Most newcomers to the flat-top acoustic guitar are likely to choose a style of instrument by selecting a size that suites their needs and which feels comfortable under the arm and in the lap. Body styles, however, have a major impact on tone, and it’s worth learning a few things about this dark art so that you’re able to acquire a guitar that suits your sonic needs, rather than merely buying one of a size and shape that have caught your eye for purely esthetic reasons.
Certainly a flat-top acoustic guitar’s volume-producing capabilities are in direct correlation to its body size, and more specifically, its overall internal dimensions. All else being equal, given two equally well-made acoustic guitars — one large, one small — the bigger one should put out more volume and hit you with heavier low notes, while the smaller guitar will be mellower, but (ideally) with clear highs and good balance and articulation. While size is the most obvious factor in acoustic guitar design, however, it certainly isn’t the be all and end all of the matter, and there’s a lot more to it than a simple “size equals volume” equation. As with so many things in life, the mere fact of a thing of greater quantity – in this case, more volume – might imply a certain superiority, but any good acoustic guitar should be at least loud enough for the listener in a medium-sized room to hear it clearly, so a players selection of one type of guitar over another comes down to many other considerations. Certain styles of music might indeed demand a big, booming guitar with a lot of punch, but in many other situations the small yet clear and articulate flat-top might win the day.
Many makers have their own differing designations, but the different sizes of acoustic guitars mentioned here are generally referred to, in ascending order from small, to medium, to large, to extra large, as parlor, concert, grand concert, and jumbo. In 1916 Martin developed a body type for the Ditson company (adopted as a genuine Martin in the early ’30s) that would become known as the “dreadnought”, and this wide-wasted style, which can vary in size from approximately similar proportions to that of a grand concert and a jumbo, has become one of the most popular types of flat-top acoustic guitar in the world. Later in the ’30s Gibson would produce its own over-sized flat-top, one with more rounded body lines, that would become known as the Super Jumbo (model designation J-200 or SJ-200). Gibson has also produced several legendary dreadnoughts, and is particularly known for a body style referred to as the “round shouldered dreadnought”.
Whatever the instrument’s size, the majority of design factors pertaining to the body of an acoustic guitar work toward the perfect ratio of strength and lightness. The lighter—or more to the point, thinner—a guitar’s top (soundboard), the more it will vibrate when a string is plucked, and convert the string’s own vibration into acoustic volume. In addition to being thin enough to resonate well, though, this top has to be strong enough to hold up to around 175 lbs of pressure that the strings exert upon it when fully tuned up. A top thick enough to stand up to that all on its own would be a veritable slab, so to keep the top thin and tuneful makers use braces glued to the underside of the top to strengthen it.
Some entry-level guitars are made with simple parallel braces that run from the end pin to the neck heel either side of the sound hole, while others have something called “ladder braces” that run side to side across the underside of the top. The standard for quality bracing patterns was established as far back as 1850, however, when C.F. Martin developed a pattern known as “X-bracing”. The X-brace greatly increases a soundboard’s strength without excessively hampering its ability to vibrate freely, and it remains far and away the preferred bracing system for flat-top acoustics today (nylon-string classical guitars have different structural requirements and use other patterns, which we will look at below). As the name implies, X-bracing uses two braces carved from light but strong wood that cross each other in an “X” pattern somewhere on the bridge side of the sound hole. This technique also employs a number of small struts glued at other points around the top, to add further support. Many manufacturers—Gibson among them—will use a wide range of different X-bracing patterns across their model range, tailoring the braces to suit the style of guitar and the tone they seek to achieve. In addition to this, some makers will “scallop” braces by shaving some wood away from their sides, a practice that reduces the weight of these “support beams”, while retaining the majority of their strength.
While certain design standards for “the quality flat-top guitar” have certainly been established, different makers use many variations on the template enhance different sonic goals. A big dreadnaught or jumbo-sized acoustic that will be used mostly for booming rhythm playing might benefit from a top made from slightly thicker wood and heavier bracing, one that can withstand the stress of aggressive strumming, project a lot of volume when driven hard, and resist the vibrational distortion that a thinner, lighter top might succumb to when whacked in anger at a beer-hazed pub gig night after night. At the other end of the scale, a smaller-bodied concert or parlor guitar designed for fingerstyle playing might require a thin top and carefully designed light bracing so that the soundboard is livelier and the strings don’t have to be hit hard to produce adequate volume. Such guitars will still produce a little less volume than a big jumbo-bodied flat-top, but their added clarity and woody richness make for a fair trade off. In between these, a big-bodied dreadnaught designed to excel at flatpicking for speedy bluegrass soloists might use scalloped braces and a slightly thinner top; a medium-bodied flat-top such as a grand concert model intended to excel at hybrid (pick and fingers) picking while also being suitable for strummed rhythm might employ a slightly more rigid top than a fingerstyle guitar of a similar size.
Up the Neck
After discerning any flat-top’s body style, we usually look to the design and “feel” of its neck. The feel of any guitar neck in the player’s hand is a highly subjective matter that each guitarist needs to assess for himself. Neck construction, on the other hand, and in particular the way in which the neck is attached to the body, is an objective consideration and another important factor in any flat-top’s design. While the neck/body joint does play a part in transferring resonance and string vibration—and makers will therefore argue the tonal merits of their respective techniques—it’s perhaps more interesting to examine a few of these with respect to maintenance and set-up issues. Glued-in necks using a dovetailed or mortise-and-tenon joint have long been the standard, and remain so today. But many pioneering manufacturers have used variations on a bolted- or pinned-in neck joint, arguing that many acoustic guitars will need some neck adjustment in their lifetime regardless of how well the neck/body joint is constructed, so a removable attachment will make that maintenance a lot less painful when the time comes. Variations on the removable neck are used by highly respected makers both large and small, from Taylor to Froggy Bottom, neither of whom are likely to be accused of building guitars that are tonally inferior to those with glued-in necks.
The majority of flat-tops made today have necks that attach to the body at the 14th fret, usually with six further frets laid over the top of the body on a fingerboard extension. Up through the first third of the 20th century or so, however, the 12th-fret neck joint was the norm, and some players and manufacturers alike still prefer this variation. More than just giving you fewer frets to work with—which would seem to make the 12-fret neck more a negative than a positive—attaching the neck at the 12th fret shifts the speaking length of the strings further into the body, resulting in a bridge that is centered further in the “meat” of the lower bout. Some players feel this gives such guitars a fuller, richer tone, or at least one that is somewhat different from a similar guitar with a 14-fret neck joint.