Acoustic Guitar Anatomy

author: Steven Bayes date: 02/25/2013 category: the guide to
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Acoustic Guitar Anatomy
Even in the 21st century some still make guitars out of wood, claiming wood gives the best sound. Unfortunately, there is nothing the general consumer can do except to obey and try to find the least bad option at the least bad price. The wood goes by a few important parameters: hardness, density, even spread of wood fibers, size of wood pores. Hardness and Density: Hardness is the most important parameter and shows how hard the wood is. A measure for hardness of wood is accepted to be the Janka test which is well described in Wikipedia. The density of the wood is directly connected with the hardness measured by the Janka test but is a different parameter because hardness of wood may also mean how hard a different "rope" of wood with, theoretically, 0 diameter and infinite length is whereas the density would mean how many of these ropes there are per cubic meter. In other words, if there are 100 sticks of copper, each of them with 1cm diameter and 1m length and 100 sticks of iron with the same size, when each of these two sets of metal are put together, the density of the two sets is the same: each contains 100 sticks with exactly the same size, but the hardness is better with the iron set because of the strength property of the iron. However, if a set of 2 sticks of metal are compared with a set of 100 sticks of copper and if these two sets are compressed to form the same shape of, say, two cylinders with equal sizes, then the strength (the hardness in more global sense) of the copper set is higher because of the higher density of the copper cylinder. However, if the Janka test is applied, this will not be shown and the result of the Janka test would be to show the hardness of the material copper versus the hardness of the material iron rather than evaluating the two cylinders. Thus Janka test would mainly measure the hardness of a given type of wood, rather than the strength. However, the strength and hardness are related with the density of the wood fiber making the difference. Hence Janka test is good enough for most applications. For others, the best way to go is to apply the knowledge and experience from metallurgy into wood assessment and the various tests they perform to evaluate a given type of metal or alloy: these are tests of bending, shearing and twisting, stretching and so on and so forth. These would mainly be important for guitar neck manufacturing. Pores: The imperfections of the silicon or silicon oxide materials are an imperative limitation in the semiconductor industry. So are pores in wood. Not exactly for guitar manufacturing, though. Pores are holes or bubbles inside the wood. Guitar manufacturers claim pores prevent attenuation, absorption and filtering of the bass frequencies. Pores must not be mistaken with wood knots. Knots are highly undesirable in most any wood application and must not exist in wood material used for guitars at all. Even spread of wood fibers and pores: Unlike metal, wood is not made out of the same atomic combinations evenly spread all over the place, giving one monolithic chunk of material (chunk of junk) but, rather, wood is made out of different parts, fibers, which go from a place to a place with others going from another place to another place all of them connected to one another by a gluing force of "connecting" wood or by tangling or, kinda, pressing these wood fibers together. Hence the wooden needles which go into carpenters skin and hands. Hence the chisel work along the fibers in one direction only. Guitars can be made out of solid wood or ply wood. Solid wood guitars would have thin veneers of real wood bend and cut in the desired shape why the plywood guitars would have veneers of plywood which are made out of pressed saw dust with, in some cases, epoxy resin or synthetic glue added to the saw dust. The quality of the plywood guitars depends on the wood from which the saw dust has come as well as the technology of pressing and manufacturing which would result in different density and size of pores. As a gross generalization, plywood guitars cannot perform as well as their full wood counterparts. Some claim otherwise on sound of tones and resistance to dryness. As far as the parameters of wood and their reflection on sound goes, the harder the wood the better the high frequencies. The bigger the pores and the more the pores the better the low frequencies with a tiny negative reflection on the high frequencies. The higher the density the better the high frequencies. Thus, there are two basic types of wood used: low frequency wood which gives a better low frequency performance and high frequency wood which gives a better high frequency performance. Some guitar manufacturers prefer to use one single type of wood which gives the best performance for the two sets of frequencies others simply prefer to use different types of wood. The single type wood method would give a better performance but these types of wood are, usually, extremely expensive and exotic, in some cases, outlawed for harvesting in some countries. Combination of low frequency and high frequency woods would be the most used method for manufacturing of inexpensive guitars. Usually, the manufacturers would put the low frequency wood as the tone wood of the body (the one which goes at the back) because, to get a good base notes out of a guitar is much more difficult, hence the most important reflector is made out of low frequency wood. The manufacturers would