The Acoustic Guitar Body
– Part 1
All the sound of an acoustic guitar originates with the plucked strings.
The rest of the guitar modifies that sound and makes it louder, but
it cannot add anything to it. Strings, and the pick, picks, or the fingers/nails
they are plucked with, are fundamental to the sound of a guitar. Since
sound travels through the air and the small surface of the strings moves
very little air, something is needed to move more air and make the sound
from the strings louder. This is the main function of the guitar body.
It acts as a mechanical amplifier, coupling the energy of the vibrating
strings to the air around the guitar.
This first part, of a two part feature about the body of the acoustic
guitar, takes a look at the history and significance of body shape,
construction materials, the soundboard, bracing, the bridge, the soundhole
and the application of the principle of Helmholtz resonance to control
bass response.
Body shape
The shape of the very earliest musical instruments came from the naturally
occurring materials, such as animal shells, sections of bamboo and gourds,
used to build them. The round bowl backs of lutes and mandolins for
example echo the natural shapes of earlier stringed instruments. These
rounded shapes do have some advantages, their curves give them strength,
minimise standing wave resonances and help acoustic projection, but
they are difficult to construct out of flat, processed woods. Today
the common shape of the acoustic guitar is a compromise between cost,
acoustic perfection, practicality and ergonomic comfort. Given our increasing
understanding of the acoustics of musical instruments there is certainly
room for improvement in the materials and shape of acoustic guitars,
although conservative attachment to the ‘traditional’ acts
as a brake on innovation.
In the 16th century when the early guitars were being built, western
thought was perhaps even more influenced by Arabic and classical Greek
ideas than it is today. Proportion and symmetry, dictated by geometry,
were thought to be very important. The earliest known treatise on lute
making written by Henri Arnaut de Zwolle in about 1440 describes lute
design in terms of geometric proportion, rather than giving dimensions
and this approach to instrument design was still used over 200 years
later. Today we know that symmetry is not necessarily desirable in acoustic
design and some modern makers have produced very beautiful and practical,
asymmetric guitars.
Early guitars had small bodies, with only a slight waist and vaulted,
or even fluted backs. These weren’t comfortable shapes to hold
(although they exhibited desirable acoustic properties), particularly
with the vaulted back and as the size of the guitar was increased for
more volume, the back flattened and the waist tightened. The idea of
a waisted body, with a square back and sides, may have been inspired
by the design of the bowed instruments of the time, where symmetric
waist cutaways were introduced to allow clearance for the bow. Some
versions of the vihuela (an instrument that is related to the guitar)
were played with a bow (see www.lutesandguitars.co.uk for more info
on early guitars).
The depth of the modern guitar can cause problems for the players right
arm and some makers are now building wedge shaped bodies, based on the
original designs of Linda Manzer (www.manzer.com, although credit for
the idea may also be due to noted luthier William Cumpiano - www.cumpiano.com),
where the top rim is narrower than the bottom.
While the accepted central position for the soundhole satisfies symmetry,
structurally it weakens the soundboard, so some modern designs like
the McPherson (www.mcpheresonguitars.com) place it off to the side,
or in the upper bouts like the Ovation guitars.
The cutaway is also a relatively modern variation and only became desirable
as string technology improved enough for the highest notes to be worth
playing. There are two methods of constructing the cutaway, the sharp,
pointed, Florentine, where two sections of rim are joined by a block
that forms the point and the smooth Venetian, where the rim is bent
around the cutaway. A cutaway reduces the body volume and therefore
affects bass response. It should be compensated for by slightly increasing
the body depth and/or changing the size of the soundhole.
Body materials
The top or soundboard woods are usually chosen for their strength and
acoustic properties while the sides, or rims, and back woods are chosen
more for their appearance and ability to be steam bent into curves for
the sides.
Early guitars were made by the same craftsmen – luthier’s
(lute makers) that made lutes and from the same materials. Materials
used varied a lot and often whatever material was locally available
and seemed suitable was used. Lute backs have been, and are being made
from ash, cherry, cypress, ebony, holly, ivory, mahogany, maple, plum,
rosewood, sycamore, walnut, and yew. Tops were made from European spruce
(Picea Abies also known as Picea excelsa).
Today there is a common belief, reinforced by marketing, that only
a very limited number of woods are ideal ‘tone woods’ and
that the ideal flat-top, steel-strung guitar should use Brazilian rosewood
for the back and sides and engelmann spruce for the top. This is despite,
or possibly because of, the protected status of Brazilian rosewood,
the scarcity of good quality engelmann spruce and the consequent relatively
high cost of both woods, although it has to be said that if they can
be found in the right quality, these woods can make a very attractive
looking and fine sounding guitar. Cheaper alternatives include Indian
rosewood, mahogany, maple, nato or even cherry for the back and sides
and sitka or another spruce variety for the top.
