The Fretboard Hump

Much as the so-called common cold is painfully common to some people while remaining pleasantly uncommon to others, so the fretboard hump, also called a body joint hump, describes a condition that is all-too-familiar to some acoustic guitar players or owners, while others may have never heard of it.

Question: What is a Fretboard Hump?
Answer: When viewing the fretboard from the side, instead of seeing a perfectly flat plane from one end to the other, we may see what looks like a raised area, or hump, at the point where the fretboard intersects the body (usually at the 12th or 14th fret). A straightedge laid across the tops of the frets can be helpful to visualize the issue.

Question: What causes this hump and will it happen to my guitar?
Answer: In order to answer this we need some background information.

Neck Design

One of two approaches is taken to incorporate the neck and body of an acoustic guitar. Either the body is added to the neck, such as we see in a Spanish Heel design, or the neck is added to a completed body as an independent component and may be held in place by glue (as is seen in dovetail joint designs) or using bolts (often used with a mortise and tenon design).

Using either approach to connect the neck and the body, a section of the fretboard extends beyond the heel of the neck (or the point where the neck intersects the body) and out onto (or over) the body of the guitar.

One of two distinct neck designs determines whether or not the fretboard makes direct contact with the soundboard. In an elevated fretboard design, a section of the neck wedges between the fretboard and the soundboard, much as we might expect to see on an electric guitar. The neck may approach the plane of the soundboard at an incline, or the upper bout may be sloped sharply down toward the back of the guitar, or both.

The most common neck design used on today's acoustic steel string guitars attaches an unsupported fretboard extension directly to the soundboard. Although some instruments feature a bolt-on attachment to secure the fretboard extension, most are simply glued down to the soundboard.

The Spanish Heel neck, the Elevated Fretboard neck and even the common bolt-on electric guitar neck do not suffer the condition known as the fretboard hump. It is only seen in guitars having a glued-on fretboard extension.

The Hump

The word hump refers to what appears to be a raised area of the fretboard where the neck joins the body. However, all is not quite what it appears to be.

To visualize this, think of the fretboard as a line-up, with each fret represented as an individual. Someone shouts, "I need a volunteer!" and all the frets take a step backward, except for the 14th fret (or whatever fret is residing above the intersection of the neck and body - the fret at the body joint). Just as that lone individual did NOT step forward (but ends up prominently portrayed as the "volunteer"), the fret above the body joint has not raised. It remains fixed, right where it was when it was originally installed.

The frets that "stepped backward", so to speak, creating the illusion of a "lone volunteer", a hump in the fretboard at the body joint, are the recipients of two independently controllable factors:

1. Forward neck bow
2. Fall-away

On many acoustic guitars, a deliberate design decision called fall-away is introduced to the fretboard extension, where the fretboard material is tapered to slope away from the path of the strings from the body joint to the end of the board closest to the soundhole. Lay a straightedge across the frets to verify whether or not your guitar has been built this way.

Question: What is the purpose of this fall-away?
Answer: To mitigate string buzz, just like introducing relief to the fretboard between the nut and the heel.

Question: Do all acoustic guitars feature this "fall-away"?
Answer: No.

Question: Is this fall-away effective? Is it absolutely necessary?
Answer: It depends on several factors, including the angle of incidence at which the fretboard plane intersects the string path, how low the action is set, and how aggressively the strings are plucked/strummed.

Contrasting a steeply pitched neck with a neck that (more closely) parallels the top surface of the guitar may make this easier to visualize. These drawings are of two popular electric guitar designs, not acoustic guitars, but they illustrate the point:

Fall-away may be deliberately engineered to keep the strings off the fretboard, but it can also occur unintentionally, such as after a neck reset where, in lieu of shimming the fretboard extension, it is just bent down and re-attached to the soundboard.

With fall-away accounting for the visually prominent hump on the soundhole end of the fretboard, forward bow in the neck on the other side of the hump can really help to exaggerate the effect.

Forward bow in the neck readily occurs as a result of the tension of the strings relative to the stiffness of the neck. Under tension, the neck will tend to bow, or curl forward.

