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The Acoustic Guitar Body – Part 2

Part two of this feature on the history, development and construction of the acoustic guitar covers the principles of Helmholtz resonance, 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.

The guitar body as a Helmholtz resonator

The soundhole, along with the volume of air enclosed by the body, forms a low frequency resonator that smoothes and extends the guitars bass response. Scientifically this type of resonator is referred to as a Helmholtz resonator, named after the German physiologist and physicist, Hermann Ludwig Ferdinand von Helmholtz (1821-1894). In its purest form a Helmholtz resonator is a hollow sphere with an opening, often a tube inserted into it. Air will flow or oscillate through the opening or tube at the resonant frequency, which is determined by the volume of air enclosed by the sphere and by the length and diameter of the tube. Helmholtz used sets of these resonators, spun from brass and tuned to a range of frequencies, in his research on the analysis of the harmonic content of complex sounds.

The Helmholtz effect helps to flatten out the main resonant peak of the soundboard and extend the bass response. Without this the main resonance of the soundboard could make the instrument sound unpleasantly honky and lacking in bass response. Obviously before the 1800s body volume and soundhole size would have been chosen empirically. Since then practical experience shows that the application of the principle of Helmholtz resonance has not always been well understood by luthiers.

The resonant frequency is given by the following formula -

where f = frequency of resonance in Hz, c = speed of sound in air (approx. 343 m/s), S = area of tube, V = volume of air in the chamber and L = the length of the tube.

For a guitar the length L of the resonator tube might seem to be simply the 2 or 3mm thickness of the soundboard. However due to the behaviour of air as a gas something called ‘end effect’ must be allowed for. The effective length of the ‘tube’ is approximately 1.6 times the soundhole radius, plus the thickness of the top.

The great guitar maker Don Antonio de Torres Jurado introduced a device he called the Tornavoz that is often quoted as being intended to increase the guitars projection. According to Jose Romanillos, tornavoz literally means 'turned voice' and is a term originally used to describe an architectural feature found in some old Spanish churches. This consists of a dome or hemisphere arranged to reflect the preachers voice. In the Torres guitars the tournavoz is a conical steel or brass tube extending back from the soundhole into the body of the guitar. Rather than increase projection by reflecting sound it’s obvious that it acts as the tube in a Helmholtz resonator and lowers the main resonant frequency of the body. After experimenting with it for some years Torres eventually abandoned it.

The resonator volume is the volume of the inside of the guitar body, which isn’t easily measured. The simplest method to find the volume of any guitar body is probably to lay the guitar down on a sheet of squared graph paper, pencil around it, count the squares and multiply by the average depth of the rims. Luthier’s lucky enough to do their design work on computer may have the ability to calculate body volume in the software.

There is a further factor to take into account when calculating the resonance of an acoustic guitar body. For the Helmholtz equation it's assumed that the walls of the resonator are completely rigid. The soundboard and frequently the back of the guitar body vibrate in and out, so the internal air volume is increasing and decreasing rather than remaining constant. As a result the resonant frequency is lower than predicted by the basic Helmholtz equation.

It’s the low frequency resonances of the acoustic which seem to cause the most problems when the guitar is amplified. This is why blocking the soundhole with a feedback buster helps stop feedback because it stops the body working as a Helmholtz resonator and drastically alters the bass response of the guitar. In many amplified situations this tonal alteration actually helps the guitar to cut through and sit better in a live sound mix.

Much of the basic tonal character of the different types of acoustic guitar is defined entirely by the bass response and therefore is directly a consequence of the fundamental body resonance. Generally speaking the resonance for a steel strung acoustic guitar is around A2 (55Hz), an octave above that at G#2 (103.8Hz) for a Flamenco guitar and between F#2 and G2 (92.5.to 98 Hz) for a classical nylon strung guitar.

The Rose and the Rosette

The Oud always has an intricate geometric fretwork carving instead of a simple open soundhole. This feature of covering or screening the soundhole is probably for religious and cultural reasons. The Islamic influence carried over from the Oud to the Lute, although Lute carvings gradually started to show more representational themes, for example of branches and leaves, than the purely geometric patterns of the Oud. In the European tradition this carving is referred to either as the rose, or as the ‘Fenestrum’ (window) due to its strong resemblance to a church window. Over time the rose developed into an extremely intricate decoration, often sculpted in three dimensions from parchment, ivory and wood. These elaborate structures effectively blocked most or all of the soundhole, suggesting that understanding of its acoustic function had been lost. This decorative function carried over to other instruments and sometimes instruments of the Viol family even had both f holes and a purely decorative parchment sculpted rose under the end of the fingerboard.

