Ever wondered why your acoustic guitar has that big old hole right in the middle?
Acoustic guitars have soundholes to amplify the sound of the guitar by allowing the internal resonance that would otherwise remain trapped inside the body to be released. The vibrations of the soundboard increase air pressure within the body of the guitar. As the air is released via the soundhole the pressure drops forcing more air into the body. This process then repeats.
How sound is created on the acoustic guitar
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.
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 the 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 sound to be amplified, as in the case of the acoustic guitar.
The soundhole 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 soundhole, the weight of the soundboard is reduced which increases its ability to vibrate with greater intensity.
Secondly, the soundhole allows a point of exit for internal resonance that would otherwise be contained within the resonance chamber that is the guitar body. And as we discussed at the beginning of this article, air pressure is increased due to the vibrations of the soundboard which compresses the air. As it is released the air pressure is lowered which draws more air into the body of the guitar, and this cycle then repeats. This phenomenon is known as Helmholtz Resonance.
Directional sound waves
Have you ever been listening to a song on your smartphone and noticed its 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 soundhole 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 soundhole but lowers on the back of the guitar due to the absence of a soundhole.
Soundhole Size: Why some guitars have different size and shape soundholes
Most acoustic guitars typically feature a round sound hole approximately positioned toward the upper bout, at approximately the center of the soundboard. This is the simplest in terms of manufacturing, provides a balanced frequency spectrum, and is what we guitarists have become accustomed to over the years, but soundholes don’t actually need to be circular.
Some acoustic guitars feature far more intricate soundhole designs, or in the case of some semi-acoustic Godin guitars and Ovations such as the Ovation Applause, multiple, strategically placed smaller holes.
Another variation on the soundhole 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 soundhole designs seen on the guitars listed above, it’s also true that altering the location and shape of the sound hole/s can influence the frequency spectrum of the guitar along with projection and volume.
What’s the difference between projection and volume?
Projection describes the distance sound travels, volume describes the amplitude or loudness of sound.
It’s generally accepted that the smaller the soundhole 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 perhaps 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 tonewoods the guitar is constructed from all contribute to the sound produced when the guitar is played
As we can see, the soundhole of your guitar does a lot more than just swallow guitar picks. The main reason acoustic guitars have sound holes is 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.