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The Diversity of Modern Classical Guitar Tone and Volume due to Plate Thickness and Bracing.

26th December, 2006

Dane Matthew Hancock, BEng



In this article the effect of varying top and back thickness and using different rigidities of top fan bracing will be explored.  Though it is true that top, back and sides material and grade will alter the tone and volume of a guitar, this article relates to the thickness of the top and back material and not the type or grade of material used.  It is generally believed that the back and sides contribute only a small amount to the overall sound quality of a guitar and that the top is the single most important element.  This statement is more correct for a guitar with a thinner top, i.e. typical Spanish classical guitars and modern contemporary ‘lattice-braced’ guitars.  With an instrument that is tuned so the top resonates, to an extent, in sympathy with the back, i.e. the German style concert classical guitar, the thickness of the back and sides is somewhat more critical. 



Mechanics of the Classical Guitar


The strength of a top with bracing must fully counter the forces produced by the strings.  The main component of this force is a bending moment caused by the force of the strings acting over the eccentricity of the bridge.  There is also compression in the top between the bridge and fret-board and a similar amount of tension between the bridge and the lower bout.  The top also contains shear force due to the bending moment caused at the bridge.  These principals are shown in Figure 1.  A slight convex radius can be built into the the top to better counter force from the strings.  A convex radius is built into the back of all fine classical guitars.   


As the strings are set in vibration by the player, the saddle transfers the vibration to the bridge.  The bridge in turn transfers the vibration to the top and top bracing.  The bridge vibrates by ‘rocking’ back and forward on the top, in the direction of the strings.  The axis that the bridge rocks on is known as the ‘fulcrum’.  A typical classical guitar bridge is essentially a string anchor and brace.  With a fan braced top, the bridge sits on top of the central fan braces.  These braces help to transfer vibration from the bridge to other areas of the top.  As the bridge rocks back and forth, the top vibrates and sets the air around it in motion.  This ‘air in motion’ is actually air in compression and rarefaction, (Siminoff 2002, p.11) or sound waves.  The back and sides reflect the sound produced inside the guitar back through the sound-hole and in the general direction of the audience.

The higher treble frequencies of the guitar can be slightly improved by making the treble side of the top stiffer.  Higher stiffness can be achieved by using stronger bracing or adding bracing to the treble side of the top.  Higher stiffness can also be achieved by setting the lower harmonic bar at an angle to keep the treble side of the top in effect shorter as can be seen in Figure 2.  A Miguel Rodriguez top has a third transverse brace running at an angle under the sound-hole.  This brace gradually stiffens the top on the treble side only.  A 'Rodriguez style' top by Kim Hancock is shown in Picture 1. 


The major part of the sound energy is produced inside the guitar and will project through the sound-hole.  A substantial amount of the sound energy produced by the guitar is from top vibration alone (as per the harpsichord or pianoforte).  (This concept can be demonstrated by totally covering the sound-hole of a stringed instrument and playing it.  A small part of the sound is produced by the strings alone.  Too much ‘string noise’ is undesirable for the concert performer.






The Effect of a Thinner Top


Obviously, the thinner a top is made, the stronger the top braces will have to be to counter the constant force produced by the strings.  The material used for the braces in traditionally constructed classical guitars is usually the same material used for the top.  (Sometimes spruce bracing is used with a cedar top.)  The grain in the braces and top should be orientated so that it is quarter-sawn, (see Figure 3).  This gives the top and bracing the most available strength when counteracting the forces produced by the strings.  Simply, for the same material, a stronger brace will be either thicker, wider or both.  Most top braces are rounded, vaulted, or some are left square, (see Figure 4).


Within limits as a top is made thinner the tone will become crisp, dry and more fundamental.  ‘Fundamental’ refers to the sound character produced when the principal mode of vibration of a string (full length) is more prevalent than the secondary (half string), tertiary (third of string), and so on, (Bacon et al 2002, p.94).  This concept can be seen in Figure 5.  This type of ‘fundamental’ tone is more suited to polyphonic music.  


Antonio de Torres Jurado, (born 1817, Almeria, Spain), was the first guitar maker to combine fan bracing, a larger plantilla and the now standard 650mm (25.5 inch) scale-length.  Torres is considered to be the father of the modern classical guitar and rightly deserves ‘a place on the Olympus of instrument makers’, (Grondona & Walder 2002, p.14).  Guitars such as Torres tend towards a thinner top and thicker back.  Torres believed that the back and sides contributed very little to the sound quality of a guitar (Romanillos 1987, p.81).  An example if a 'Torres Style' braced top by the author is included in Picture 2.   


A concert classical guitar with a thin top will generally lead to a louder guitar.  This characteristic is shown to its limit in contemporary ‘lattice-braced’ concert classical guitars.  A thinner top will vibrate more freely and the sound will reflect off the back and sides, rather than vibrate in sympathy with the back.  The back and sides on a lattice-braced guitar are typically very thick and in some cases laminated.



