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More than you ever wanted to know about strings – Part 1

Strings are the heart and soul of the acoustic guitar and indeed of any stringed instrument. All the rest of the instrument does is make the sound of the strings louder and allow the musician to select and sound different notes.

Strings may seem like such simple objects, but it has taken thousands of years of technological development to produce the musical strings of today and the development and design characteristics of stringed instruments have been determined entirely by the type, limitations and quality of the strings available at any given time.

Strings and stringed instruments seem to have started their development with the bow. As far as we know, the first bows appeared in the late Palaeolithic or early Mesolithic. The earliest evidence for bows in Europe comes from Stellmoor in the Ahrensburg valley north of Hamburg, Germany and dates from the late Palaeolithic Hamburgian culture (9000-8000 BC). Although it’s often assumed that the bow was first developed for hunting and perhaps used as an impromptu musical instrument, which then developed into stringed instruments, there is now evidence that things may have developed the other way around and that the earliest bows were crude musical instruments.

Bowstrings have been made of a range of natural materials, such as plant fibres and animal sinew. Most Neolithic bows are made of yew, a material that was later commonly used for the ribs of lute bowls. Ötzi the Iceman found in the Ötztaler Alps carried an unfinished yew longbow, with a bowstring of nettle or flax fibre.

The myth of catgut strings

Although gut strings have been found in the tombs of the pharaohs, gut string making as a European industry didn’t get started until the early 14th century. Gut guitar and violin strings remained in general use right up until 1946.

There is a common misconception that gut strings are made of catgut and at least two explanations of how this myth got started. One story has it that the early string makers borrowed a technique from rope makers for twisting strips of sheep gut together to make the thicker string gauges. The rope makers used this method to make the thinner cords for ships rigging and the nautical term for these cords was ‘catlines’.

Another story is that back in 1300 AD the town of Salle in Pescara, Italy, was famous for saddle making, partly because the saddles were stitched together using a strong thread made from the intestines of the local mountain sheep. Tradition credits a man called Erasmo for first thinking of using the thread for strings. Erasmo was later sainted for starting an industry that became very important for the town and Saint Erasmo is now, not only the town’s saint, but the patron saint of string makers.

When people asked about the secrets of the fine musical strings made by the string makers of Salle they were told by the townsfolk that the strings were made from catgut. The string makers thought they would protect their trade by circulating this story because, at the time, cats were regarded with a great deal of superstition and it was believed to be extremely bad luck to kill a cat.

Two of today’s string companies, D’Addario and E. Mori (LaBella strings), both now located in the USA, are proud to trace their history all the way back to their family business in the 14th century town of Salle.

The history of metal strings

Wire is made by starting off with a thick bar of metal and then drawing the metal through conical holes in a series of progressively smaller ‘dies’ to make wire of different diameters. This process goes back to around 400 A.D. The earliest currently known die plate is one discovered in Viking Scandinavia that has been dated sometime between 500 and 700 A.D. Each time the metal is passed through a die it can only be reduced by a certain amount or it will snap. This is the origin of the various wire gauges and string making has its own gauge standard of Music Wire Gauge or M.W.G.

Early metal strings, for example harp strings, were made, either from single wires, or rope-twisted strands of gold, brass or iron. This placed restrictions on the thickest strings that could be made and limited the range, particularly in the bass, of any given instrument.

Wound bass strings, with a core of gut or silk, wound with silver or silver plated copper wire, weren’t invented until around 1650 and high quality, polished steel wire, suitable for strings, became generally available in the 1830s. Gut strings were replaced by nylon in 1946 due to a shortage of sheep (and therefore guts) following the Second World War.

The characteristics required of musical instrument strings

Musical instrument strings must possess quite a number of demanding characteristics.
They must have sufficient tensile strength to be tensioned to the required operating pitch and remain elastic, be uniform in diameter and density and be unaffected by temperature and humidity. Strings need to be as flexible as possible, resistant to friction wear and resist corrosion. By any standard, even given the materials science of today, this is a punishing list of requirements. Of course, real strings aren’t perfect and only satisfy each of these requirements by varying degrees.

