Frequently Asked Questions about Classical Guitars

Construction and repair of Classical Guitar and related instruments
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James Lister
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Frequently Asked Questions about Classical Guitars

Postby James Lister » Fri Dec 23, 2011 3:38 pm

The following is a compilation of Frequently Asked Questions in the luthier section of this forum. The answers have been put together from the collected knowledge of a number of the luthiers who are members of Delcamp. Thanks to all the luthier members (and others) who have given advice and information on this forum that has helped to compile this list.

This thread is now locked - if you wish to ask further questions, or make comments, please do so on the "FAQs about Classical Guitars - Discussion" thread, which contains all posts made in response to the original FAQ topic. Any useful questions and answers will be transferred to this thread by the moderators.

James




1. What’s the difference between spruce and cedar tops?
2. How does humidity affect my guitar?
3. What is action?
4. What is the correct action for a classical guitar?
5. Should you change all your strings at once?
6. How often should you change your strings?
7. Which strings are best?
8. What are the advantages of different tension strings?
9. How should I change the strings on my guitar?
10. What's the difference between a classical and a flamenco guitar?
11. Why does my guitar appear to be out of tune in higher frets?
12. How can I get rid of buzzing on some frets?
13. What is neck relief and what is it good for?
14. What are the advantages/disadvantages of a cut-away?
15. Why are some classical fret boards wider than others?
16. What is a double-top guitar?
17. What is the effect of using different woods for the back and sides?
18. How does the bracing pattern used affect the sound of a guitar?
19. What is French polish?
20. Why is French polish (shellac) preferred for classical guitars?
21. What's a double-back guitar?
22. There is a crack on my guitar, what should I do?
23. What makes a concert instrument?
24. How long does it take to hand-build a classical guitar?



1. What’s the difference between spruce and cedar tops?

Spruce tends to produce a brighter, more focussed tone with strong trebles and good clarity/separation. Cedar gives a warmer, fuller tone but with less clarity. It is generally thought that cedar opens up quicker than spruce. Cedar perhaps gives a more traditional "Spanish" sound, and spruce is more suited to Baroque or Renaissance music.
Although the choice of tonewood for the top of the guitar does have a significant effect on the tone of the guitar, there are many other factors that are of greater significance.


2. How does humidity affect my guitar?

Wood is hygroscopic, that is it absorbs water from the surrounding air. As wood dries, it shrinks (mostly across the grain). Wood used in instrument making is always either air- or kiln-dried before use. However, if a guitar is constructed in conditions of high humidity, then there is a risk of cracking if the guitar is then kept in very dry conditions. Luthiers tend to try to make their guitars at a relative humidity (RH) of about 45%, unless the guitar is destined for somewhere with consistently high RH.
High humidity levels are less of a problem in terms of damage to the guitar, but can affect the tone and the action of the instrument. If the humidity where you live is frequently above 70%, then it may be worth investing in a de-humidifier.
It is always worth investing in a hygrometer so that you know how the RH varies where your guitar is kept. These can be purchased fairly cheaply from good music stores, and are accurate enough for the purpose.


3. What is action?

The action of a guitar is the height of the strings above the fingerboard. The higher the action, the harder the guitar is to play, but if the action is set too low, then the strings will buzz against the frets when plucked. The correct action will depend to some extent on the player, the guitar, and the strings used. The action is set by adjusting the height of the nut and the saddle.


4. What is the correct action for a classical guitar?

Normally 3mm at the first string (top E) and 4mm at the sixth string (bottom E). These figures can vary up or down slightly depending on the player, but not normally by more than 0.5mm.
The action is measured at the 12th fret, from the top of the fret, to the bottom of the string.

Action at the nut is measured by fretting at the third fret, so that the string is stretched between the second fret and the nut. Now measure the gap between the string and the first fret. This should be about 0.25mm for the bass E, 0.2mm for the A and the G strings, about 0.1mm (a piece of paper) for the D and B and less (almost 0.0) for the top E string.


