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The Workshop


Wren Guitar Works

Here is a brief video tour of my current shop space with some footage showing some of the processes involved in hand-fabricating my instruments.




Wren Guitar Works

    Hi everyone! I'm going to be running a bit of a blog here that will be updated fairly regularly. Right off the start, let me apologize for my horrific grammatical atrocities. I'm no writer, but here goes!


Blog Index

Sept. 15/11 : Mark Kett Interview

Sept. 1/11 : Guitar Neck Considerations

April 9/11 : Soundholes and Sideports

Feb.27/11 : Anatomy of an Inlay

Dec.20/10 : Deflection Testing

Oct.14/10 : The Guitar Bridge

Aug.28/10 : Wood Terminology

July21/10 : Resonant Frequencies

June 14/10 : French Polish Finishes

April 27/10 : Why Handmade?


Wren Guitar Works


Building Blog Entry Sept.1/11

Guitar Neck Considerations


Nut Width, Material & Slot Spacing:  One of the first topics of discussion when mapping out a commissioned guitar, is the width of the neck at the nut.  As most veteran players have found, the fretting hand of the guitarist can detect quite small increments in variation of neck width, particularly at the nut.  In the steel string world, the two main nut widths are 1 11/16” and 1 ¾” (43mm and 44.5mm respectively). Popular wisdom dictates the narrower nut for flatpickers, and the wider nut for fingerstyle players … but of course this is a very subjective call that can’t be generalized.
Less talked about, is the spacing of the strings at the nut with regards to the positioning from the edge of the fingerboard, and in respect to each other. Some builders space the strings evenly from their centers, while others keep the spaces between the strings even … a very fine distinction at best.  More important is the distance from the high and low E strings to the edge of the fingerboard. Most builders allow around 3.5mm space on the treble side and approximately 3mm on the bass side. Why the discrepancy? The majority of steel string players need a bit more room on the treble side of the neck so that they don’t pull the high E off the edge of the fingerboard when tackling some of the more difficult “grips” or chord inversions.

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Heel and Peghead Joints:   Long before any of the player’s specifications are considered, the luthier has to decide whether to produce a one piece, two-piece or three-piece neck. This decision, in my mind, is a function of wood conservation rather than a structural consideration. One-piece necks are certainly easier to make (less joinery and no problems with colour matching) but waste a substantial amount of wood.  Since most steel string guitar necks are fabricated utilizing increasingly rare mahogany from South America, I’ve decided to stop making one piece necks and have opted for the two piece alternative (with a glued on heel block) that involves very little waste.

Truss Rod:  The internal structural reinforcement in steel string guitar necks can be roughly divided into non-adjustable and adjustable.  In the ‘70s and ‘80s many builders, including myself, utilized square tubular non-adjustable truss rods similar to what C.F. Martin used for decades. This type of reinforcement added significant rigidity to the neck and also offered resistance against twisting, if installed correctly. The disadvantage was, of course, that it was not adjustable to take into consideration different string gauges and tunings.
Today, most builders have switched to an adjustable truss rod. I am currently using a double action truss rod that requires only a ¼” wide slot to be machined in the neck and offers the maximum amount of flexibility with regards to adjusting neck bow. If the guitar owner is touring South America or South East Asia (where the average humidity can exceed 80%) or Scandinavia or Northern Canada in the winter months (where the average humidity is lower than the Sahara Desert) … the truss rod can be adjusted to give the neck the perfect relief.

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Neck profile/contour:  There is no standardization when it comes to steel string guitar neck profiles.  Walk in to your local guitar store and pick up a dozen different guitars and you will probably notice a twelve different neck shapes … everything from low profile “fast” necks, to large “boat neck” retro style soft-V contours.  Again, this is a subjective call by the player. I typically carve a medium elliptical profile and then “roll” the shoulders of the fingerboard to create an overall pleasing neck shape that will not fatigue your fretting hand after hours of playing. I also offer the option of replicating the neck of an instrument that you currently own and love.

Fingerboard Radius:  Back in the ‘70s, I planed a very flat radius into my fingerboards, but my taste, as well as the current trend in general, has dictated more of a convex curve on the top of the playing surface. Most players feel that (up to a point) a nicely radiused fingerboard adds a degree of ease-of-playability and an element of comfort to the instrument.