The Effect of a Thicker Top


A thicker top will have more inherent strength and, therefore, need less support from bracing.  This style of instrument was perfected by Hermann Hauser I ‘the first non-Spanish luthier to make truly great modern classical guitars’, (Evans T & M 1977, p.65).  This thicker top is the reason that the fan bracing on a Hauser style guitar is typically very fine in comparison to the top bracing of a typical Spanish guitar.  It is interesting to note that the Viennese guitar has only transverse top bracing, with no fans at all (Jahnel 1981, p.152-3).  The back on a Hauser style guitar is thicknessed so that the top resonates with the back in sympathy.  This can become a tedious process for the luthier when building the instrument, as not only the top is tuned, but also the back is tuned to the top.  This thicknessing/ tuning process can also be very fine and only small fractions of a millimeter will have a huge effect on the finished sound of the guitar.  This thicknessing will not only change the volume, but to a great extent the tone.  Perfecting the tuning process requires minute adjustments to previously built instruments based on experience and intuition.  However, the results are well worth the effort as this style of concert classical guitar has a rich, sweet, brooding and diffused tone.  The timbre of these instruments can also be coloured, to a reasonable extent, by the type of material used for the back and sides, more so than a Spanish style instrument.  Another great quality inherent of this style of guitar is the balance obtained across the full range. 



Thick versus Thin


An instrument has to be as light as possible to take the force of the strings and also have enough top and back material to generate a full rounded tone (not too dry, not too diffused). 


If the top is too thin the tone will be harsh and ‘honky’.  This is indeed a problem with contemporary lattice-braced guitars.  It is true that great volume can be obtained with these instruments, and some of the world finest classical guitar virtuoso’ play this style of instrument with much success, most notably John Williams who now plays a Greg Smallman guitar exclusively in his concerts.  However, many artists find the sound quality produced by lattice-braced guitars is somewhat inappropriate for the traditional classical guitar repertoire.


If the top is too thick the tone will be too diffuse and the projection will be low.  Therefore, tuning a fine concert classical guitar is an essential and time-consuming task. 


There are guitars made from the finest master-grade quarter-sawn top material and stunning Brazilian rosewood back and sides, that have not been tuned properly, and do not have a great sound.  In some cases these instruments can look stunning and be built extremely well, however, they are completely deficient in tone and projection when played.  Sometimes, in factory-produced guitars, tuning is to an extent overlooked.  This is due to these instruments being built to plans that are restricted to accommodate a wide range of material strengths.  Generally this will lead to a heavier guitar, which is greatly lacking in projection, particularly in the treble strings.  


Conversely, there are some great sounding instruments built with unmatched, lower grade tops, with low-grade locally available back and sides.  Of particular note, Torres instruments were built from materials available to Spain in the 19th century.  Many of Torres tops were not book-matched, (see Figure 6.).  However, this was possibly done on purpose as the finer grade, stiffer material was commonly used on the treble side.  Some back, sides, neck and even bracing material contained visible knots.  For example a knot is visually apparent Torres SE111 (Opus 111) on the lower right side of the back. This particular instrument is considered by some classical guitar authorities to be finest surviving Torres (Grondona & Waldner 2002, p46.).  These examples serve to illustrate how important, and sometimes overlooked, the tuning process is for a fine concert classical guitar.     




References used in this article


Bacon T, Cooper C, Van Eik J, Fowles P, Jeffrey B, Johnston R, Miklaucic T,  Morrish J, Rebellius H, Richardson Dr. B, Sparks Dr. P & Wade G, Zern B 2002, The Classical Guitar-A Complete History, Backbeat Books/ Outline Press Ltd, London.


Evans T & M 1977, Guitars - Music, History, Construction and Players From the Renaissance to Rock, Oxford University Press, London.


Grondona S & Waldner L 2002, La Chitarra Di Liuteria (Masterpieces of Guitar Making), l’officina del libro, Sondrio.


Jahnel F 1981, Manuel of Guitar Technology, Bold Strummer Ltd, Connecticut.


Romanillos J.L 1987, Antonio de Torres Guitar Maker-His Life and Work, Bold Strummer Ltd, Connecticut.



Other Useful References


Brosnac D 1978, An Introduction to Scientific Guitar Design, Bold Strummer Ltd, New York.


Courtnall R 1993, Making Master Guitars, Robert Hale, London.


Romanillos J & Harris M 2002, The Vihuela de Mano and The Spanish Guitar – A Dictionary of the Makers of Plucked and Bowed Musical Instruments of Spain, The Sanguino Press, Guijosa.


Siminoff R 2002, The Luthier’s Handbook A Guide to Building Great Tone in Acoustic Stringed Instruments, Hal Leonard, Wisconsin.


Urlik, S 1997, A Collection of Fine Spanish Guitars From Torres to the Present, Sunny Knoll Publishing Company, California.


Wade G 2001, A Concise History of the Classic Guitar, Mel Bay, Missouri.


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          © Dane Hancock  2006. All rights reserved