Stringed instrument building: Making waves and resisting entropy.

Guitars and fiddles have a lot of similarities and some major differences.  Comparing the two is an interesting exercise which sheds some light on how each instrument works. Stringed instrument building is essentially concerned with two things: modifying vibrational energy to produce sound, and resisting the destructive forces of that energy.  Making a string vibrate requires that the string be held under tension. The universe being what it is, that tension is gradually released into the structure which holds the string, distorting it until the tension is gone.

I’m going to compare and contrast the the essential features of guitars and violins, starting with a list of their similarities. For this discussion I’m mainly considering flat-topped guitars, with bridge-anchored strings. There are also guitars with arched tops and with end-mounted strings. Also, what I say about “violins” applies equally to violas, cellos and basses.

Guitar and violin essentials

  1. A string or strings under tension
  2. An amplifying  sound box consisting of
    – A vibrating top plate or membrane
    – A resonant air volume
  3.  A neck and fingerboard, allowing the vibrating length of the string to be varied
  4. Tuning pegs

I’ll take each of these in turn and consider the differences, but first, to understand where those differences come from, it is essential to consider:

How the strings are excited.

Guitars  – the string is plucked, the sound box amplifies the vibrations, The energy imparted to the string is at its greatest the moment that the string is plucked and that energy dissipated in  moving parts of the guitar.

Violins  –  the string is bowed. Bowing is effectively a very rapid succession of plucks where the sticky rosin on the passing bow hairs grabs the string and holds it until the pull of the string overcomes the adhesion and the string returns towards its starting position. As the string relaxes, the rosin again sticks to the string, grabs it and releases it, in a continuing cycle.  Unlike the guitar, the moving bow is constantly imparting energy to the string so that instead of producing a strike-and-fade note, a violin can hold a note indefinitely, increasing and decreasing the volume at will. In fact the pitch and tone can also be varied in a single note

As will be seen, these different methods of imparting energy to the string greatly affect the design and construction of the two instruments.


1. The Strings

Strings are, obviously, the heart of stringed instruments.  Development of string technology has contributed more to the evolution of acoustic instruments than anything else in the last 150 years.

String evolution

Length, tension and mass are the factors that determine the frequency that a string vibrates at. Think of the strings on a harp or piano, the long heavy ones play the low notes, the short skinny ones play the high notes. Originally strings were all gut. It was later found  instead of just making the low strings thicker and thicker (and less and less playable) their mass could instead be increased  by wrapping them in wire.  Gut strings were not very durable and were sensitive to moisture, tending to go out of tune quickly. As various filament producing technologies were developed gut began to be replaced, starting with steel wire.  Steel strings were stronger and could be tuned to higher tensions which drove the instruments harder and produced more power. The tone tended to be harsher and less colorful.  The advent of plastics allowed strings to be made that were more durable and moisture stable than gut but warmer and more colorful than steel.  By now people were hooked on the power of steel strings and attempts were made to take and blend the best qualities of the materials available.

For guitars the advent of steel strings meant a sharp branching in the evolutionary tree. Guitars were developed to take advantage of the tonal power of steel. Steel strings could produce much more volume and a more cutting sound that could be heard in ensembles with louder instruments. But this came at a price, if you put steel strings on a classical guitar you have a tiger by the tail*. Its very exciting for a while but they will destroy your guitar. So steel stringed guitars were build with heavier tops and heavier bracing.  Steel strings could also drive a larger body for a boomier sound.  The necks could be narrower because the strings don’t vibrate as widely.  Steel strings were also magnetic which allowed the development of the electric guitar.

The violin didn’t evolve in the same way in response to the invention of steel strings. It has been largely unchanged since the nineteenth century. While classical guitarists switched to nylon strings the steel E string became an almost universal fixture on violins. Because it is so thin and under such high tension, no other material could match its durability.  The higher tension available with steel created a powerful penetrating sound, able to rise above an orchestra. Many players lament its loss of warmth and the tonal inconsistency with the other, synthetic strings on the instrument.

Strings for the violin family instruments have evolved to a much greater extent than guitar strings (with the exception of the steel E-string). Bowing the string leads to problems that guitar strings don’t face such as a tendency to twist and untwist as they are bowed, leading to an unwanted “whistling” sound. Violin strings and cello strings come in a huge array of combinations of solid, twisted, plaited cores of a variety of metals or plastics. These are wrapped in one or more layers of windings of exotic  metals and insulating materials.  All of this comes with a cost.  The most expensive set of guitar strings will costs about $40, that will buy you less than two thirds of a good cello C-string. The whole set costs about $400

*there’s a cat-gut / tiger-gut joke in here somewhere.