Neck binding:  Historically, with steel string guitars, high-end instruments had some kind of binding on the fingerboard, both as a structural and cosmetic consideration. I personally bind my ebony fingerboards with two thin strips of matching ebony … not because I have anything against the more traditional plastic binding (I quite like some plastics as binding material), but because I like the simple understated elegant aesthetic. A bound fingerboard also can minimize the feel of the fret ends if the instrument is subjected temporarily to a dry atmosphere.

Neck-to-Body Joint:  Historically, the neck on steel string flattop guitars has been attached to the body by means of a glued dovetail joint.  The last couple of decades have also seen the proliferation of the mortise and tenon utilizing bolts instead of glue for ease of removal.  Since the late seventies, I’ve been using what I call a “pinned dovetail” joint, i.e. a dry dovetail joint (no glue) that provides a secure mechanical joint … and a bolt for added insurance and ease of removeablility.

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Position Markers: As opposed to the violin and classical guitar world (how do those people do it?), some kind of fingerboard position markers are typical in the steel string sphere.  Most players that I come in to contact with agree that a side dot that is easily visible on stage is more than adequate. I often get requests to place my “Wren” inlay at the 5th fret, to further differentiate my work from the other builders out there, and to provide a visual position “anchor” to the front of the fingerboard.

Neck and Fingerboard Materials:  In my mind, the decision to use ebony for a fingerboard material is dictated by the fact that I use the mass of an ebony bridge to get the sound I’m after … and I like to match the fingerboard material to the bridge.  This is sort of backwards logic, I know.  I like the sound that an ebony bridge imparts to my guitars … I like the fingerboard to match the bridge … therefore I use an ebony fingerboard to match the bridge.  Ebony also provides a nice dense fingerboard material that resists wear and contributes to the classically understated overall aesthetic that I strive for.
In the past and currently, I use South American or “tropical” mahogany for the neck. This species is stable with regards to twisting and humidity fluctuations and has the right mass for the physical balance I like in a guitar.

Fret height and profile: I use a medium sized “6230” fretwire manufactured by Dunlop on my flattop acoustic guitars. Some electric guitarists prefer a higher wire that helps facilitate the string bending and lateral vibrato techniques used with slinky strings and amplified instruments … but I prefer the medium height wire that facilitates the slurs and slides often utilized in flattop playing.
I spend a good deal of time pre-bending, seating, dressing and recrowning my frets.  In my opinion, simply seating the frets does not get the most accurate results, even on a brand new guitar.  Frets that have been professionally dressed, recrowned and polished inevitably provide the most accurate playing surface.
I also pay special attention to the treatment of the fret ends … rounding them enough to feel smooth to the players fretting hand, but not so bevelled to allow the string to slip off the fingerboard edge.

Scale Length:  As most players know, scale length is basically the distance between the nut and saddle i.e. the overall “speaking length” of the string.  This distance not only determines the fret spacing, but also helps determine how much tension is exerted on the top and the “stiffness” or feel of the actual strings.
For me, the main determining factor in choosing a scale length, are the sonic implications (as opposed to ease of playability offered by frets that are very slightly closer together).  I use what has come to be termed as the “long” or “normal” scale length, namely 25.34 inches, on my Concert and Remuda models.  I like the initial aggressive snap it gives to the attack transient … especially on the lower strings. This longer scale also tends to give these models slightly more power and projection.  On my small NL-1 model I am going for a funkier midrange-heavy overall tonality with a softer point to the attack transient and a slightly more intimate voice … so I use the shorter 24.9 inch scale.

Peghead Angle:  Some builders theorize that steeper peghead pitches provide added string tension but in practical terms, it takes quite a radical change in angle for it to be perceptible to the player.  In my opinion, a guitar neck is designed properly as long as all of the strings have an adequate (but not extreme) break-over angle on the nut.

Sonic Considerations: Most of the chatter you hear about guitar necks involves the physical issues I’ve raised above, but there are certainly sonic considerations involved in fabricating a successful neck too. As with all of the components of the instrument, the neck has its own resonant frequency that can work with and enhance the other parts of the instrument. The overall mass of the neck/truss rod/tuning machine assembly has an influence on how much energy from the box is absorbed or reflected by the neck … ultimately affecting the power, projection and sustain of the instrument.