put the high frequency wood as a top wood on the front side of the body where the hole is. The sides of the body would be made out of the low frequency wood to boost the bass performance. The neck would usually be made out of hard wood to avoid breaking, although, these days, the manufacturers would put a truss rod on every acoustic guitar, thus some fluctuation of this rule can be used in order to improve the sound performance although the neck, the head and the finger board are not supposed to affect the quality of the sound. Generally, they would put inexpensive wood for neck wood on truss powered guitars. There is no way around and the fingerboard must be made of hard wood to assure mechanical stability. Some, usually the manufacturers of the least expensive guitars, would not put fingerboards at all but would bang the frets into the neck directly. Not a big deal but the fingerboard can be changed once in a while and is less expensive than the whole neck. Then, when the neck is made out of inexpensive wood, how expensive would the neck be? Shouldn't. Then why would one want to rebuild an inexpensive guitar, why wouldn't one get a new one once every a lot of years. The head must be strong enough to bear the pressure of the strings. The best, the head must be a continuation of the neck, made out of the same piece of wood. I've never seen assembleable heads although I wouldn't be surprised to see. Better be metal. The design of the head is rather important for the strength thereof. The strongest heads would be these where the axle of each of the tuning machines goes through a hole in the head perpendicular to the head plate. Other heads have two cutouts for the axles to go through parallel to the head plate. These aren't strong enough. Some would prefer these because the string goes down and cannot easily jump out of the groove of the nut. This can be compensated with making the head go back as compared to the neck and placing the thinnest string the most to the top of the head thus to the back. Some use string lockers at the nut, mainly for electrical guitars. Haven't seen lockers on acoustic yet. Also, each string must be wound over the axle in such a way as to make the string go as low as possible and as straight into the grove of the nut as possible. I.E. the 3D navigation of the string must go straight into the grove from as low point as possible. And must be approved by NASA and the US Air Force. The machines are not so important but people have made a science out of them. This is because the machines may not hold the tuning very well for a long while. Mainly important for concert performers. There are two types of tuning machines: open and closed. Closed machines look like Lamborghini's transmission. They have a cover over. Preferred option these days because they are dust and dirt proof as well as stronger because of the mechanical protection provided by the cover. The open machines have one great advantage, though: The screw which keeps the gear to the axle can easily be tightened up whenever one wants as opposed to having to unscrew the 4 or 8 or more screws of the cover with a special tiny watchmaker's precision screw driver to gain access to the axle gear screw which must be tightened as much as possible to keep the string in tune yet the axle has to be able to rotate in order to allow for a possibility for to perform a tuning technique on each string. Funny, eh! As far as the material goes, a good consideration is to use material only from the best age of the wood. Planks can be cut in parallel to the imaginary axle of the stem. The deeper to the center they are cut from, the older the wood. Different age of wood has different qualities. Careful selection may gain some results in the sound of the guitar. Unlikely to have ever been carried out nor any research to have been done. Wood aging plays a significant role in sound. This is also known as wood opening. One needs to wait for the wood to age to open to provide a better sound. This aging factor makes a selection very difficult because no one knows how a given type of wood would sound like in 10 years in order to be able to select now. As a gross generalization, the unscientific scientific approach says: if the wood sounds good now, the wood would sound more good in a 10 years even better in 100 and 1000. The idea of the guitar body is to provide sound reflection, called resonation, by internal sound concentration. The guitar body is a passive object, I.E. the guitar body does not amplify the sound but rather concentrates and focuses the sound in order to project the result through the guitar hole in a direction towards the audience. Thus the guitar body would conform to the general scientific principles and parameters such as frequency response and volume. A very important consideration in sound concentration, is the fact the hard materials REFLECT sound and the soft materials ABSORB sound. There is no way to avoid this basic rule of physics. Thus, regardless of what the sound buffs say, the harder the wood, the greater the density and the tinier the pores, the stronger the sound. Also, the greater the thickness, the stronger the sound. Hence, high thickness hard wood guitars would be preferable. Marble guitars would be the best, as well as metal guitars, but marble guitars are not manufactured due to the high cost and the difficult manufacturing process as well as the weight of the final product. Obviously, diamond guitars would be the best because diamond is the hardest material but there is no such a huge diamond to be able to cut the necessary shape unless the diamond