5. Should you change all your strings at once?

There have been a few heated discussions about this on the forum. Suffice to say, some believe that removing all the strings at once (and hence all the tension) can in some way damage the guitar. Others believe that this is complete nonsense. If you take all the strings off, it allows you to clean the fingerboard, and the top between the soundhole and the bridge. If you change them one at a time, it may save some time in tuning up.


6. How often should you change your strings?

From previous discussions, it seems people on this forum change their strings after anywhere between 20 hours and 200 hours playing. 40 to 60 hours would seem reasonable for most players. The bass (wound) strings tend to need changing more frequently.


7. Which strings are best?

This of course comes down to personal preference, and some strings are more suited to certain guitars than others. A poll on this forum showed that by far the most popular are D'Addario (48% of replies), with Savarez second with 19%.


8. What are the advantages of different tension strings?

As with makes of strings, the best tension strings for your guitar will depend both on your own playing style, and on the guitar itself. Hand built guitars are often made to suit a particular tension of strings, and will not work as well if you change the tension.

Generally:
Hard tension strings can give more volume, they tend to buzz less, and allow for greater dynamic range. They are slightly more difficult to play, particularly for the left hand.
Mediums can give more tone colour variation, and are slightly easier on the left hand, but buzz more than the hard tensions. Vibrato is also easier with medium tension strings.
Low tension strings are rarely used, but may be well suited to period instruments, or guitars with very thin tops.


9. How should I change the strings on my guitar?

There are many different ways to tie the strings on your classical guitar, but a very good description (of one method) with detailed photos can be found here:

viewtopic.php?f=43&t=55428

Note: As the above post makes clear, the use of a match to create a ball end on the string is optional, and great care should be taken not to allow the match, or the melted end of the string, to touch the guitar.


10. What's the difference between a classical and a flamenco guitar?

Flamenco guitars traditionally have cypress back and sides, and a spruce top, although in recent years, “flamenco negras” have increased in popularity, having rosewood back and sides.
Flamenco guitars tend to be more lightly built, and the top is thinner than most classicals. The combination of light, thin top, and lower string height (action) give the flamenco guitar it’s characteristic fast, percussive attack, and quick decay (i.e. less sustain). The height of the strings above the top is around 4mm less than on a classical, and the action at the 12th fret can be over 1mm lower.
Traditionally, flamenco guitars used tuning pegs (similar to violin pegs) rather than machine heads, but many modern falmencos are now fitted with machine heads. Falmenco guitars have tap plates, usually clear, self-adhesive plastic, to protect the top.


11. Why does my guitar appear to be out of tune in higher frets?

There are a number of possible reasons for these intonation errors:
a) Defective strings. If a string is not a constant diameter along it’s length, or it’s density varies, then it will not play in tune in all positions.
b) Poor fret placement. This is pretty rare these days, when factory guitars have their frets slots cut by machine, and makers of hand-built instruments (hopefully) take great care over fret placement.
c) Incorrect compensation. The position of the saddle needs to be set accurately to allow for the effects of the stiffness inherent in the string, and the stretching of the string caused when the finger presses it down behind the fret. On a classical guitar, the correct compensation is about 2mm further away from the nut than the “theoretical” correct position, but it varies from string to string. The 3rd (G) string usually has the largest error, as it is the stiffest string. Many classical guitars have
d) Too high action. If the action is too high, then the string is stretched more when pressed down, requiring increased compensation.
e) Changing string type, and/or tension can also change the amount of compensation required.


12. How can I get rid of buzzing on some frets?

Buzzing occurs when the string vibrates against the frets further up the fretboard than the fretted note, and is usually due to either too low an action, or poorly levelled frets. If the action is set to 3/4mm (see above for action measurement), and there are still buzzes when the guitar is played with normal strength, then the frets may need levelling. Sometimes back-buzzes occur when the string vibrates against the frets between the fretted note and the nut. This is usually due to the action at the nut being too low, but can also be caused by resonances within the guitar.

viewtopic.php?t=16472&highlight=buzz


13. What is neck relief and what is it good for?

Neck relief is a slight curvature of the fingerboard (strictly the top surface of the frets) - usually between the nut and the 12th fret. This curvature is designed to allow the action to be set lower at the saddle without the strings (particularly the basses) buzzing.