Mahogany is probably the most frequently used wood for the back and
sides and sometimes is even used for the soundboard, although as noted
in a previous article, the description ‘mahogany’ may be
applied to quite a wide range of species (e.g. nato). Traditionally
flamenco guitars, originally built as low cost instruments, have used
cypress for the back and sides. Cypress helps impart that bright percussive
flamenco sound, although modern flamenco guitars are also made with
rosewood back and sides. Archtop jazz guitars almost always have maple
or sycamore back and sides for a loud, bright, percussive sound.
Almost all of the species used for making guitar soundboards are large,
tall, straight and relatively fast growing trees. Such trees naturally
produce straight grained timber that is lightweight and flexible, but
with strength along the grain, in widths suitable for soundboards. These
include spruces like the European or Alpine spruce (Picea abies), Adirondack
or red spruce (Picea Rubens), Engelmann (Picea Engelmannii), Sitka (Picea
Sitchensis) and the state tree of Colorado; Colorado Blue spruce (Picea
pungens glauca). At one time Adirondack spruce occupied the same prized
position that Englemann does today, until it became too scarce. Today
managed replanting means Adirondack may be making a comeback.
In recent years cedar has become a popular choice for its warm tone
and appearance and because it doesn’t need to be ‘played
in’ like spruce. Western red cedar (Thuja Plicata) the official
tree of British Columbia, is the most frequently used, although it’s
not a true cedar at all but related to the cypress, as is Port Orford
or White cedar (Chamaecyparis Lawsoniana) another wood that is sometimes
used.
Other woods occasionally used are Spanish cedar (cedrela odorata or
mexicana), mahogany and the quite scarce redwood and koa.
Wood for soundboards and backs is supplied in ‘sets’ –
two pieces made by separating one piece of wood into two by ‘opening’
it down the middle. The two pieces are then glued together with the
join running along the centre line of the guitar. The best wood sets
for soundboards are split from the log, rather than sawn, since splitting
maintains the integrity of the grain.
Laminates – plywood, are frequently used to build guitars by
the bigger manufacturers. Laminates are cheaper than solid woods, they
are more consistent to work with and with a good quality surface veneer
can provide a high quality appearance. Laminates have been used in the
past for soundboards, but this practice is now rare apart from pressed
tops for archtop guitars. However it’s increasingly common to
find mass produced guitars at even quite high price levels, made with
laminated back and sides. The most disturbing thing about this practice
is that manufacturers frequently refer to such guitars as having ‘mahogany’
or ‘rosewood’ backs and sides when only the surface veneer
is made from these woods.
From its early use for cheap soundboards, plywood or laminated wood
gained a very poor reputation as a guitar building material. On reflection
this seems a little strange because you might think that plywood’s
qualities of strength, low weight and ease of steam bending would be
ideal. The problem was that the plywood used was intended for general
construction work and not for building guitars. This type of plywood
is made from three 1mm thick sheets of either birch or low quality mahogany.
The three sheets are held together with relatively thick glue layers.
Such plywood is too thick and stiff for building soundboards and the
glue layers damp and adsorb vibration. The laminates used today for
guitar construction are thinner, use better quality woods, are limited
to the back and sides of the guitar and are custom made using less acoustically
adsorbent glues.
The Soundboard
The design and construction of a soundboard has three main and conflicting
requirements (such conflicts are almost universal in the design of many
of the objects that we use). A soundboard needs to be light, strong,
rigid and flexible. A light soundboard can respond or ‘speak’
quickly; it exhibits a fast note attack. Because it requires less energy
to get it to move, a light soundboard is more efficient and louder.
On the other hand the soundboard must be strong and rigid enough to
withstand the considerable tension placed on it by the strings. However
the soundboard also needs to be flexible in order to vibrate and project
sound.
Bracing
The top back and sides of an acoustic guitar are usually strengthened
by a pattern of bars of wood called braces (typical height 16mm, width
6mm), glued to their inner surfaces.
Soundboard bracing serves two purposes; it adds strength and rigidity
to the soundboard where it’s needed and it allows tonal tuning
through control of the resonant modes of the soundboard. Most of the
strengthening is provided by the larger main braces while smaller subsidiary
braces are added for tonal modification.
Another way to look at bracing is to say that it introduces a necessary
amount of asymmetry, in terms of mass and stiffness variation, to a
soundboard that otherwise follows the very symmetric outer shape of
most guitars.