Tone, scale length and picking position

As stated in the introduction, the strings are the source of all the sound that a guitar makes. The rest of the guitar may modify the quality of that sound but it can’t add anything that isn’t already there. The harmonic content and purity of string vibration is quite dramatically affected by scale length. The longer a string is, the less it is affected by its own stiffness and the wider the range of accurate harmonics it produces when it vibrates.

To put it another way, the character of the sound a string produces is quite dramatically affected by its length. So much of what we think of as the sound of a guitar is a direct result of the scale length. The characteristic ‘guitar’ sound is also affected by the natural picking position, at about a fifth of the string length, accentuating some harmonics and damping others.

Doubling a solid (unwound) string's diameter quadruples its strength and the tension required for a particular pitch, but increases its stiffness 16 times!


Materials

Today, after thousands of years of evolution, musical instrument strings are made from only a few materials. The only major change in materials in the last hundred or so years has been the substitution of nylon for sheep gut and even then gut strings are still being made by specialist string makers. What has changed is the quality, accuracy and consistency, of the materials and of the manufacturing processes, to produce the finished product. In recent years alloys have been fine tuned by the use of small amounts of additives and by changes in manufacturing methods.
String cores are almost without exception made of tin plated Swedish steel for metal strings and multi-strand nylon for wound nylon strings. A relatively small range of metals are used for the wrap wire of wound strings and acoustic strings are almost always wound with copper or one of its alloys. Electric strings (and often Dobro strings) are wound with harder materials like nickel, nickel plated steel or stainless steel. Bowed instrument strings are sometimes wound with aluminium wire and monel metal tape and flattened nylon are used to wind tape wound strings. Silk is also used as an inner wrap on some strings to modify tone.

Metal string construction

Musical instrument string manufacturers do not make the wire from which metal strings are made, but buy it from specialist wire mills. For example most string manufacturers buy at least some of their wire from the Mapes Piano String Company in the U.S. or the German company Roslau. Wire used for strings is known in the industry as ‘music wire’ and the wire used for the plain strings and for the cores of the wound strings is made from high carbon steel, that is spring tempered and often referred to as ‘Swedish steel’. Although music wire is available un-plated, steel music wire for guitar strings is usually tin plated to minimise rust. Other plating materials are sometimes used, such as bronze or even gold.

Here’s a picture of a typical wire drawing machine, this one’s made by Morgan-Koch, with the wire being drawn through successively smaller diameter dies via a system of wheels.

A section of the wire store at Rotosound


To make plain strings (there is no difference between acoustic and electric plains) the string maker cuts off a length of wire of suitable gauge and twist locks a small brass cylinder, know as a ball, onto one end. Length cutting and string ball attachment in most string factories is a totally automated machine process. Strings for some instruments such as the mandolin may have a simple loop twisted onto one end. Nylon guitar strings, with their greater flexibility, are usually tied to the guitar bridge, but recently have also been available with ball ends.

Plain steel string ball end and twist.

Wound strings start with the same process, but in addition are wound with a suitable gauge wrap wire to create a finished string of the required gauge. Large diameter strings may be over wound with a second layer of wrap wire, but this is very unusual for standard acoustic guitar gauges and is normally only found on the bass strings for baritone guitars.

Hand winding at Rotosound. The round weight on the rod gravity tensions the core via a mechanical linkage to the string drive chuck.