Additionally, an otherwise rigid or very stiff neck can be deliberately induced to bow forward by adjusting a dual-action truss rod, such as when adding relief (Relief refers to an ever-so-slight but deliberate forward bow in the neck intended to accommodate the path of the oscillating strings, preventing or mitigating the incidence of contact with the metal frets).

For an acoustic guitar neck whose fretboard extension is secured directly to the soundboard, it is a mistake to assume that the endpoints of the curve of the forward bow exist at the nut and the end of the fretboard. They do not; that is not where the bow is occurring.

On these guitars, with or without truss rods, the endpoints of the curve of the forward bow only exist from the nut to the heel of the neck (which does not extend past the body joint). In other words, the forward bow of the neck is limited to an area of the fretboard that is shorter than its entire length.

So what? Why does this matter? An exaggerated forward bow (curl) in the neck, along with an exaggerated fall-away at the fretboard extension will create the illusion of a hump at the body joint.

In the image of the horse, below, do you see humps? Are its withers (above the front shoulders) raised? Is its croup (above its back hips) raised?

Or, rather, does this horse suffer from "sway back" (or lordosis)?

The two factors responsible for the optical illusion referred to as the Fretboard Hump are:

Fall-away: On guitars whose necks attach at a steeper angle, the fretboard may be ramped slightly, away from the string path in an effort to mitigate string contact with the frets on the fretboard extension. On some guitars, failure to introduce fall-away will result in unwanted string to fret contact, especially where action (string height) has been set as low as possible and the strings are strummed aggressively. Additionally, when a neck on one of these guitars is reset, the angle of incidence is most always increased (the plane of the neck tips back away from where it was prior to the reset). The fretboard extension must be shimmed to compliment the change in geometry, and the fall-away must be corrected (increased) so that the board does not protrude up into the oscillating string path. Simply bending the extension back down to be glued to the soundboard will really exaggerate the fall-away. This common repair misstep will surely exacerbate the illusion of a fretboard hump.

Forward neck bow: Forget the truss rod for a moment, and recognize that the strings are pulling the neck (most necks) into a forward bow (however slight), and that bow is anchored at the body joint - not at the end of the fretboard. Imagine looking at a profile view of the side of a 14-fret guitar, strings tightened: Starting at the nut, your eyes follow a concave curved path between the nut and the 13th fret where the 6th fret is at the bottom of the curve. Deliberately adding (or being unable to counter) an excessive amount of forward bow will introduce an obnoxious wave into the fretboard, a wave that will appear to crest at the body joint, contributing to the illusion of a hump.

When viewing a neck having both exaggerated forward bow and fall-away, the area between the 14th and 15th fret will appear to be raised into a hump.

If you will keep any relief below 0.01″ (0.1 mm) and minimize fall-away, there will be little to no apparent hump effect. The more gap you introduce between the strings and the fretboard, the more hump may appear.

In the case of a severe fall-away condition, it may be feasible to properly shim the fretboard extension and thereby diminish or remove the hump illusion.

Extremely slender or supple wooden necks may suffer deformation over time, where nothing short of fretboard (or sometimes even neck) replacement can correct the wave effect.

I Swear it's Not an Illusion - It's a Hump!

We know that environmental conditions can exacerbate the illusion of a hump, especially for instruments that are more prone to movement relative to the moisture content in the air.

We know that the most significant movement in wood occurs tangentially. For more information on this, see my article » Neck Blocks, Tail Blocks and Linings.

Most neck blocks are cut in such a way that their tangential movement, their propensity to swell and shrink, is in the up and down direction ... directly beneath the fretboard at the neck-to-body joint.

(See! I knew it was a hump!)

In the presence of humidity high enough to cause the neck block to literally swell sufficiently to actually push the fretboard up into a hump, you would very likely be dealing with a host of issues, including splits in the sides. Under such extreme conditions, the neck joint, which controls the angle at which the neck encounters the body, would be subject to movement. If a tiny shift in the neck block/body connection were to cause the headstock to drop back, ever so slightly, this would both lower the action as well as make an existing hump condition to appear to be more prominent. The lowered string height can introduce fret buzz that wasn't present before.

With wood swelling and shrinking to varying degrees on such a guitar throughout the seasons, and having an understanding of what factors are contributing to the illusion of the hump at the body joint, I hope this can help to explain why the issue of the fretboard hump can appear to be more pronounced on some days than others.