Some builders shape the top surface of the fingerboard/frets slightly concave to achieve this, while others make the fingerboard perfactly flat, and allow the tension of the strings to pull the neck up slightly. Others build flat, but to put in a very slight relief between the 1st and 4th frets on the bass side only. From the 4th fret onwards the fingerboard/frets are made perfectly flat, and any further relief is just due to string tension. This adjustment at the first few frets helps to eliminate buzzing on the F and F# on the bass E, which is the place most classical guitars buzz first.


14. What are the advantages/disadvantages of a cut-away?

A cut-away allows easier access to higher frets, and is far more common on steel string guitars than classicals. It is extremely difficult to make direct comparisons between cutaway and non-cutaway guitars, as no two guitars (even by the same maker, or from the same factory) are ever the same.
There are 2 possible reasons for loss of tone and volume, first the loss of some area of the upper bout, and second, the change in shape, and reduction in volume, of the guitar’s air cavity. Generally, the upper bout plays only a small part in tone production. In some guitars (e.g. Smallmans) the upper bout does not vibrate at all, so they would be no different with a cutaway (in terms of soundboard vibration). The change in air volume will have some effect, but not necessarily either good or bad – after all, some small guitars are better than some larger guitars, and vice-versa. The effect of the change in shape is difficult to predict, but is unlikely to be significant.

It is certainly possible to make very good instruments with cutaways, but not many top makers make them (Gary Southwell is one exception). Some of the poor reputation of cutaway guitars may be in part due to the fact that most of them are factory made instruments.


15. Why are some classical fret boards wider than others?

The fret board (or fingerboard) on a classical guitar is normally around 52mm wide at the nut, and 10mm wider at the 12th fret. The width at the nut varies between about 49mm and 54mm. A narrower width makes it easier to reach across the strings, but makes it more difficult to play cleanly, since the strings are closer together. Builders of hand-made instruments try to match the width of the fingerboard to the particular player.


16. What is a double-top guitar?

There was some confusion for a while when some makers were referring to double backs as double tops, but double top usually means a top made from two very thin pieces of spruce, or cedar (or one of each), with a thin layer of material called Nomex in between, glued together to form a sandwich. Nomex is a very light but strong woven material used in the aviation industry. If you look carefully at the top of a "double-top" you can usually see the mesh pattern of the Nomex imprinted into the spruce.

pepe wrote:... the confusion about double top comes from a bad translation of "DOBLE TAPA" OR "DOUBLE LID", which is the back as usually, Spanish builders put the back the last to "close" the guitar. In English, it should be DOUBLE BACK.



17. What is the effect of using different woods for the back and sides?

Many different tonewoods have been used for the back and sides of classical guitars with good results. Rosewoods are probably most commonly used these days, with Brazilian rosewood perhaps still the most sought after, in spite of restrictions on it's trade (CITES - Convention on International Trade in Endangered Species). The woods used for the back and sides of the guitar have only a fairly subtle effect on the tone of the instrument. Many other factors are of far greater significance - notably the treatment of the soundboard. There have been a number of discussions on alternative tonewoods on the forum, including these two topics comparing rosewood and maple:

viewtopic.php?f=1&t=17027

viewtopic.php?f=1&t=20668


18. How does the bracing pattern used affect the sound of a guitar?

It has been argued that the exact pattern of the bracing has little effect on the tone of the guitar, and there is some truth in this. There are certainly many factors that have a greater effect on tone. A good example of this is lattice bracing. Few would argue that a Smallman type lattice guitar sounds different to a traditional fan braced instrument (although there is much less agreement about which is “better”). However, it is not really the bracing pattern itself that makes these guitar so different - other factors such as the lightness of the lower bout area of the top, and the considerable thickness/weight of the back and sides (and upper bout) are perhaps more significant.

