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Hump FAQ
When viewing the fretboard from the side, instead of seeing a perfectly flat plane from the nut to the last fret, 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.
I am so glad you asked. I am going to address that very issue in this article.
For accurate answers, we need to go deeper ... Please read on ...
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. An independent neck may be held in place by glue (as is seen in dovetail joint designs) or by using bolts (often used with a mortise and tenon or butt joint design).
Spanish Heel
The body is added to an integral neck/neck block to complete the guitar.
Bolt-on / Glue-on
The neck, an independent component, is added to a completed body.
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. 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 second, and 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.
Elevated Fretboard
The neck supports the fretboard throughout its length, as a single unit.
Unsupported Extension
The section of the fretboard unsupported by the neck is directly attached 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.
Acoustic vs Electric Necks
Is it Really a 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, imagine a line-up of 22 individuals, all standing next to one another in a row. Each person is wearing a sequential number, with # 1 standing on the far left and # 22 standing to the far right. Someone shouts, "I need a volunteer!" and everyone takes a step backward, except for # 14.
Very often, just as that lone individual did NOT step forward, yet 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:
Forward neck bow (naturally occurring or induced "Relief")
Fallaway
It might be easier if I just draw it:
The Fretboard "Hump"
Fall-away
Fallaway is a deliberate design decision that affects the fretboard extension. Advocates of fall-away suggest that, even though the string path is already diverging distinctly from the fret surface plane, in order to prevent the oscillating strings from contacting the frets, it is necessary to increase the distance between fret and string over the fretboard extension.
This is accomplished by tapering the fretboard material away from the string path, from the body joint to the end of the board nearest the soundhole. Lay a straightedge across the frets to verify whether or not your guitar has been built this way, or was modified after a neck reset or a fret job.
It is a generalized approach to mitigating string buzz, similar to introducing relief to the fretboard between the nut and the heel.
Like "relief", it depends on several factors. These include the angle of incidence at which the fretboard plane intersects the string path, how low the action is set, how consistently a player frets high up the fretboard, and how aggressively the strings are plucked/strummed.
A fretboard may feature deliberate fallaway and still suffer string buzz, for more than one reason (not the least being that the fret positioned above the neck/body joint is closer to the path of the widest swing of the oscillating string than are any of the frets positioned along the fall-away).
Contrasting a steeply pitched neck with a neck that (more closely) parallels the top surface of the guitar may make the whole thing easier to visualize. These drawings are of two popular electric guitar designs, not acoustic guitars, but they illustrate the point.
Below is an illustration of the side view of a Gibson Les Paul. Notice the distinction between the plane made by the string path (red line) and plane of the body (green line):
Gibson Les Paul Illustration
Compare the Les Paul with the illustration, below, of a Fender Stratocaster. Notice how much more parallel those two planes are.
Fender Stratocaster Illustration
Acoustic guitars whose neck-to-body geometry more closely resembles the Les Paul often feature the fall-away fretboard extension modification.
Fallaway may be deliberately engineered to keep the strings off the fretboard, but it can also occur unintentionally, such as after a neck reset on an acoustic guitar. When the neck is re-attached (often at an increased incline), in lieu of shimming the fretboard extension, the section of fretboard is just bent down and re-attached to the soundboard. This dramatically increases the fallaway effect.
Forward Bow
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.
NOTE : 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.
I know that I have waxed philosophic on this topic of the "belief in relief," ad nauseum , including having dedicated a section to it in my article, » The Truss Rod . I fear I must digress yet once again to make a relevant point.
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 in the neck exist at the nut and the end of the fretboard. They do not. That is not where the bow is occurring.
The Range of Forward Bow
On these guitars, with or without truss rods, the endpoints of the curve of the forward bow only exist from the nut (and sometimes as far forward as the 2nd fret) 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 the metaphoric equivalent of a fretboard hump? It is a stretch, I realize, but humor me. Allow the horse's croup (above its back hips) to represent the headstock, and its neck to represent fall-away (taper) deliberately planed into the fretboard extension.
Are the horse's withers (above the front shoulders) raised higher than they were, yesterday? Does it exhibit a shoulder "hump?" Or, rather, does the "sway back" condition this horse suffers from (called "lordosis") create the illusion of a hump?
Swayback Horse (see the hump?)
It bears repeating. The two factors responsible for the optical illusion often mistaken as a Fretboard Hump are:
Fallaway : 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, for some players, failure to introduce fallaway may result in unwanted string to fret contact, especially where action (string height) has been set as low as possible, fretting is occurring high up the fretboard, and/or the strings are strummed/plucked 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 fallaway may need to be 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 fallaway. 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 typically 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 (about) the 10th or 11th fret where the area between the 5th to 7th 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 an exaggerated forward bow as well as fall-away, the area between the 14th and 15th fret will appear to be raised into a hump.
The Wavy Fretboard
If you will keep any relief well below the "standard" 0.008″ (0.025 mm) ,and minimize fallaway, there will be little to no apparent hump effect. The more gap you introduce between the strings and the fretboard, the more pronounced the "hump effect" may appear.
In the case of a severe fallaway 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.
It Really is 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 titled, » Neck Blocks, Tail Blocks and Linings .
We know that most neck blocks are cut and installed 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, all along!")
In the presence of humidity high enough (and wood vulnerable 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 cracks and splits in the sides and possibly even in the soundboard. 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 potentially cause an existing hump condition to appear to be more prominent. The lowered string height can introduce fret buzz that wasn't present before. Conversely, if the neck block were to rotate forward, the action would be raised and the neck would likely require resetting.
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.
If your guitar is being subjected to these conditions, a "hump" in the fretboard is likely the least of your concerns.
Summary
IDEA
: Instead of continuing to build rubbery, curl-capable necks for our acoustic guitars, relying on internal steel machinery to make up for the shortcoming, we could build stiff, curl-free, immovable necks with flat fret surfaces right from the outset.
IDEA
: We could properly support the fretboard extension to maintain a flat plane across the length of the board (such as is done with an elevated fretboard), controlling string action height using the neck to body joint.
IDEA
: Instead of continuing to build flexible body shells that become victims of humidity, we rigidify our guitar bodies utilizing lamination techniques for neck and tail blocks, linings, and even the sides, if desired.
We could say, "Good-bye" to a host of issues.
For one: no more fretboard hump.
It's just a thought.