is synthetic, even though may not be possible. Another consideration is the assembly of the different plates of the body: the tone wood, the top wood and the side wood. The best, the body would be monolithic, drilled into with machines. Manufacturers do not make this also because the manual precision drilling is difficult and take a long while and is very imperfect yet resulting in a bad tone and machines either do not exist or no one wants to use them mainly because of the price and the low speed of manufacturing. Monolithic body would have different age of the same wood all over the place as well as different seasons. Even distribution with differently aged parts of the tree is an important consideration as well unless the planks are cut vertically and not horizontally. Then some may argue the orientation of the cut is also of importance. More complicated, within every age part of the wood, sub parts which grew in the spring would have different parameters than these which grew in the summer mainly as far as the pores are concerned. An important consideration is the selection or the quality of the selected piece of wood out of a given type. There are very many parameters which affect the actual tree of a given type: environment, climate, minerals, water, etcetera. Hence, just to say a guitar is made out of a given type of wood doesn't say much. The important thing is the quality of this wood. Hence the guitars manufactured in China are low quality regardless of what they are made out of because the Chinese manufacturers would select the lousiest wood possible out of a given type to try to save every penny for materials. And because the consumers in the US and Canada are either stupid or do not have information, the consumers go by features rather than by how stuff is made. Hence they purchase lousy products just because the features are better. As a consequence, a Chinese guitar made out of the best wood or sets of wood may be even worse than a paper guitar someone made in the basement. Another example would be: a Chinese V8 car would definitely be worse than a Japanese V6. The features and parameters may be better BUT what's the point of having a car at all which would break one hour after purchase, even, on occasion, before the few seconds to reach 100km have elapsed. As a gross generalization, the quality of the used wood cannot be generalized upon very much. Thus, the principle "Each plank is different" applies to an extent. Scientific research can be carried out and would give some results, though measurement of the parameters of each individual piece, being a plank or a veneer or the neck half cylinder may give better results. Yet, one must remember, these parameters will change in the future. Thus, the engineering, the design of the guitar may prove to be more important than the material. Here is a quick reference of the types of woods used for guitar manufacturing:
  • Mahogany: hard, high density, large pores. Ideal for a single wood design. Combines the requirements for high and low frequencies. Extremely expensive. Dubbed "Black Gold" same as oil.
  • Ebony: harder, high density, some size pores, not as big as mahogany, and oilier before dry. Oily pores filter sound. Even more expensive.
  • Rosewood: hard, high density, some size pores, oilier before dry. Expensive, yet not as expensive as mahogany and ebony.
  • Swamp Ash: hard and soft layers with large pores.
  • Maple: hard. Inexpensive. Preferable for inexpensive models.
  • Walnut: hard with large pores. May filter some frequencies. Inexpensive.
  • Spruce: soft but high density. High frequency wood. Inexpensive.
  • Others: Alder, Koa, Basswood, Korina, Lacewood, Wenge, Pao Ferro, etcetera. There are other hard woods even harder than ebony but these are impossibly expensive or protected by law. The hardest wood in the world is the bull oak which grows in Australia and is protected for conservation purposes as well as for conservation of some endangered birds which prefer to nest on this tree. Most likely, the most expensive guitar as far as the material goes is the guitar fully made of ebony. These guitars are extremely expensive and often offered as a custom option by some of the large manufacturers. Just the material for the neck of the guitar may reach price of more than $200. Thus the price of the materials of an ebony guitar may as well be in excess of $300. When you add manufacturing and distribution cost, the price may exceed $500. When you put the standard 10 fold for a hefty profit, these guitars would exceed $5000. Some sell for much more because of fashion, name, model, signature by a famous guitar player such as Eric Clapton for example and so on, so I would not be surprised to see ebony guitars sold for $6000 to $10000 even more. When made in limited quantities and are to be sold to famous rock stars only, these may exceed $20000. Generally, a good idea is to use full mahogany guitars. Some put spruce as a top wood while everything else is mahogany except the rosewood fingerboard. The addition of the spruce top reduces the price. The manufacturers may claim they would mix mahogany and spruce to get all frequencies. Some may claim they would use rosewood fingerboards because rosewood is hard. For those around the spruce: there are three types of spruce used for top wood mainly: Sitka, Engelmann and Adirondack. Sitka is the most used and the least expensive. Adirondack is the most expensive of the three. Here is an excellent article on the spruce tops. And here is an interesting article for all woods used in a guitar. Here are some design consideration: Material wise: The whole guitar better be made out of full wood and not out of plywood. The body must be as large as possible to produce a good sound reflection yet possible to be played on. Thus a body with straight shoulders would give more sound as well as a body with oval tone wood. A cut out on the body under the neck improves the playing ability with only negligible decrease in volume of the sound. This is more expensive to manufacture hence the manufacturers stay away from this option. Cut out option is available in only the more expensive models. The material must be as thick as possible although excessive thickness may affect the low frequencies, most likely, negligibly. Yet the thick material may add extra weight which is not desirable although bearable. As far as the principle of sound concentration is concerned a spherical body would give the best results but these would be very difficult to play on. The neck must be made out of hard wood. Still, truss rod is necessary for aging bending prevention and not for tenability. Tiny tuning of the action ( distance between the strings and the fingerboard ) may be possible but is highly undesirable because this loads the neck and acts on the gluing point between the neck and the body. Bad news is most of the manufacturers still glue the neck to the body as opposed to the better way of screwing. Also, neck bending affects the intonation: the tuning of every fret. The head must be very strong and strongly supported onto the neck. Thus to use a single piece of wood for the neck and the head is advisable. Ideally, the truss rod would go all the way through the head and even the body but the manufacturers do not consider this to be necessary because the bending of the head is a slow process and does not affect the performance because the effect of bending can be compensated with tuning. For a good strength, the truss rod must be positioned tightly into the neck hole and not to be able to move inside. Most likely, no manufacturer would use such a design because they do not care and rely on the bolt strength and because they want to introduce some kinda tuneability of the neck angle by variation of the bolt tightness. The fingerboard must be made out of the hardest wood possible. Some consider the neck and the fingerboard to have an effect on the sound. May be negligible. Usually, rosewood has been decided to be used with ebony for the more expensive models. After all, the frets bear the wear and to change the fret board is not so expensive. Some inexpensive models do not have fret boards and the frets are banged into the neck directly. Cannot be changed nor readjusted. I am a great fan of a full mahogany guitar would be nice, even the fingerboard. The bridge must be made out of hard wood, ebony is preferable, rose wood and mahogany are often used. Ideally, the bridge must be screwed into the top but most of the guitar manufacturers rely on glue. The saddle and the nut are usually made out of synthetic plastic with bone nuts and saddles for the expensive models. Ivory is no longer allowed to be used. If there was a way to ensure ivory was derived from naturally dead elephants, then there would not be a problem to use these. However, to ensure fair trade is almost if not impossible thus ivory better be disallowed. Mechanical: The guitar manufacturers are afraid from machine and wood screws like a vampire from silver bullet. They believe the screws and nuts would loosen up and vibrate. How stupid of them. The guitar would rather disintegrate than the screws and bolts and nuts loosen up. I do not know what screws, nuts and bolts there were during Stradivarius era but these components are extremely reliable now and, probably, much more reliable than any wood and glue in the whole guitar. A metal frame (titanium alloys or dural) for the body was going to be nice but the manufacturers do not want to use this idea. They are afraid from metal too like a vampire from a metal crucifix too. A suspended to the bottom of the body, metal string attachment and a movable wooden (or metal) saddle is a better option but only a few manufacturers do so. Movable bridges allow for a better intonation as well as action adjustments. Manufacturers may be afraid again from vibrations. With the strength of the tuned strings upon the saddle? Fully adjustable bridges, similar to the ones of the electric guitars are not available with the acoustic guitars. Unknown why. Generally, there aren't very many choices with the acoustic guitars except for the materials, the availability of truss rod and fingerboard and the width of the neck. There are a few sizes of frets with the electric guitars. The thicker the frets and the more oval (jumbo frets) the easier for solos and the more difficult for chords and vice versa. Acoustic guitars would not offer much of a choice in fret sizes. The position of the frets on the fingerboard defines the intonation of the guitar: how accurate the corresponding tone to every position is. There is a very accurate theoretical physics formula which tells how long a give string with a given diameter and a given tension should be to produce a given tone. Thus the theoretical position of every fret is 100% defined. However, the action of the guitar (the height of the string above the fret) requires extra tension to be applied at the string thus a fret compensation which depends on the action which is adjustable as well as the size and type of the string as well as the length of the neck. Since strings are chosen by the customer as well as the action can be slightly tuned by the truss bolts, the