One factor in the bracing pattern that does have a significant effect is the balance between the amount of transverse bracing and longitudinal bracing. Extreme examples are the ladder bracing found on many romantic guitars, where all of the top struts are transverse (i.e. across the grain), and some varieties of fan bracing, where all the struts run almost parallel to the grain. As a general rule, guitars with more transverse bracing tend to be brighter and punchier, whilst those with mostly longitudinal bracing tend to be warmer and sweeter in tone.


19. What is French polish?

French polishing is actually a method, rather than a type of polish. The material itself is shellac, which comes from the secretions of the lac beetle. These secretions are harvested from the bark of trees, and processed to produce flakes of an orange/brown colour. These flakes are then dissolved in alcohol to produce the liquid “polish”.

The method of French polishing involves applying the shellac using a “rubber” – a wad of cotton wrapped in a clean, cotton cloth. The rubber is soaked in the shellac solution, and then squeezed out so that the rubber is just damp to the touch. The finish is then applied to the wood using the rubber, in a variety of motions – circular, figures-of-eight and straight lines. The process is very slow, the finish being obtained by very many applications of very small amounts of shellac. Each “coat” needs to be thoroughly dry before the next coat is applied, and the number of coats may be anywhere between 10 and 50, depending on how much shellac is applied with each coat.


20. Why is French polish (shellac) preferred for classical guitars?

Shellac can be a very beautiful finish when well executed. Because the finish is extremely thin, it does not inhibit the vibration of the soundboard, which a heavily applied lacquer can. Although lacquers are generally far more robust than shellac, they are very difficult to repair if damaged. Once the skill of French polishing is acquired, it is relatively straightforward to repair, or touch up the finish. From the luthiers point of view, a further advantage is the non-toxic nature of the material – shellac has even been used as a coating for pills.


21. What's a double-back guitar?

A double back guitar has a second “back”, mounted inside the guitar body. The second “back” is normally made of softwood, and is in some ways like a second top (hence the confusion of names). The idea is that this second back vibrates in sympathy with the top, possibly improving projection. As this second back is not in contact with the player, it’s vibrations are not damped by the player’s body (which the real back is).


22. There is a crack on my guitar, what should I do?

Crack repairs are really best left to luthiers, preferably someone experienced in repairs (not all luthiers are). A poorly repaired crack will detract from the appearance of the guitar, and may open up again. Sometimes, if the crack is not too bad, there is no effect on the tone or playability of the guitar, in which case an acceptable option is to leave it. It is worth trying to determine what caused the crack. The most common cause is low humidity. When the wood dries out, it shrinks (mostly across the grain), causing stresses in the wood which can lead to cracks. Check the relative humidity (RH) where the guitar is kept, and if it is much below 40%, it may be worth investing in a humidifier.


23. What makes a concert instrument?

There is no precise definition of what constitutes a concert guitar. When a classical guitar is described as a “concert” instrument, it is usually intended to indicate that the guitar is suitable for professional performance. However, many so-called concert guitars would not be up to the standards required by professional musicians. Conversely, many top makers do not use the term “concert guitar” at all, but their guitars are easily up to professional standards.

Some guitar makers offer “standard” and “concert” models, and perhaps spend more on the materials for the concert guitar, but this does not necessarily make it a better instrument.


24. How long does it take to hand-build a classical guitar?

This question often comes up and like many things associated with the guitar, there's no one answer. So much depends on the individual makers methods, approach. general speed of working and the complexity of the inlays/construction.
Some claim to build a guitar in 50-60 hours, which is pretty fast and obviously using a lot of power tools and/or buying some ‘off the shelf’ parts. Others may take 160 hours or more, with complicated design elements, and every part made by hand, and French polished. The average is probably between 100 to 120 hours work on the actual guitar (excluding time spent on other activities associated with any business.)
James Lister, luthier, Sheffield UK

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