String winding

Wound strings have the wrap wire applied, not surprisingly, on a string winding machine. These vary in complexity from simple machines that just hold the length of string core wire under tension and spin it, while the wrap wire is fed on by hand, to machines with lead screw or belt driven heads that feed the wrap wire on, under electronically controlled tension. At present even the most sophisticated string winding machines are not fully automated and require a human operator to load the cores and unload the finished wound strings. Although it is possible to buy complete string winding machines from a few companies that specialise in them, many string makers are proud to design and make their own string winding machines and say that their strings exhibit special qualities due to the way their particular machines tension the core and the wrap wires. Some string makers have also improved their productivity by building machines that wind a number of strings at the same time.


A Rotosound, 2 string, automatic winder in action.

Hex cores versus round cores

The core wire used for the vast majority of modern metal-wound guitar strings has a hexagonal cross section. This stops the wrap wire from un-coiling when tension is released as the newly made string is removed from the winding machine. The six points of the hexagonal wire bite in to the inner side of the wrap wire as it is wound on and lock it into place. However wound metal music strings weren’t always made this way. Because of the pianos great range in pitch and dynamics, piano design has been a rich proving ground for string technology. For example, most wound piano strings are still wound on round cores because this construction produces a slightly more flexible string, so harmonics generated by round core strings differ from those generated by hex core strings. The resultant tone, at least of round core piano strings, is felt to be superior.

Wound piano strings also differ from guitar strings in that the wrap wire stops just short of the bridges at either end of the string, to minimise string stiffness. This means that piano strings have to be custom made to fit each model of piano. Windings on these strings are locked in place by hammering or rolling flats onto a section near either end of the round core before winding, effectively making a short, square cross-section of core for the last turns of the wrap wire to bite onto.

Hex core ‘universal’ replacement piano strings are made, but only for use as quick, cheap replacement strings. The hex core construction allows the piano technician to unwind the wrap wire to expose the core at either end of the string, without the risk of the rest of the wrap becoming loose.

British string makers, Rotosound (www.rotosound.com), make both bass and guitar strings with ‘contact cores’ that, like piano strings, have the windings stop short to expose the core where it passes over the bridge, although the nut/fret end of the string of course remains wound.

A few string makers, for example Newtone in Debyshire (www.dwmee.34sp.com), do still make round core wound guitar strings. Newtone use the piano string technique of creating flats on the core of wound strings, near the end of the string, to lock the windings. Even using this technique, because of the risk of the wrap wire uncoiling and becoming loose on the core, it’s harder to produce round core strings of consistent quality than it is using hex cores. String consistency and stability, throughout the industry, improved dramatically when hex cores were adopted.

Wound string tone and the wrap material

The harder the wrap wire material used, the brighter the tone of the finished string. Stainless steel wound strings, normally only used on electric instruments, are very bright and the availability and use of bright sounding, stainless steel strings on the electric bass has lead to the development of a range of new bass techniques and the adoption of the bass guitar as a solo instrument. The shape of the winding also has a tonal effect and a round wrap wire produces a bright string, while tape or half-round wound strings have a warmer sound. This is because the round wire winding, like a spring, flexes quite easily and doesnt damp the higher frequencies. The tape winding is stiffer. Monel metal tape, another alloy of nickel and copper, is used on most tape-wound strings.

Silver was the first material used as a wrap wire for wound strings, but this soon changed to silver plated copper, retaining the expensive appearance, at lower actual cost. The silver plating was well within the scope of available technology at the time wound strings were introduced and it helps protect the copper against corrosion, although silver itself also tends to tarnish in today’s polluted air. Copper is a ductile metal that is relatively easily formed into wire and is also heavy enough to add significant mass to the lower pitched strings.

Wound nylon strings are still wrapped with silver plated copper wire, partly to obtain the accepted tone and partly because the soft copper can be wrapped over the filament nylon core, without it being necessary to apply very high tension to the core during the winding process.

Steel core, silver plated copper wound strings are available and are usually regarded as specialist strings only used for the gypsy jazz style and sound of Django Reinhardt. These shouldn’t be confused with the silk and steel ‘folk’ strings that include a layer of silk or nylon filaments between the core wire and the wrap wire, to produce a softer, almost nylon string sound in the bass.

