All About Flat-Top Acoustics

April 1, 2009

MOST PLAYERS KNOW THAT HAVING A SOLID TOP on a flat-top acoustic is a good thing, and that having solid back and sides is even better, but relatively few take their contemplation of acoustic guitar construction to the point of considering exactly how these slivers of wood are put together. Sure, the wood itself will play a big part in any flat-top’s signature sound, but the way in which that particular guitar is constructed— and its overarching design—are equally crucial to shaping its voice. This All About will provide an overview of the most popular tone woods used in the construction of steel-string flat-top acoustic guitars, and also explore some of the most common bracing and construction methods, all to the end of helping you better understand what’s going on inside the box. 

DESIGN AND CONSTRUCTION

One factor of guitar design that most newcomers to the art will notice first is sheer size. The size and overall internal dimensions of a flattop’s body are in direct correlation to its volume- producing abilities, and, to some extent, its low-end response and high-end clarity. In short, given two well-made instruments from either end of the scale, the big boy should really hurl out some noise, with thumping lows and impressive projection, while the smaller model should be softer and sweeter, but with clear highs and good balance and articulation. As fundamental a consideration as size is, however, there’s a lot more to guitar design than the “size equals volume” equation.

To get a little more technical about it, 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 (or soundboard), the more it will vibrate when a string is plucked, and convert the string’s own vibration into acoustic volume. At the same time, however, this soundboard has to be strong enough to withstand the force of the tight, tuned-up steel strings pulling upon it, which is around 175 lbs of pressure on the average flat-top. If you make a guitar top thick enough to withstand this string pull all on its own, you won’t get much sound out of it at all, so luthiers use braces glued to the underside of the top to strengthen it, which enables the use of a soundboard that is thin enough to vibrate freely.

While some very basic budget or beginner guitars are made with simple parallel braces that run from the endpin to the neck heel either side of the sound hole, or so-called “ladder braces” that run side to side across the underside of the top, the real craftsmen discovered long ago that more careful and considered bracing patterns and techniques could greatly improve the resonance of the instrument. As far back as 1850, C.F. Martin developed a pattern known as X-bracing that greatly increases a soundboard’s rigidity without excessively hampering its ability to vibrate freely, and this remains far and away the standard for flat-top acoustics today (nylon-string classical guitars have different structural requirements and use other patterns, which won’t be discussed here). 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 soundhole. This technique also employs a number of small struts glued at other points around the top, to add further support. Precisely where that crossing of the “X” occurs, the acuteness of the angles it forms, and the number and positioning of the supportive struts are factors that vary widely, and are often part of any respected guitar maker’s secret tonal recipe. Many manufacturers—from Taylor to Martin to Gibson to Santa Cruz—will even use a wide variety of X-bracing patterns across their range, tailoring the braces to suit the model of guitar and the tone they want it to produce. 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.

As we are already beginning to see, rather than there being some ultimate standard of construction that works best for all good flat-top acoustic guitars, different 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 honky-tonk night after night. On the flipside, a smaller-bodied 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. In between these, a big-bodied dreadnought designed to excel at flatpicking for speedy bluegrass soloists might use scalloped braces and a slightly thinner top, whereas a small- or mediumbodied flat-top, intended to excel at hybrid (pick and fingers) picking while also being suitable for strummed rhythm, might want a slightly more rigid top than a fingerstyle guitar of a similar size.

Neck construction, and more specifically the way in which the neck is attached to the body, is another important factor in any flattop’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 setup issues. For many years, glued-in necks using a dovetailed or mortise-and-tenon joint were the standard, and these do continue to proliferate today. But plenty of adventurous manufacturers have used variations on a bolted- or pinnedin 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, such as Taylor and Froggy Bottom, neither of whom are likely to be accused of building guitars that are tonally inferior to those with glued-in necks.

The wood used to construct an acoustic guitar’s neck and fretboard do play a small part in forming its tone, but this contribution is usually dwarfed by that of the tonewoods used in its top, back, and sides.

TOP WOODS

The most significant slice of wood in your entire acoustic guitar is its top, or soundboard. An acoustic guitar purporting to be anything other than a budget option for beginners ought to at least have a solid top, which costs more than a laminated or plywood top. The good news is that the range of entry-level models carrying solid tops is wider today than ever before, and these instruments are also more affordable than they were just ten years ago. When we talk about solid woods in acoustic guitars, we’re still talking about very thin cuts of wood— pieces of wood almost of a thickness that could be used as a veneer in other types of construction—but they are sawn from solid timber and used all on their lonesome, without being glued back-to-back with other cuts of wood. The term can be confusing, as there’s a lot of hollow space inside an acoustic guitar, such as, for example, a Martin D-28 that’s made from solid spruce and solid rosewood. But the top, back, and sides of such a guitar—however thin—are indeed made from solid wood.