2 The Sound box function and structure

The top

The top of the instrument is where most of the sound amplification takes place. The job of the top is to take the tiny vibrations of the strings and spread them over a larger area so that they can set up vibrations in the air around us which we can hear.  Violinmakers and guitar makers share a common problem: how to make the top light enough to vibrate easily while still being strong enough to resist the destructive pressures of the strings. Violin makers and guitar makers agree on one part of the solution to this problem: use a material that is both stiff and light. The best known natural material with a high stiffness to weight ratio is spruce, or similar species of wood. Consequently the tops or sound boards of most stringed instruments, including pianos and concert harps, are built with spruce tops. The major exceptions to this are banjo type instruments. Their drum type construction can lead to very powerful sound production but with perhaps less tonal nuance than is possible with wooden sound boards.

Other than a shared choice of material, the two instruments have taken different routes in trying to solve this problem of responsiveness versus structural integrity:

In the violin the strings are anchored at the top of the neck and at the very end of the body. The strings rise up and bend over the bridge, more or less at the center of the body.  The bridge is not glued to the top, but held in place by the downward pressure of the strings.  This downward pressure is fairly considerable and the arched shape of the top resists the deforming pressure of the strings by distributing that pressure more evenly across the span of the arch, in much the same way that an arch in a building spreads its load. In addition to the arched shape of the violin top, it also gains some strength from a supporting column, the “soundpost”,  standing under the treble bridge foot, while a longitudinal strut, the “bass bar”, supports the other foot. Supporting the bridge area with extra wooden struts like the bass bar is an obvious solution to preventing the top from sinking while leaving other areas of the top light so as to still be responsive to low energy impulses from the string. The use of the soundpost is a radical departure from this and has profound consequences. The anchoring of one foot causes the oscillations of the other foot to almost double in amplitude, resulting in a more powerful instrument. Furthermore, by making the soundpost movable, the tonal color and other playing attributes can be adjusted relatively easily, without making invasive structural changes to the instrument.

The string forces acting to distort a violin

In the guitar, on the other hand, the strings are anchored at the bridge which is glued to the top, not in the center of the body, but in the center of the widest part of the body.  Whereas the violin bridge presses directly down, perpendicular to the top, the guitar bridge twists, pulling up on the top behind it and pushing down on the top on the string side of the bridge.  To resist these destructive pressures, instead of just making the top thicker, which would make the top vibrate less easily, reinforcing struts of wood are placed strategically at stress points, again with the aim of distributing the load more evenly throughout the structure.  On more sophisticated guitars, there is also some slight arch into the tops in a manner to resist the bending effects of the bridge.

The string forces acting to distort a guitar.

Jazz guitars and mandolins are a hybrid of these structures with arched tops and end mounted strings like a violin, but without the sound post.  Instead they have two “bass” bars, one under each foot.

Playing with tone. It has been found that the exact shapes and sizes of all the elements of the instruments top, plus their positioning relative to each other, can significantly effect both the power of the sound that is produced by the instrument, and, more engagingly, the tonal color of that sound. Once we’ve made an instrument that doesn’t quickly collapse when the strings are tightened, the fascination of instrument building is playing with those tonal flavors. The arrangement of bridge supports in the violin was pretty much fixed hundreds of years ago. The precise placement and dimensions of the bass bar and soundpost are crucial to the overall tone and playability of the instrument, but the variations are very subtle and nuanced. In the guitar, on the other hand, bracing systems are still very much open to interpretation and wildly different systems are out there to be tried or invented.

An interesting spice in this process of fine tuning for optimal sound is that many of the desirable tone qualities are to be found at or near the point of structural collapse. Part of the art of successful instrument building is knowing how far you can safely push those limits. This is one area where the difference between master-built and production instruments really shows. Production instruments tend to be overbuilt, keeping the price down, trading sound quality for stability, while a master builder relies on experience to know how far each element of the instrument can be safely taken. This also shows up in the price that an established maker can command for her instruments; a maker who can show instruments that are twenty years old and still structurally sound and tonally vibrant inspires more confidence in a buyer.