Brass and phosphor bronze, are alloys based on copper as the main ingredient. Alloying elemental metals and other elements together produces metal alloys with characteristics that don’t exist in the pure constituents. Phosphor bronze is an alloy of copper with tin, to produce a tougher, harder metal. Brass string wrap wire has the least copper at 80% alloyed with 20% zinc and strings wrapped with this alloy are often misleadingly referred to as 80/20 bronze. The relatively high percentage of zinc gives it a light yellow-gold appearance and this harder alloy gives the wound strings a bright sound.

Phosphor bronze is mostly copper, alloyed with between 3.5% and 10% of tin. Alloys with slightly different proportions of the component metals are used by different string companies. Less than 1% of phosphorus is used in the alloy and it was originally added to remove oxygen from the mix during melting. This softer metal wrap wire results in a string with a mellow, warmer tone than the 80/20 brass alloy. Phosphor bronze wound strings have a slightly red appearance compared to brass (bronze) wound, due to the higher proportion of copper in the alloy.

Nickel wrap wire is used mainly on electric guitar strings and is normally regarded as producing too bright a tone for use on acoustic guitars, although nickel wound strings are used by Dobro players and some mandolin players.


The upper string in this picture is phosphor bronze wound and the lower is brass wound.


A Nylon, silver-plated copper, wound string.

Wound string wrap to core ratio

There is one ‘hidden’ parameter which string companies do have the opportunity to vary and which may be the biggest difference between otherwise similar string sets and that is the ratio of wrap wire diameter to core wire diameter, of the wound strings.
Up to a point using a larger core diameter gives a string greater volume and sustain, but string stiffness is increased, making the string harder to play and its harmonic purity is compromised. Using a thin core results in a more flexible string that is easier to play, with good harmonic purity, but lower volume and sustain. Hex core dimensions for standard string set gauges and tunings don’t normally exceed 0.016 across the flats for the low E string. Hex cores normally increase by about 0.001 per string from the wound g string upwards, although some manufacturers may use the same core size for two successive wound strings in a set.

String and String Set design

Designing a musical instrument string, or even a string set, is quite a complex problem because of the number of variables involved. There is the required pitch, material, mass, tension and breaking strain of the string itself, but because the string interacts with the instrument it is fitted to, factors such as the scale length and resistance to tension of the instrument must also be considered. In the dim and distant past the instrument and strings would have been made by the same person, by a process of trial and error. Today there is little interaction and guitar and string makers tend to develop their products separately. Much of the time each takes the others products pretty much for granted.

If you ask a string manufacturer what criteria they use in designing strings and string sets you are unlikely to receive a useful answer (I know because I asked them all!). Answers you do get range from “we don’t know, we just make them to the formula set years ago” to “we could tell you, but we would have to kill you” or “we make them pretty much like the other string companies because it’s what the customers expect”.

String set tensions

Until about 150 years ago string design was mostly trial and error. Then the general level of engineering knowledge and technology developed to the point were it could be applied to the existing practical experience and clear design principles were established. For example the following formula shows the relationship between tuning tension, physical properties, and the fundamental vibrating frequency of a string.

T = (UW x (2 x L x F)2) / 386.4

Where T is tension in lbs (strictly speaking, Pounds are a measure of mass and not tension. String tensions given in pounds can be converted to Newton’s by multiplying by 4.45), UW is unit weight of the string in lbs per linear inch, L is vibrating length in inches and F is the fundamental vibrating frequency in Hertz.

To make use of this formula you need to know one key property of a string; it’s Unit Weight. This can be arrived at using two different methods, either by accurately weighing a known length of string or by measuring the diameters of the core and wrap wire (if the string has one) and calculating unit weight using the known densities of the materials.