Although modern guitars have open soundholes, the decorative aspect of the rose is echoed in the decorative border around the soundhole, known as the rosette.

The artistic intricacy of this three dimensional rose is astounding but it must affect the bass response.

Body construction

The ‘joints’ between the top sides and back of the guitar body are simple butt joints and on their own would have little strength. The guitar body is actually held together by the linings, thin strips of wood shaped to the sides and glued to the insides of the joints between the top, back and sides. The linings are commonly referred to as ‘kerfing’ or ‘kerfings’ although correctly this term applies to the repeated saw cuts made part way through the linings to allow them to bend and follow the shape of the sides. Some modern makers are using solid linings where, rather than relying on flexibility imparted by the kerf cuts and clamps to get the linings to follow the shape of the sides, the linings are steam bent into the same shape as the sides. Solid linings help increase the rigidity of the body.

This view inside a Taylor guitar shows the kerfed lining joining the top to the rim inside the upper bout.

Any direct connection between the edges of the top, back and sides is further reduced by binding and purfling. Although the two are often confused, strictly speaking binding protects the body edges, while purfling is mainly for decoration. The edges of the guitar body are vulnerable to knocks and to moisture seeping into the otherwise exposed end grain. To protect these edges a shallow route is cut around the body and a thin strip of binding wood, bone/ivory/tortoiseshell or usually plastic, is glued on. Further decorative purfling, in shell, exotic woods or precious metals is often added inside the binding. Binding and purfling aren’t essential, but add manufacturing cost and value to a guitar, so on cheaper models are minimised or are even left off altogether.

A cross section through the edge of a guitar body showing the inner black and white purfling on the top and side and the outer white binding strip.

Bodies are usually built by first shaping and bending the sides or rims. The heel and end blocks are glued to the sides and clamped into a frame cut to the required body shape. Linings are glued to the top and bottom edges of the sides and the edges and linings sanded flat. The back is centre joined, braces fitted and glued to the sides, or rather to the linings on the sides. Lastly the top is also centre joined, braced, the soundhole is cut and the top is glued onto the sides. A certain amount of tuning of the back and top may be done by thickness sanding and shaving the braces before gluing them in place onto the sides.

Once all the glue has set the body is cleaned up, the routes for the binding and purfling are cut and the purfling and binding are glued around the edges.

Alternative materials

Although Rickenbacker used bakelite to build their ‘frying pan’ electric lap steel guitars in 1935, and Mario Maccaferri made plastic guitars from Dow Chemicals Styron in 1953, it wasn’t until 1966 that the next and really successful, commercial use of composite plastics appeared in the ‘Lyrachord’ woven glass fibre and resin bowl backs of the Ovation guitars. The Lyrachord back is now a standard feature of almost all Ovation guitars, although they have phased out glass fibre in favour of random laid carbon fibre and in their LX series they now add tiny glass bubbles to the resin for a lighter back. Ovation has gone on to produce both entirely carbon fibre instruments and guitars with laminated carbon fibre and wood soundboards.

Two other American companies; Rainsong and CA Guitars produce carbon fibre guitars. Here in the UK, Emerald Guitars (www.emeraldguitars.com) build in carbon fibre and Cool Acoustics, a business owned by Loughborough University, has developed guitar designs based on the use of foamed polymers, although it hasn’t yet gone into commercial production – www.coolacoustics.com.

It’s a common observation that the rising cost and diminishing availability of many woods used in guitar construction is driving manufacturers to look for alternative synthetic materials. Most of the time this means plastics, however some consideration shows that this is a ridiculous state of affairs. The majority of commercial plastics are manufactured from oil. Oil is a non-renewable natural resource that is also in diminishing supply and current conflicts over supplies of oil are causing a great deal of misery. Wood from trees is a natural, renewable, bio-degradable, resource that, while it grows, benefits the environment. What’s needed is better management of the forests we have and to plant more trees than are cut down.

Current acoustic guitar design approaches

There are two common approaches to guitar design at present.