In any event, hopefully I’ve given you some food for thought with regards to the neck of the instrument.  Until next time … keep your stick on the ice and keep on picking!

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Building Blog Entry April 9/11

Soundholes and Sideports

    One of the things that a luthier thinks a lot about when designing a guitar is the soundhole … the size, its position, the reinforcement around it, how it affects the main air resonance of the instrument and ultimately, how efficiently it allows the internal sound waves to project from the box.

   Over the years there have been a number of theories put forth explaining how the internal sound waves contribute to the overall sound of the guitar.  From a physics standpoint, it has been suggested that the many and complex internal sound waves do not tend to interact with each other from a phase relationship standpoint … rather, they combine at the soundbox’s aperture and resolve any phase relationship issues at or near the soundhole, contributing a very positive sonic element to the guitar. The fact that the internal sound waves of a guitar box contribute significantly to its overall response can easily be demonstrated simply by taping a piece of cardboard over the soundhole.  The result is a noticeable attenuation in power and tonal nuance.  Even partially blocking a guitar’s soundhole can have a detrimental affect on the overall sound quality, which illustrates why the size of this aperture is so critical.

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   For starters, the size of a guitar box’s soundhole is one of the predominant factors that determines the main air or Helmholtz resonance.  The general rule of thumb is, for a box with a given internal air volume … the larger the soundhole (or soundholes plural), the higher the air resonance.  Some builders maintain that a higher main air resonance results in a guitar that more efficiently produces higher frequencies.  This makes sense on some levels, but I’ve not found this to be the case in my own instruments.  My instruments with a lower Helmholtz resonance have just as prominent a treble register as my guitars tuned to a higher air note. I do however, like to control the note that my main air resonance falls on, for two reasons.   I find that there is an optimum range of aperture sizes that maximize the sound-producing efficiency of the guitar box.  I also have specific resonant frequency notes that I have found from experience make the most evenly responsive instruments.

    When I was initially building guitars in the seventies and early eighties, the only aperture in the guitar box was of course the front soundhole. Since that time, side ports have been quite popular in high end instruments. One of the reasons I am so enthusiastic about side ports, is that they allow the builder much more flexibility in achieving different target notes for the main air resonance (because they, like the front soundhole, enlarge the overall aperture of the box). For instance, comparing a guitar with no side port and an instrument with multiple side ports can reveal a difference in the main air resonance of two full tones. That's a lot more flexibility than simply making your front soundhole 3 3/4" or 4" in diameter! Since all of the main resonances on the guitar are "coupled" or affected by each other, this simply opens up a whole new world of possibilities with regards to tuning the instruments various components!

    I believe the first builder to use a sideport on a guitar was Sergei de Jonge. At first I was a bit skeptical about the concept. Intuitively, you would think that if there is more sound coming up at the player then there has to be less sound projecting forward. To test this theory, we would have someone sit down and play a side-ported guitar, with another person behind the player reaching over and closing the port, then opening it, then closing it etcetera. We would have a number of "auditors" out front who would listen for a period of time from a vantage point of six feet in front of the instrument, then 12 feet, twenty feet and so on. It was amazing to me that by the time I got around eight to twelve feet back, it was a virtually imperceptible volume and tone difference between the closed and open side port from in front of the instrument ... and yet the player hears a dramatic improvement in response ... a definite "win-win" situation!

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Building Blog Entry Feb 27/11

Anatomy of an Inlay

    To a guitar player, the words "mother-of-pearl" or "abalone" conjures up images of Martin D-45s, PRS "dragon" inlays, or simple Gibson crown fingerboard markers. Inlay designs can consist of simple geometric shapes, or they can be almost surrealistic scenes from nature and life. Most of us don't give much thought to the origins of this complex art form, or the source of materials utilized by the luthier or inlay artist creating the patterns adorning our prized instruments.

   There is often some confusion between the terms "inlay" and "marquetry". The later is the art of making patterns with wood veneers and gluing them onto a surface, and historically is more prevalent in western society, especially in furniture making. Inlay, on the other hand, has its origins in the Far East and utilizes a variety of shells and materials into a cavity and then sanded flush with the surface. 