You might expect the gauges of a string set to be chosen so that, when tuned to pitch, all six strings would be at the same tension, providing the guitarist with a consistent feel across the set, or at least for there to be an obvious relationship in the tension values across a set, for example, a smooth increase in tension from treble to bass. In practice all string sets exhibit large differences in tension across the set.

Apart from Thomastik Infeld, who publish tensions for their string sets on their website, only major manufacturer D’Addario, publishes a complete string tension table and prints individual string tensions on their set packets. Without published tensions for every available string brand it takes a bit of research to find out just how widely string tensions vary and that, although there is a consistent pattern to the tensions within sets, across all manufacturers, there is no obvious logic behind it. It is quite common for there to be at least a 30% variation in tension across a string set.

If you compare the set gauges of commercial acoustic guitar string sets from all the string manufacturers you will find they are made up of almost the same gauges. For example the maximum gauge variation for light (0.012 e string) acoustic set, out of all strings in a set is only 0.004 between a 0.022 and 0.026 g string and the average differences in gauge for the same string in sets from different brands are even smaller at only 0.001 of an inch difference in diameter. These tiny differences do have more of an effect than you might at first think because tension is related to the square of string diameter. Doubling the diameter of a string quadruples the tension. Greater differences occur in the custom and signature sets and in the sets designed for playing certain styles such as bluegrass.


In this graph of string set gauges the curved brown line shows the gauges required for an equal tension set (starting with an 0.010 e’) compared to the actual gauges found in the D’Addario commercial sets.

In this graph the corresponding tensions are shown for the same sets plotted in the gauges graph. All the set tensions exhibit the same characteristic ‘sickle’ shaped plot. Notice the ‘crossovers’ for the custom light and bluegrass sets with the slightly heavier lower strings. The flat ‘perfect equal tension’ line is included for comparison.

What is significant is that string set tensions for standard acoustic steel strung guitar in EADGBE tuning, follow the same patterns, across the industry. In general for any medium set starting with a 0.013, the top two plain strings are at roughly the same tension, the third string (usually a wound string) forth and fifth strings increase in tension by about 30% and finally the bottom string tension is only 6 to 10% higher than the first two plain strings. As the overall gauge of the sets varies from extra light to light to medium (the term ‘heavy’ for an acoustic string set isn’t normally used now, perhaps because marketing departments feel it might put customers off) this pattern of relative tensions across the set remains, although of course all the tensions across a set increase as you go from extra light to medium sets (see the tension graph above for a clearer picture of string set tensions).


This graph shows the tensions for strings set for different guitar types. Tensions shown are for D’Addario phosphor bronze EXP sets, plus a Newtone resonator set and two La Bella Baritone sets for comparison. D’Addario’s resonator set is intended for the d’bgdBG lap Dobro tuning and so exhibits very different tensions to the Newtone set.

It is perfectly possible for the string manufacturers to assemble equal tension sets for standard tuning, or for any of the common open and alternate tunings. It is even possible to buy off-the-shelf, standard gauge, single strings to make up a set that comes within a few pounds of equal tension, without resorting to custom gauges. So why do the string manufacturers compose their string sets with such uneven tensions? Since string manufactures aren’t prepared to reveal their reasoning we can only make an informed guess.

The most obvious transition is between the top two plain strings (all my examples given here are for 2 plain 4 wound sets) and the other four wound strings and this is clearly evident in all the graphs. The core dimensions for the 3rd and 4th strings are commonly smaller than the plain 2nd. Bigger gauge strings are louder, so it seems that the bigger gauges (core plus wrap) for the g, d, A and E strings compensate for the effect of the small cores and are necessary to obtain a volume balance across all six strings. This seems considered to be more important than an even string tension across the set. Since volume balance will depend to some extent on each particular guitar, presumably string set tensions are chosen as a best guess compromise to suit the ‘average’ guitar.


A plot of the gauges and core diameters for both an ‘equal tension’ set and the D’Addario EXP16 set, show that the core diameters for the 3rd and 4th strings are smaller than the plain 2nd.