manufacturer has no way to compensate for the applied tension due to action and string type. As far as the action is concerned, the manufacturers would usually specify something like: 2mm height of the thick string and 3mm height of the thin string at 12th fret with 1mm of the thick string at the 1st fret. No manufacturer I know of specifies the strings. Manufacturers use the desire of the customer for a choice to avoid the guarantee of the accuracy of the guitar. Old trick. I've used this trick as well for instrumentation. A real manufacturer should say something like: the intonation of this guitar is 100% guaranteed with this action and this set of strings. Keep waiting... The length of the neck is another important consideration. The longer the neck the more frets but the more the string tension because the longer the string the lower the sound at a given tension thus tension must be increased. The increased tension, however, can be compensated with thinner strings which would require a lower tension to play a given tone at a given length. The thinner the string the higher the tone. Thus, looks like, the longer the neck the better. Not exactly. The longer the neck the more difficult to play. Thus, regardless of the neck length and the position of the bridge and the design of the shoulders and the body: the closer the 1st fret to the body of the player (not necessarily to the guitar player) the better. Thus people with long hands as for example John McEnroe can choose long necks if they so desire. As far as the design is concerned, one must look at this shopping guide: 1. Materials 2. Truss Rod 3. Size of Neck: Classical guitars have wider necks which can go as wide as 5" or 12 to 13cm but the acoustic guitars have narrow neck similar to the electric guitars and there isn't much of a choice. Length of the neck allows for more fret positions but requires more tension to be applied at every string which may not be a problem with thin strings now available. 4. Action: the distance between the strings and the neck. This is probably the most important parameter which, at the 12th fret, should be approximately 3mm for the thin strings and 2mm for the thick. More than this becomes difficult to play and even this is a lot. The playability depends on the strings as well as on the length of the neck: longer and thicker strings require more tension hence more strength to press but allow for a lower action without fret buzz which, to some extent, compensates for the required extra strength. Light strings as well as nylon strings require low strength. 5. Intonation: although to provide the non adjustable acoustic guitar with a perfect intonation is a must for the manufacturer, most do not. 6. Are the strings parallel to the long sides of the neck or there is more room at the end and is there more distance between the first string and the side of the neck at any fret than between the 6th string and the opposite side of the neck at the same fret. I.E. are the strings in the middle of the neck? 7. Is there enough room from each end strings (1st and 6th) to the side of the neck or the string would roll out easily. 8. Is the spacing between the strings even? 9. Are the cuts on the nut and the saddle even? 10. Most importantly: Are the holes for the strings at the bridge evenly spaced? 11. Are there any scratches on the neck, body under the strings or the pick protector on the body? Has the guitar been played or the guitar is brand new, manufacturer sealed? Has the guitar been tuned by the shop or the tuning is original by the manufacturer? Are there any scratches or dents on the body or throughout the whole guitar, has the guitar been "on the wall"? Are there any scratches, shiny parts on the frets? 12. Is there any fret buzz? Be aware, this may be because of the strings. 13. Where is the guitar made? If in China, be aware: there is most likely poor design, manufacturing, materials. 14. Are the machines covered? Are they loose? If yes, can they be easily tightened up? 15. Are the knobs on the machines and the axels casted out of the same metal or the knobs are somehow attached? If attached, what are they made out of? Metal (OK) or plastic (bad)? 16. Are the strap hooks stable? 17. Are the body plates unglued? Is the neck unglued from the body plate? The sales representatives may try to pull the "truss rod tuning method" on the customers. Be aware: this type of tuning is undesirable and may be damaging in a long or not so long terms. Make sure the factory adjusted truss rod provides perfect action and intonation. Price: This is the most difficult topic as far as the choice is concerned. As a conclusion: Good materials are good to have but these do not make a SIGNIFICANT difference as far as the sound goes. A simple $40 Chinese pine guitar and a mahogany / spruce $120 Epiphone sound the same as a $3000 Martin. The same difference. And the Martin body is bigger. I am not talking of a Martin Backpacker. But WE ALL PAY FOR THE MATERIALS, DO WE NOT. I could not care less whether a Texan red neck working for Fender in the Great State of Arizona can feed them children. We all have children, do we not? Shall I mention the douchebags of Gibson in the center of Country Music, the fabulous state of Tennessee? Look at the design: wideness of neck, length of neck, action, fret size, intonation, truss rod availability, adjustability, straightness of the neck, strength And remember: when I make mistakes, this is not the guitar to blame, this is the guitar player! Enough typing for now, though. By Steven Stanley Bayes
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