By: Aaron Schulman
The acoustic guitar body, or “sound box,” is where the sound is perfected, enriched and enhanced.
It is also the piece of guitar construction that helps suit a player’s size, sound desires, and playability.
In this guitar anatomy article, we go into detail of its construction to help any guitar enthusiast gain a greater appreciation for the art and skill of crafting a fine acoustic guitar.
In the first 3 parts of this series, we talked in depth about the overall anatomy or gross anatomy of the acoustic guitar.
Universally, they are essentially the same with a few differences on models, makers, and styles (whether it’s a beginner’s guitar or a professional model).
The second part of this series discussed the guitar headstock in fair detail.
The third part of this series discussed the making and design requirements for a solid neck and fretboard.
In this next installment, we will talk about the basic anatomy of the 6-string dreadnaught guitar body (with a few exceptions).
Though body styles and sized vary greatly from manufacturer to manufacturer, most of the acoustic guitar bodies have the same conceptual anatomical components.
By learning about the basics of the most commonly used body style, one can easily build upon that knowledge when learning about different sizes and styles (such as the arch top acoustic, the jumbo, or the tenor).
The body is a sound-box
Luthiers (guitar makers), often refer to the body of the acoustic guitar as the “sound box.”
Essentially, they are not referring to a square or rectangular shape, but a quasi-4-sided body (with curves and bracing) that acts as a box to amplify and enrich sound vibrations that come from the steel strings.
The body or “sound-box” of the guitar is as varied as any other instrument in design, size, tonewoods and materials that are used, as well as finish coats, paint, and other designs and inlays.
All of these variables are necessary for a finished acoustic guitar body, but they vary greatly between luthiers.
All of the different elements create different tones, projection, overtones, and EQ spectrum between various guitars.
An easy way to demonstrate this would be to take a dreadnaught sized guitar from 5 or 10 different companies and play them in the same environment.
This is the easiest way to feel and hear that all acoustic guitar bodies are not created equally.
If you take a look at the outside of the acoustic guitar body, they all have many common anatomical terms (parts), in common, regardless of the manufacturer or size. (See figure 1 for visual aid on these terms.)
The unique shape of an acoustic guitar helps shape its specific tones and overtones when played, as well as the tonewood composition of the top, backs and sides.
Imagine the guitar body as a drum. The top of the guitar body does most of the vibrating and amplifying for the string vibrations (like the drum head for a snare drum – vibrating from the hits of the sticks).
If the shape of the drum were restricted with a waist or shaped out of round, the vibrating head would have more restriction and therefore the sound would “close up” a bit.
Likewise, with an acoustic guitar, the less severe the slope of the waist is, the less restricted or more “open” the sound and EQ response will be in all 3 registers (bass – mid- treble – separated for clarity).
The upper bouts (outward curving areas / shoulders) of the guitar accentuate and amplify the higher tones in the Eq spectrum.
The lower bouts amplify the lower (or bass) tones. If one were to compare a jumbo guitar model with a Dreadnaught model, the accentuated (deeper) waist in the jumbo model would be visually obvious (see figure 2).
A deeper waist makes for a more ergonomically friendly design (easier to keep it rested on the leg while playing), while restricting the vibrations of the top more so than on a shallow-waisted guitar, as in the Dreadnaught shape.
In contrast, the shallow waist of a dreadnaught makes for a more difficult guitar body to steady on the leg, while opening up the sound profile (EQ balance) of the guitar body.
The overall dimensions of the body as well as the depth of the body (created by the width of the sides) affect the tonal qualities and amplifying capabilities.
Top, sides, back and tonewoods
In another 2-part article on GuitarInternational.com, we talked in depth about tonewoods that are used for the top, sides and back.
With respect to the acoustic body, a good top will be made from a tonewood that has a high strength to weight and thickness ratio (high strength with – low weight and thickness).
The top is usually “book-matched” from 2 pieces of identical or mirror images of wood, with a glued seam from top (neck area) to bottom (between the lower bouts or heel block area), dividing the rosette in the middle.
It is also ideal to have a wood with low internal damping of vibrations (reduces sound vibrations naturally) and a high velocity of sound (vibrations travel efficiently through the wood).
Some of the most popular woods that naturally fit this description are from the spruce and cedar families: Sitka spruce is the most popular followed by Engelmann Spruce, Adirondack Spruce and some others – while Western Red Cedar is popular for many classical guitars and many steel-string guitars.
When the guitar strings are strummed or plucked, the vibrations travel through the saddle and bridge, and are amplified by the top tonewood.
The sides and back help in amplifying and giving overtones to round out the sound and give more complexity to the tones (kind of like adding spices or seasonings to a recipe to give it more flavor).