Early lutes, with their low string tension and edge positioned bridge,
used simple ‘ladder’ bracing, with four or five braces running
across the width of the soundboard. This simple bracing pattern is the
most obvious approach to make the soundboard as stiff across the grain
as it is along the grain. Later lute designs incorporated a ‘J’
brace with the tail of the J curling under the bridge to balance the
volume between the bass and treble strings. Some of the Stella guitars
made by the Oscar Schmidt Company of Jersey City, N.J. and played by
many legendary blues players of the 1920s apparently used a simple ladder
bracing for the top.
The ladder brace pattern shown here, although still used for guitar
backs, has been abandoned for soundboards in favour of bracing patterns
that allow more volume and tonal complexity.
Modern classical guitars use variations on the fan bracing pattern
used by Torres. Like the basic ladder pattern, thick cross braces run
above and below the soundhole, but the belly is strengthened and stiffened
by several braces radiating in a fan pattern below the bridge. With
this bracing pattern it’s clear that the area of soundboard between
the neck and the waist doesn’t contribute much to the overall
sound through vibration. The thick lateral brace at the waist, re-enforcing
plates around the soundhole and one or even two lateral braces under
the end of the fingerboard plus the fingerboard itself make the upper
section of the soundboard much stiffer than the area below the bridge.
The classical fan bracing shown produces good tone and volume with
sufficient strength to resist the tension of nylon or gut strings.
Almost all flat top soundboard bracing patterns now use diagonal braces.
These stiffen the soundboard both along the grain to resist string pull
and across the grain where it’s naturally at its most flexible.
Most acoustic steel-strung guitars today use a variation on the ‘X’
bracing pattern. X bracing was developed by the Martin Guitar Co. in
1840’s. Its strength comes from two long braces, running at opposing
angles across the soundboard to form an X, crossing just above the bridge.
The two ends of the bridge lie above the lower limbs of each brace.
The angle between these two braces and the distance between their crossing
point and the bridge has a strong effect on tone and volume as does
the placing and carving of the ‘bass bar’ braces in the
lower quadrant of the X. This is one reason the older 12-fret-to-the-body
designs sound quite different to modern 14-fret-to-the-body guitars,
since the bridge on the 12 fret is further from the brace crossing point
and drives the belly of the soundboard in a different way.
Martin changed from centre scalloped braces to heavier braces without
the centre scallop in 1944. This seems to have been in response to an
increase in guitars having soundboard problems due to a swing towards
players using heavier strings. Players noticed the reduction in bass
due to the heavier braces and Martin now makes guitars with and without
centre scalloped braces. This shows that guitar owners need to be aware
of the string tension limits their guitars are designed for.
Here’s a picture of a factory demonstration soundboard for a Martin
D28 showing the main X and subsidiary braces (without centre scallop)
The other ‘common’ bracing pattern is the ‘A’
brace, although it is rarely used today. In this pattern the two main
braces run from either side of the heel block and pass on either side
of the bridge and there is often a cross brace across the width of the
soundboard above the bridge.
With both these bracing patterns there are usually two or three short
diagonal braces across the belly of the guitar and two or three small
braces for the rest of the soundboard. A fairly thick cross brace is
usually found just under the end of the fingerboard for stability and
to lend strength to the neck joint. A wooden plate is also added to
strengthen the soundhole and a hardwood plate is fitted under the bridge
to provide a firm surface for the string ball ends and stop them biting
into the spruce top.
The backs on most guitars are strengthened by a simple ‘ladder’
pattern of usually four parallel braces across the width. Sides on many
modern guitars are not braced, often because the sides are made from
laminated wood. Solid rosewood sides, particularly Brazilian rosewood
with its uneven grain, need vertical braces across the grain, to resist
splitting.
Braces are commonly made of spruce, mahogany, or sometimes red cedar,
are taller than they are wide and have tapered or scalloped ends. The
scalloping allows the soundboard to flex at the edges. Sometimes braces
are also scalloped in the middle to tune and control stiffness.
Bracing is where you find a significant difference between factory
built and handmade guitars. Factory instruments are often overbuilt
with thicker bracing than is really needed to allow for the weakest
materials that might be used. With a custom hand built guitar the luthier
can select and test his bracing material and the top is often hand tuned
by shaving the braces once the guitar is assembled.
Archtop guitars have less top bracing since they rely on the arched
carve or moulding of the top for its strength. Normally only the two
main braces are fitted and the tradition is to use two long parallel
braces or tone bars running from heel to end block. Some archtops are
made with an X brace pattern, giving a quieter, warmer sound with more
sustain. The arched top is either press moulded from spruce laminate
or carved from solid wood, sometimes with the bracing bars as part of
the carving.