One approach seems based entirely on traditional intuitive craftsmanship and practical experience, where it’s accepted as unavoidable or even desirable that the entire instrument vibrates when the strings are plucked, and that therefore all parts of the instrument contribute to the overall sound. Builders following this approach usually build only in wood and will choose the wood for the back and sides for tonal contribution. They will tune the back, usually for an octave higher tap tone, to complement the top. Necks are built in the ‘normal’ way without extra stiffening, apart from a truss rod. With this approach the character of the woods chosen for the back and sides may have a discernable effect on the tone of the finished instrument.

Acoustically the junction between the edge of the soundboard and the sides is a critical point because energy at the edges of the board is either adsorbed by the sides or reflected back into the board. It’s at this point that the characteristics of the sides affect the sound of the guitar. It seems likely that the linings at this point are also critical and that their effect on tone and sustain may have been generally overlooked.

The second approach is perhaps a more recent and cerebral approach, where it’s reasoned that the vibration of some components is perhaps far more important than others. Therefore it’s desirable to reduce vibration in some components as much as possible and to enhance them in others. Luthiers following this approach concentrate on focusing all of the energy from the strings into the soundboard, making a light, strong soundboard for efficiency, often adding extra stiffening to the neck and making the back and rims as rigid as possible. Various extra structural elements are often added to stabilise the guitar and although much of the instrument may be made of wood, carbon fibre and other new materials are often used. Sometimes such instruments show visible design changes, although some makers take pains to maintain a completely traditional outward appearance.

Body size and type

Some musical instruments are made in a range of pitches – contra-bass, bass, baritone, tenor, alto, soprano and piccolo and their size very clearly relates to their pitch range. This is not the case with 6 string acoustic guitars, althought it seems there was a brief period when the guitar was made in eight different sizes ranging from the Contrabass up to the Soprano or Octave Piccolo. Just to confuse things the so-called tenor guitar has four strings and is really a banjo with a guitar body. The acoustic bass guitar is a fairly recent invention, is usually a four string instrument and, although its neck is longer, doesn’t have a noticeably larger body. Baritone acoustics, also a recent introduction, differ only in having a longer scale length than a standard pitch guitar. Like the bass, their bodies are usually equivalent to a standard pitch jumbo.

The C.F. Martin Guitar Co. originally used a numbering system to indicate body size starting with the 0 size in the 1850’s. Currently Martin makes guitars in three body sizes ranging from the smallest 00 guitars (36cm across lower bout) to the biggest with 0000 bodies (40cm across lower bout). Over the years a few model names indicating body shape as well as size have come into common use.

The early or baroque guitar was a small high tuned instrument with a body measuring roughly 30cm long by 15 cm wide. Guitars stayed around this size until in the 1850s Don Antonio de Torres Jurado and his contemporaries increased the size of the classical guitar for greater volume. Typical body size for a nylon strung guitar is 40cm across the lower bout by 10cm in depth. Even then steel strung Parlour guitars continued to be made up until the 1920s and in recent years have come back into fashion. Typically the lower bout on a parlour guitar measures 33cm and maximum body depth is 10cm.

A beautiful example of a Lacote parlour guitar from 1836 featuring the Lacote concealed tuners referred to in the first article of this series. On the right is a full-frontal shot of a Martin parlour guitar from the 1840s. The relationship of the soundhole and the bridge to the sides clearly shows how small the parlour guitars are compared to most modern instruments. It's often a surprise just how loud these small guitars can be.These are both 12 fret to the body guitars, which places the bridge in the middle of the lower bout where the energy from the strings will create the maximum volume.

Baby – the baby guitar was first introduced by Taylor as an economy/travel guitar. Their version is a size Dreadnought and is useful both as a very portable practice instrument and as means of adding a different tone colour to recordings or live performance, sometimes in high tunings.

Travel – the travel guitar is an abomination and should be banned. Seriously, travel guitars are normally low cost instruments, usually with a full scale neck but with the smallest possible, often oddly shaped, body, designed for portability. Unfortunately they balance badly, sound unimpressive and are difficult to hold and therefore to play.

