internal resonance that would otherwise be contained within the guitar body.
How sound is created on the acoustic guitar
While the answer above is a decent summary of the role of the sound hole, there’s a lot more detail we can go into that explains the purpose of the humble sound hole more clearly. To really grasp the concept though, and how it relates to the acoustic guitar we first need a quick overview on the physics of sound.
Ever wondered why your acoustic guitar has that big old hole right in the middle?
Forever the source of frustration, the sarlacc of plectrums, the sound hole actually plays a big role in how an acoustic guitar produces sound, or perhaps more accurately, amplifies sound.
Sound occurs when an object vibrates. The vibration, causes the molecules in the air around it to vibrate, which in turn creates audible sound waves.
If there is no air, there is no sound.
Hence sound cannot exist inside a vacuum as demonstrated by English scientist Robert Boyle.
Sound waves are made up of high pressure areas (compression) and low pressure areas (rarefaction). The height of the sound wave (amplitude) is a measure of the intensity of the sound wave, or volume and is measured from the center of the sound wave (the point of equilibrium).
The diagram below shows this in more detail.
The guitar body is essentially a resonance chamber.
When the guitar is played the vibration from the strings is transferred to the soundboard (the top of the guitar body) through the bridge of the guitar which is attached to the soundboard. This transfer of vibration is known as resonance.
The sound hole actually contributes to amplification in a couple of ways.
First it allows for greater vibration of the soundboard. By removing a section of the soundboard, in this case the sound hole, the weight of the soundboard is reduced which increases its ability to vibrate with greater intensity.
Secondly, the sound hole allows a point of exit for internal resonance that would otherwise be contained within the resonance chamber that is the guitar body.
Directional sound waves
Have you ever been listening to a song on your smartphone and noticed it’s volume appears to increase when you place it into a bowl, despite there being no additional energy introduced? This is because sound waves emanate out from the source of vibration in all directions reflecting against the sides of the bowl. Due to the opening of the bowl, the sound waves are released in one direction as opposed to multiple.
When contained as in the case of the reflected sound waves from the underside of the soundboard, the sound hole provides direction for the sound waves to travel. You are not really hearing a louder sound, just a more directed sound e.g. volume is louder on the top of the guitar body due to the sound hole but lower on the back of the guitar due to the absence of a sound hole.