Why Do Acoustic Guitars Have Sound Holes?

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.

Sound wave

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.

What is resonance?
Resonance in its most basic sense is the transfer of vibration from one surface to another. Resonance can occur by being reflected from another surface (e.g. sound waves emanate out from one surface until they come into contact with another surface causing that surface to also vibrate) or if the vibrating object is in direct contact with the initial source of the vibration.

In the case of the acoustic guitar the bridge is attached to the soundboard, while the internal walls of the guitar body reflect vibrations from the underside of the soundboard.

What confuses people is that resonance doesn’t introduce additional energy, like in the case of a powered speaker.

The increase in intensity of the vibration is due to the transfer of vibrations from a smaller surface to a larger surface e.g. the strings transferring vibrational resonance to the larger surface area of the soundboard. As the soundboard is considerably larger more air is displaced. This allows for a sound to be amplified, as in the case of the acoustic guitar.

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.

Soundhole Size: Why some guitars have different size and shape sound holes

Most acoustic guitars typically feature a round sound hole approximately positioned toward the upper bout, at approximately the center of the soundboard. Some acoustics however feature far more intricate sound hole designs, or in the case of some Godin guitars and Ovations such as the Ovation Elite, multiple, strategically placed smaller holes.

Another variation on the sound hole is the classic F hole, seen mostly on violins and other acoustic instruments such as semi-acoustic archtop guitars.

While there’s an obvious aesthetic appeal to the different sound hole design seen on the guitars listed above, it’s also true that altering the location and shape of the sound hole/s can influence tone and volume.

It’s generally accepted that the smaller the sound hole area the less volume produced. In the case of semi-acoustic guitars like the Godin or the semi-acoustic archtops, as the guitar is designed to be played through an amplifier the volume is less of a design consideration. But this is only right up to a point. The shape of the guitar along with the acoustic properties of the tone woods the guitar is constructed from all contribute to the sound produced when the guitar is played

Summary

As we can see, the sound hole of your guitar does a lot more than just swallow guitar picks. The main reason acoustic guitars have sound holes it to amplify the transferred vibrations (resonance) of the strings and reflected resonance in the body of the guitar. They also contribute to feedback when the guitar is amplified but that’s another story for another article.

As always if you have a comment or want to share your experiences on the guitar we welcome your comments below.

Leave a Comment

Stay Tuned!

Enter your email address below and we’ll notify you now and then when we publish something new.
No spam, nothing to sell,
just sharing good info.

Tweet
Pin
Share