SPRUCE Far and away the most popular top wood for flat-top acoustic guitars, spruce is light, strong, and rigid—all qualities required in a resonant soundboard. Spruce has a round, sweet tonal character and an appealing high-end response, with good clarity and definition. The most common form of spruce used today is Sitka spruce, a wood that accentuates a guitar’s fundamental notes boldly, and presents a bright, loud voice within the overall spectrum of this variety. The rarest and most prized variety is eastern red spruce, also known as Adirondack or Appalachian spruce. Along with its darker, slightly amber appearance, it has a rich voice, delicious complexity, and excellent volume. This is the wood of the most prized pre- WWII flat-tops. Few old-growth stocks remain, and any new guitars built with it will be very expensive. Another highly prized variety of spruce with a lighter grain, Englemann, has a tonality that is more harmonically complex than Sitka, but it is also mellower and less forceful. Any good piece of spruce should exhibit a tight, fairly straight grain, although some appealing irregularities do occur in perfectly good samples. A spruce top generally takes some “playing in” to reach maturity and reveal its full tone. (Note that braces on the finer acoustic guitars are often made from spruce, too.)

CEDAR Many flat-top makers have also made good use of cedar, also known as western red cedar, but it is far more common to classical guitar building. Cedar is usually heard as rich, open, and well defined, perhaps with a mellower bass than spruce and a little less clarity. Sometimes, it also has a warmer overall tone. It is a wood that reveals its voice more quickly on young guitars, rather than requiring age and playing-in to bring out its full character, the way spruce does. Cedar tops will occasionally distort more quickly when played hard, but the fact that some good cedar tops can sound louder than spruce tops without aggressive strumming helps to counteract this impression. Cedar has a darker appearance than spruce, which can also be appealing.

MAHOGANY More often used in the back, sides, and neck of an acoustic guitar, mahogany is also occasionally used as a top wood, usually in an all-mahogany-bodied flat-top. Such guitars were traditionally lower-end options (albeit from some excellent makers), but can sound perfectly good for many players’ needs. A mahogany soundboard tends to be less open sounding than one of spruce or cedar, but usually has a pronounced midrange element and a punchy voice.

BODY WOODS

Most conventional flat-tops use the same variety of wood for both the back and sides of the guitar, and although the guitar’s top plays the lead role in determining the instrument’s tone, many manufacturers agree that the wood here does play a part in shaping and fine-tuning its voice (although the debate on this subject continues to rage). Lower- priced acoustics with solid tops will often still use laminated wood for the back and sides, although they usually contain layers of the tonewoods discussed below. When used in laminated wood, the tonal characteristics of these species are lessened. Wellmade guitars with solid tops and laminated backs and sides can still sound very good, but the use of solid backs and sides indicates a jump into the big leagues constructionwise, and another step beyond the tonal improvement that a solid top represents over a ply top. The presence of solid tonewoods here also indicates a better-constructed guitar overall, and that takes you further up the quality ladder. In the better flat-tops, the characteristics of the wood used in the guitar’s back and sides will be clearly present in its overall tone, and usually heard as a degree of seasoning in the low end, overall presence, and harmonic complexity of the instrument.

MAHOGANY The more affordable of the two most common body woods, mahogany nevertheless appears in some excellent flat-top guitars. This tonewood is bright, clear, and tight. It provides excellent volume and projection, with a round, woody midrange.

ROSEWOOD Traditionally a wood used for high-end models, rosewood contributes excellent depth, warmth, and dimension to the tonal signature of a good acoustic guitar. It offers a bold, rich low end, and presents a detailed yet balanced midrange. East Indian rosewood is the most common variety used in guitar making today, although some manufacturers will also use Madagascar rosewood in their top models. Both possess similar tonal traits, although Madagascar is considered to be closer in sound and appearance to the legendary “king of rosewoods,” Brazilian rosewood, which is now on the CITES convention list of endangered species, and therefore barred from importation.

MAPLE A hard, rigid wood, maple is used far less than mahogany or rosewood, but is an important ingredient in a few noteworthy flat-tops, such as the post-war Gibson J- 200 and the Guild F-50. Maple contributes a distinctive brightness and definition to a guitar’s overall tone, and enhances cutting power and sustain.

KOA Traditionally used in Hawaiian-style acoustic steel guitars (that is, those played with a bar slide, or “steel”), where it is sometimes used for soundboards, too, this luscious, honey-colored wood is an exotic choice that is employed for its look as much as— or more than—its sound. It doesn’t make for a particularly loud guitar, but has a clear, bright tone with a little midrange honk that appeals to certain players.

OTHER WOODS Some manufacturers will make their backs and sides from other exotic or uncommon tonewoods such as walnut, cherry, bubinga, Australian blackwood, ebony, ovangkol, oak, imbuia, sapele, and others. Each of these has its own distinctive tonal qualities, but the more unusual among them are often selected on grounds of aesthetics rather than sonics.

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