Some modern makers have started building slight curves or even doming
into the tops and backs of their steel flattop and classical guitars
to give them strength and rely less on the bracing (this is really an
old technique as used by Torres that went out of fashion because of
its cost and complexity). This has the advantage of producing tops with
high strength to weight ratios, but has to be balanced against reducing
flexibility.
Lattice bracing is a fairly new technique that involves bracing all
of the soundboard below the soundhole with a very lightweight lattice
framework, usually made from selected balsa wood, reinforced with strips
of carbon fibre cloth coated in resin.
Another modern trend is to various diagonal ties, often of carbon fibre,
running across the body to stiffen the body shell and stop twisting
and long term warping.
The Bridge
The vibration of the strings is transferred to the guitar through the
bridge to the top of the guitar. On flattop and classical/Spanish guitars
the bridge acts both as a lever to drive the soundboard and as an anchor
for the strings. It’s also often overlooked as acting as one of
the larger latitudinal braces, because it’s fitted on the outer
surface instead of under the soundboard. As the strings vibrate the
longitudinal string tension rises and falls. This change in tension
pulls on the bridge and causes it to rock backwards and forwards, rotating
along its width.
The height of the bridge and saddle above the soundboard has a significant
effect on volume. This is because the bridge and saddle form a lever,
transferring the changes in string tension into wave motion along the
length of the soundboard. The joint between the bridge and the soundboard
acts as the lever pivot point. A high bridge and saddle have a greater
advantage in driving the soundboard because the mechanical advantage
of levers increases in proportion to the length of the lever from the
pivot, to the point at which force is applied. The width of the bridge
also acts as a further part of this lever system to drive the top.
Of course in practice the actual movement of the bridge is more complex
than a simple rotation or rocking motion.
Archtop guitar bridges
On archtop guitars the bridge only drives the soundboard and the strings
are anchored through a tailpiece. This design method removes a lot of
the static string tension from the soundboard, which only has to support
the downward pressure from the floating bridge. It also means that archtop
bridges don’t have to be glued on, but can be held in place by
the string pressure. The bottom of these bridges must be shaped to precisely
fit the contours of the arched top.
The soundhole
The most common shape is the simple round soundhole, but guitars have
been built with a wide variety of soundhole shapes, for example; the
famed ‘gypsy jazz’ Selmer Maccaferri guitars with the large
D hole and the later model with the small vertical oval hole. Originally
these guitars were designed with an internal resonator chamber and the
D hole was part of that feature. Unfortunately the internal resonator
tended to come loose and was often removed. Later, Selmer changed to
the small oval hole to suite their optional pickup, which wouldn’t
fit in the large D hole.
The distinctive Selmer Maccaferri D soundhole shape was originally
part of the internal resonator design.
Although the very first hand-carved archtop guitars, designed and built
by Orville Gibson towards the end of the 1800’s, had a single
oval soundhole and a glued-on, pin bridge, the archtop guitar has developed
design element in common with bowed instruments like the violin and
cello and is almost always built with the two f soundholes that are
characteristic of these bowed instruments. The f holes on an archtop
guitar work in a slightly different way to the round soundhole on a
flat-top steel or a classical nylon strung guitar. They do act as ports
for the resonator formed by the body, but they also decouple the length
of soundboard between them and allow it more flexibility. As purely
acoustic instruments these guitars reached their pinnacle in the big
jazz band setting of the 1950s. Freddie Green, regarded as the master
of this rhythm style, played either an Epiphone Emperor or the legendary
Stromberg Master 400 with its massive 48cm lower bout.
This picture of the stylish Gretsch G400 Synchromatic archtop shows
the floating bridge and tension bearing tailpiece that are so characteristic
of the archtop design, but with the unusual cats eye soundholes.
A Gibson Super 400 archtop with the traditional f soundholes
In the next issue…
In the next issue this top to bottom tour of the history, development
and construction of the acoustic guitar comes to a conclusion with part
two of this feature, which covers the body as a Helmholtz resonator,
the rose and rosette, body construction, the use of alternative materials,
the different approaches to acoustic guitar design and construction,
the various sizes and types of acoustic guitar, acoustic efficiency
and finally new directions in guitar design. ....... Click here for Part 2
By Terry Relph-Knight © 2006
Note – Guitar dimensions given are approximate
Thanks again to Frank Ford of Gryphon Stringed Instruments www.frets.com
for many of the pictures used in this article.
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