Terz (aka the tierce or tertz) – There seems to have been a brief period where composers were experimenting with the guitar in a range of sizes, including the tertz which is a small guitar. The name refers to its tuning of a minor third above the standard guitar. Old Martin catalogues show a steel strung version which has been recently re-introduced. The Martin Terz or size 5 is a small instrument with a 22 inch scale often tuned a third above regular tuning. The lower bout on a Terz measures 28cm and maximum body depth is 11.5cm.

Today’s general purpose guitar strikes a balance between body size, tone and volume. The Martin OM-45 and the Taylor Grand Concert and Grand Auditorium models are examples of this type of guitar. They suite both fingerpicked and chordal playing styles, tuck under the right arm without too much discomfort and don’t overwhelm vocals for the singer/guitarist. The lower bout on this style measures around 38cm to 40cm and body depth is typically 10cm.

Dreadnought – although not as large as a jumbo the dreadnought has a blocky square outline and only a shallow waist. The original dreadnought style guitars with 12 frets to the body were made by Martin for the Oliver Ditson Music Co. of Boston and first sold in 1916. Like the jumbo this design works well for rhythm duties. Typically the lower bout on a dreadnought measures 40cm and maximum body depth is 12cm.

An early example of the Dreadnought style, this simple Martin built Ditson from 1929 has 12 frets to the body.

Jumbo – the jumbo is the big gun of acoustic guitars, with very round upper and lower bouts and a pronounced waist. It has a loud powerful sound and works well in a band setting played as a rhythm instrument. Because of the large, deep body, a jumbo can be an uncomfortable guitar to play. Gibson’s SJ-200 super jumbo, first made in 1936 for singing cowboy star Ray "Crash" Corrigan, is perhaps the best known of the breed. Martin jumbos like the J-40 don’t have as rounded a shape as most other makes. Typically the lower bout on a jumbo measures 42cm and maximum body depth is 12cm.

Perhaps the archetypal jumbo guitar; the Gibson SJ200. Unusualy for a non-jazz guitar, maple is used for the neck, back and sides rather than rosewood or mahogany.

Body and soundboard efficiency

One of the aims of innovation in guitar design is to make a louder instrument. The quest for volume has driven the evolution of the guitar from the tiny four-course instruments of the 16th century to the Jumbo acoustic of 1936 and the solid electric guitars of the1950s. This is not just for the sake of shear volume, although that does allow the guitar to compete with other instruments, but because a guitar that converts energy from the strings more efficiently is a more responsive instrument that encourages the player to play with greater ease, dynamics and expression.

Standard acoustic designs only convert approximately 5% of the energy in the plucked strings into sound. The rest is lost to friction and just heats the guitar up. Most design effort at the moment focuses on stiffening the guitar structure and on making the soundboard more efficient. One possible approach to making a more efficient guitar is suggested by the use of levers for mechanical amplification found in the various forms of ‘resonator’ guitar. If the guitar bridge were not glued to the soundboard, but balanced on pivots supported inside the guitar by a frame connected to the sides and back, then an amplifying lever could be extended from the bridge to connect to the soundboard. This would allow the bridge to be sighted wherever the neck position and scale length dictated, but still be able to drive the centre of the lower bout of the soundboard.

New directions

The hottest new technique in soundboard construction is the double-top or honeycomb top. In this technique the soundboard is constructed as a sandwich of two wood ‘veneers’ top and bottom, with a Nomex filling. The veneers are usually around 0.6mm thick and the Nomex material is made of resin impregnated Aramid fibres in a honeycomb paper matrix. This method of construction results in a soundboard that is light but very strong, so strong that often no braces are used. Another variation is to substitute a third veneer of wood, with honeycomb cut outs, for the filling. Resonant modes can be controlled by placing the honeycombing only in selected areas of the top. More information on the double top technique can be found at www.reynoldsguitars.com – Randy Reynolds – classical ‘double top’ guitars.

A select few luthiers are busy re-inventing the nylon-strung guitar as an archtop instrument, mainly for jazz players. For information on this style of instrument go to www.henneken-archtops.com.

In conclusion

This second part of a two part feature completes my top to bottom tour of the history, development and construction of the acoustic guitar. In next months article I’ll be taking a look at those vital generators of guitar sound – guitar strings.

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.
Thanks to Stephen Barber & Sandi Harris – www.lutesandguitars.co.uk for the lute and guitar rose pictures and thanks to Joshua A. French for his information on the tounavoz - www.frenchguitars.com.