For the more exotic instruments, such as National resonators, Dobros and Baritone guitars, where there is less market pressure to conform, individual manufacturer’s ideas on the correct string set become much more evident and there is greater variation in string set make up between the brands.

Strings & the musician

All this detailed information on string construction and design is all very well but what use is it to the musician?

Well there is a considerable choice of string types and gauges on the market, with new variations being added all the time, so understanding strings at least helps you to see beyond the marketing hype to select the most appropriate standard string set for your style and instrument. If you are buying strings in a guitar shop it isn’t always evident what variations are available and you might not realise that specialist sets, like the bluegrass set, even exist. Taking it further; knowledge of string set design helps the guitarist looking for their own sound. Individual string gauges can be selected to make a custom set with a slightly different tonal balance, or to better suit playing in an open or altered tuning. For example; conventional wisdom says that you would never use electric strings on an acoustic guitar, however fingerstyle master John Renbourn says on his website that for many years he used sets of nickel electric strings when playing his acoustic using a magnetic pickup. Nickel strings could also perhaps be used to brighten up an otherwise very dull sounding acoustic guitar. Conversely it makes sense to use phosphor bronze or even copper wound strings on a naturally very bright instrument. Or perhaps a mixed phosphor bronze, bronze set could be used to obtain an unusual tonal balance. DR Strings make sets of what they call Zebra strings with alternate strands of phosphor bronze and nickel plated steel wrap wire, for a bright acoustic tone and great performance with a magnetic pickup. The so called gypsy or manouche style silver plated copper wound strings have a very interesting tone and are worth checking out for playing other styles than gypsy jazz.

The mystery of string set tensions

I have been very puzzled by the tension characteristics of commercial string sets revealed during my research for writing this article and, using the tension information from the tension chart published by D’Addario, I decided to try a limited experiment. I was able to select six individual strings for EADGBE tuning, from the standard range of D’Addario gauges, with a much closer to equal tension than their standard packaged sets. I tried this set on an Alvarez Yairi DY66 dreadnought with a cedar top (so this guitars nature is to be warm and loud). This ‘custom’ set is very comfortable to play, there is no obvious volume imbalance across the six strings and the overall sound is very good indeed, not lacking in volume and lively without being over-bright. I used the following gauges - 0.012, 0.016 plain and 0.021, 0.029, 0.039 & 0.049 phosphor bronze wound, for this set, giving the following tensions of 23.3, 23.3, 23.1, 25.2, 25.4 & 22.5. The maximum tension difference is about 14% across the set, compared to nearly 39% for a standard set of D’Addario, or in fact almost any other manufacturers, set of light gauge strings.

Browse to www.daddariostrings.com and select – Support – FAQ – Product assistance, select the seventh question down about a tension guide and you can download a PDF file of the D’Addario tension guide.

Coming up in part 2 in the next issue

In part two of this feature on strings I’ll be explaining exactly why strings break, why they wear out and lose their tone and how to install new strings to minimise tuning problems. I’ll look at long-life strings, cryogenic hardening and nylon strings in more detail
There will also be a table listing all string manufacturers, along with the types of strings they make and a graph comparing the cost of string sets.

By – Terry Relph-Knight

CREDITS - My thanks for their help in preparing this article to – Jason How at Rotosound, Frank Ford for the three string pictures, Brian Vance, Sr. Brand Manager, Fretted products at J.D’Addario, Jim Cavanaugh for the Super-Sensitive Musical String Co., Craig Theorin Product Manager Elixir® Strings W. L. Gore & Associates, Inc., Malcolm Newton of Newtone Strings, Brian Ball at Ernie Ball inc., Greg Orred at GHS, Paul Mason at Sfarzo Strings, Pete at Rohrbacher Technologies and anybody else I pestered with daft questions about strings and have inadvertently omitted from this list.