Common tonewoods that are used for the back and sides are, Mahogany, Maple, Nato (cheap but similar to Mahogany in character), Sapele, Rosewood, Koa, and many others.
Again, each wood gives its own “spice” to the tones and overtones of the amplified sound which is characteristic of the species of wood.
As a player strums on the guitar, the pick (or plectrum) guard protects the top from wear and tear of playing.
The body structure is decorated often by a rosette (decorative and structural solidifying ring around the sound hole) which can be made of many materials, including plastics, abalone or mother-of-pearl, and exotic woods.
The edges of the sound box are held together with binding and are decorated by purfling made of similar materials as the rosette. Many luthiers have taken the art of inlaying to amazing levels of ornateness and beauty.
The Guts of the Guitar Body
If one could see inside the acoustic guitar body, many simple things would be evident.
There would be somewhat of a “skeleton” which gives the guitar strength, gluing area, and structural integrity.
At the inside near the neck joint, there is a neck block which gives strength to the guitar as well as gluing areas for the top, back, neck joint, sides and kerfing (bendable, slotted wood pieces used to help fasten the top and back to the sides).
Opposite the neck block (between the lower bouts) is the heel block.
It also gives structural support, gluing surface, and a solid block for supporting an end-pin or end pin jack for a stereo cable to connect to an on-board pickup system or microphone blender.
Under the top (surface) of the guitar and the back, there have been countless ways to glue supportive bracing patterns (see figure 3) which help distribute the vibrations of the top while giving it reinforcement to maintain structural integrity over the lifetime of the guitar (due to the average of 180 lbs of tension cause by the constant pull of 6 steel strings).
Because the top and back are so thin, they must have some kind of skeletal reinforcement to maintain the body’s integrity over the lifespan of the guitar and exposure to various environmental conditions.
Various luthiers also scallop some bracings (carve them) differently than others, giving unique qualities to the sounds of the different guitars.
Some patterns are called “fan” and some are called “X” bracing, which are used for different models of guitars based on their shape and the luthiers intentions.
Also, many luthiers have used combinations and have invented some of their own bracing patterns which carry their own name.
The ladder bracing pattern is often used to reinforce the back of the guitar.
Though there is no “right” way to brace guitars internally, luthiers have their biases, preferences and techniques.
They also continue to innovate with design changes and different woods to achieve various effects as well as cost constraints (for mass producers of low to mid-range models).
The finish of the guitar body can have as much of an impact on the guitar’s sound, brightness, EQ balance and projection as the size, style and tonewoods used.
A good example and proof would be realized when playing guitar of the same exact makeup and company, yet with different finishes.
There are many ways to finish guitars, including being airbrushed / painted then finished with some kind of clear-coat.
Polyester is very common today for clear, bright and shiny, high gloss acoustics.
Lacquer is a common traditional use for glossy, steel string guitars.
Also, traditionally, shellac has been used in layers for a technique known as French polishing for many classical guitars.
These 3 are the most popularly used finish methods currently for commercial guitars.
Taylor guitars for example (makers of the Taylor 110) have moved away from nitrocellulose lacquer in exchange for a more durable, thinner polyester finish that is cured / treated by UV.
Glossy and bright: Glossy or high gloss guitars not only look bright and shiny, they also project more clearly, pronounced, and have a “brighter” sound. They will punch through and stand out more so than a satin or natural finish.
Satin or semi-gloss: A guitar with a satin finish will be a bit more mellow than a glossy finished guitar of the exact same model.
Many bluegrass players will appreciate the mid-range sound of a semi-gloss acoustic dreadnaught for its ability to punch through in the mid-range and carry a semi-mellow tone instead of a cutting, bright, edgy sound of a glossy guitar.
Natural finish: A few models will have a flat, natural finish and are clearly the most dull sounding and the least-projecting of all finishes.
Most guitars are made in the glossy to satin finish range. A natural finish guitar is selected by a player who desires an incredibly subdued, woody and mellow, reserved sounding guitar.
Some closing thoughts:
The acoustic guitar body is quite simple in design, yet painstaking detail and craftsmanship must go into each step in order to create a fine-sounding, durable instrument that tunes well and plays well anywhere on the fretboard.
Using knowledge as a guideline to find the profile you are seeking, you should be well on your way to finding the perfect guitar for your tastes, playing group, arrangements and budget.
About the Author
Aaron Schulman enjoys playing, researching, writing in-depth articles and guitar reviews. He has written many reviews and organized them by price, size, and playing level. He currently has a favorite guitar in the best acoustic electric guitar under 1000 with the Yamaha A3R (Rosewood with SRT electronics). You can read more about him at StrumViews.