Resonances

The enclosed cavity of the body produces an air resonance in the same way that a tone can be produced by blowing across the mouth of a bottle.  The frequency of this resonance is determined by the volume enclosed and the size and depth of the opening.  Any resonance in an instrument acts as both an amplifier and a tonal filter, promoting certain frequencies over others. The air resonance of the body is a large and important resonance but it is by no means the only one. The body of the instrument has its own large resonance and myriad smaller resonances coming from individual parts, or even from regions  those parts. To complicate things further, all of these resonances interact with each other, either sympathetically or antithetically. Thus the body size and shape affects the overall tone of the instrument in complex and subtle ways. For this reason  most instrument builders tend to follow the patterns of proven instruments making only small changes in order to tease out an understanding of the whole system. In general, larger bodied instruments tend to have a broader more satisfying bass end, smaller bodied instruments tend to be punchier and carry more,

Making the most of body resonances: two different approaches

When the violin is bowed, all parts of the body throughout the whole instrument become excited and resonate at different frequencies. This continues as long as energy is supplied by the bow. The violin is built to help rapidly spread the string vibrations throughout the instrument. In the guitar, on the other hand, the situation is radically different. The body receives one diminishing shot of energy from the plucked string and needs to spend that energy wisely.  It can’t afford the attempt to make the whole instrument resonate.  For this reason, the guitar is built more like a drum with an easily vibrating top and sides and back that tend to reflect back the vibrations in the top, containing and focusing them, in rather the way that spreading ripples in a bucket of water bounce off the sides and return to the center. So the guitar top is flat which is most the most easily vibrated shape, and the back and sides are heavy enough to bounce vibrations back into the top. The back and sides do vibrate and interact but not to the same extent as in a violin.


3. Neck and fingerboard

The obvious difference between the two instruments here is that guitars have a fretted fingerboard while the violin fingerboard is gloriously free of them.  The necessity of frets for a guitar goes back to the limited energy available in a plucked string. Without frets the small energy budget of the plucked string can easily be sopped up in the finger tip, very much muting the instrument,  Frets also facilitate playing complex, multi-note chords as the fingers don’t have to be placed as precisely as they do on a violin.  The disadvantages of frets start with the fact that you are dealing with a tempered scale so you may not be able to play as “in tune” as you would like.  Frets wear out strings quite quickly, luckily guitar strings are relatively inexpensive.  Frets are tricky to fit and maintain – in the opinion of this inexperienced guitar maker.

The necks on both guitars and violins are prone to warping and bending.  Broadly speaking, players prefer as little neck as possible, this is particularly true  in violins where only light pressure is needed to stop the string and the thumb provides very little opposing force. The violin neck almost only serves as a guide to hand position,  with the player subconsciously locating by feeling varying width and profile of the tapered neck. In guitars, on the other hand, much more pressure from the fingers, and an opposing pressure from the thumb, is needed to fret the notes, particularly in playing chords.  If the neck is too thin there is a tendency for the hand to cramp up.

The need to make necks as light as possible leads to problems with the necks bending.  In steel string guitars, adjustable “truss rods” have been used for many years to control the amount of bend in a neck. On nylon strung guitars, with necessarily wider necks and lower string tensions, these haven’t been as commonly used, though embedded carbon fiber stabilizing rods are becoming more standard. They are also being used more frequently in cellos which also have long necks and higher tension strings.  Makers in both trades enjoy promoting the possible tonal advantages of stiffer necks.


4. Tuning pegs

Guitars originally had the same lute style, tapered, friction pegs that violins still use. Flamenco guitars still use friction pegs.

Pegs on classical and steel-strung guitars are geared. This is particularly necessary with steel strings which are less elastic than nylon and more sensitive to change in length: a small pull on a steel string results in a relatively big change in pitch.  Gearing manages this sensitivity.  Nylon string tuners migrated so that their barrels are parallel with the face of the head stock, rather than perpendicular to it as they are in steel string guitars. This tucks them out of the way and provides support to both ends of the barrel, but it does make access a little more difficult for changing strings.

In the violin we find elements of classical, flamenco and steel string guitars. The skinny friction pegs are side mounted into a pegbox.  String access is even more restricted as the pegbox is not pierced through.  Modern cello strings have tensions and tuning sensitivity more similar to steel guitar strings and modern cellos are fitted with mechanical fine tuners in the tailpiece.  In recent years, a very good mechanical peg was developed that looks and weighs the same as an ebony peg but provides a 4:1 gear ratio, making tuning easier and preventing peg slipping.  I’ve used Pegheds on many cellos as well as on violins and violas.


Body construction and longevity

One other difference between violin and guitar construction methods has implications for the longevity of the instruments. The top and back plates of a violin are attached to the sides with simple butt joints. The top is glued on with a weak glue and this allows it to be removed relatively easily for repair by inserting a thin blade into the joint.  On guitars, on the other hand, the plate to edge seams are hidden by protective bindings which make a complex joint that can’t easily be opened.  Maintenance suffers.


Guitar versus violin body edge construction

 

 

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