august 2018

One of the questions I’m often asked as a guitar maker is if a cutaway changes the sound of an acoustic guitar. I think this is often a polite disguise for a different question: “Do I lose any tone by having a cutaway?” To better understand this, it helps to be familiar with a few of the factors involved.

Of the many variables that create an acoustic guitar’s sound, two significant elements are akin to lungs and vocal cords. The air inside a guitar’s body is like lung capacity. This air mass has a resonance determined largely by its size and the size of any openings. Scientists know this as the Helmholtz resonance. The effect can be demonstrated by blowing air across the top of a bottle. The larger the cavity is, the lower the pitch will be. So, as a beverage is emptied from the bottle, blowing across the top will result in progressively lower frequencies.

The top and back of the guitar are like the vocal cords in a voice box. Their sound is largely governed by their bracing and shape, which affects the tautness and motion to make vowel sounds and syllables, or the notes we play. And just like vocal chords, the tighter or stronger a guitar is, the higher the frequencies it will tend to emphasize. It’s easy to imagine the internal braces changing the stiffness of a top, but the perimeter shape is also significant. As a rule, the more extreme the curves are, the stronger and stiffer the parts become. To get a better picture of this effect, imagine a sheet of paper held on edge with a gentle horizontal curve. Placing the smallest amount of pressure on the paper causes it to wrinkle or flex. If this gradual curve is made tighter, it can withstand far more pressure. If the curve is made even tighter by rolling the paper into a tube, it can withstand a great deal of pressure without crumpling, even if something proportionately heavy were to be placed on top of the open tube-end. This is one reason why the more extreme curves of a jumbo-shaped guitar typically produce a brighter sound than the deep and warm sound of a more broadly curved, dreadnought-shaped guitar. The tight-radius curves of a cutaway on either of these guitar shapes will result in extra strength on the guitar’s upper bout.

These two primary aspects of a guitar’s sonic signature—the lungs and voice—work in tandem with each other. When a portion of the guitar’s body is removed to form the cutaway shape, the air in the guitar body’s lungs is made a little smaller. As long as the size of the soundhole remains the same, the pitch of this air will rise a little compared to the same guitar outline without a cutaway. At the same time, the sharper curves of the guitar’s silhouette make the top and back a little stronger—like vocal cords pulled taut—which further emphasize a higher tonal range.

Now, let’s revisit the question of whether a guitar with a cutaway loses tone compared to an identical instrument without one. No: The tone simply changes a small amount in a way that is perfectly appropriate for what a musician wants to do with the instrument. Practically speaking, a cutaway encourages easy fretting of the highest notes on the fretboard. Meanwhile, the slightly smaller lung capacity and extra vocal-cord tautness from the cutaway guitar body shift the guitar’s frequency response up, which helps to make those high notes sound good. By comparison, a non-cutaway guitar with the same outline will tend to shift its preferred frequency range down to the lower notes on the fretboard.

I feel a more helpful question for a player to ask is, “What will result in the best music?” After all, you could say we hear the relationship between the player and the guitar. So, when a player’s repertoire contains a lot of high-note passages, the music will be better if the performer has the dexterity and high-note fretboard access a cutaway instrument allows. If a player’s style focuses most heavily on low, open-position sounds and chords, the music may sound better with a non-cutaway instrument.

For those who simply want to know how big a difference there is, well, the change is small—small enough that other factors, such as the exact pieces of wood each guitar is made from, the strings, the pick a player uses, and how much coffee they drank that day all seem to matter more. In the end, there is no loss of tone—only a small change. The cutaway option gives players a choice based on their preferences, how they play, and what they like to hear.