    The earliest known inlaid object is a Mesopotamian bowl (3000 BC) with shell pieces embedded into it. Other documented examples include an inlaid coffin from the Yin Dynasty (1300 BC) and many examples from the Shoso-in Repository are still in pristine condition 1200 years later and held by experts to be unexcelled in design and execution. Asians historically have developed two distinct methods of inlay: thick-shelled 'atsu-gai' inlaid into the surface of wood, or thin shelled 'usu-gai', only 5/1000" thick, fixed directly onto the lacquer under coat.  Contemporary North American inlay is thick shelled and done mainly on musical instruments, although it is starting to be seen now on pool cues, furniture and objets d'art. Some modern inlay artists utilize techniques and tools similar to the ancient eastern masters, while others have modernized with the use of computer controlled vertical mills.

    Different cuts, sections, grain patterns, chemical treatments and refractive alignments of a huge variety of shell types, wood species (natural and dyed), bone, ivory, metals, stones, gems and vegetable nuts are all often put to use in modern inlay. Some of these materials, such as abalone shell, have irregular characteristics that change as you sand them. There are thousands of shell varieties but commercial slabbing of shell for inlay is limited to a narrow range of species because of shell size, curvature and colour. Often brilliant patterns on the surface are only thousandths of an inch thick and when sanded, become less attractive. Abalone and mother-of-pearl shells are fairly abundant in the Pacific Basin, but supplies are getting scarce due to otter depredation and 'killer tides'. 

    Abalone shells are gastropods (literally 'stomach foot') and have only one arched shell over their body to protect them from predators and a sucker-like foot that attaches to rocks on the ocean floor. There are four main types of abalone (genus Haliotis) suitable for inlay. The largest of the four is Red abalone, which has been known to grow as big as 10-12" across (more commonly 9"). The largest slabs available for inlay work are from this species, although many are spoiled by parasitic wormholes, stress cracks or disease. Green abalone is smaller and more curved than red, producing smaller pieces that are extremely vibrant in colour and in some cases, if cut near the outer 'bark', very rippled in appearance. Black abalone shell is fairly rare and generally has the most depth and pearlescence. The most valuable shell comes from New Zealand and is called Paua (pow-a). Paua shell is characterized by it's electric blue colour with veins of green and rose running though it.

    Mother-of-pearl (genus Pinctada) is a bivalve with 2 luminous white shells facing each other and can grow up to 12" in width. The shell is 'mitten' shaped with the 'thumb' as the thickest part (historically used for knife handles and pistol grips!). The 'nacre' closest to the bark exterior is sometimes a beautiful rippling golden hue, especially in pearl harvested near the Philippines. Black mother-of-pearl is available in small quantities from Tahiti and displays iridescent blacks, purples and browns with streaks and ripples throughout. 

   Years ago I used to do a lot of custom inlay work on my instruments, but these days I keep it to a minimum. Here's a few shots of my "Wren" inlay that I sometimes put at the 5th fret position on my Concert Model that utilizes abalone and mother-of-pearl shell, turquoise stone, brass and copper.

Wren Guitar Wren Guitar Wren Guitar Wren Guitar Wren Guitar Wren Guitar Wren Guitar

  The first shot is just showing some of the tools and raw materials, namely a jeweler's coping saw and a Dremel (miniature router). The second image shows drawings of the different design elements of the bird glued on to their respective inlay materials, while the third slide is me in my optavisor cutting the pieces out. The fourth image shows all of the pieces lightly glued together and the fifth file shows the fingerboard with the inlay outline scribed on. In photo number six, the fingerboard has been carefully dremelled out to accept the inlay and of course the final image is the completed inlay.

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Mark Kett Inlay

   Well folks, after doing all of my own inlay work for a couple of hundred guitars, I've decided to enlist the help of my buddy Mark Kett to cut my "Wren" inlays as well as some possible custom work. Most of you have had Mark on your radar for years, but just in case, I've done a brief interview with him (see below). My introduction to Mark was years ago when I was partners in the Twelfth Fret Pro Shop. Mark would come in occassionally to show us his latest guitar and it would completely knock me on my keester ... MAN this guy is talented. Even though Mark is a whiz with programing CNC machines, all of his inlay work is cut and inlaid by hand and is it EVER accurate ... this guy is scary! You know when Linda Manzer hires him to inlay all of her Manzer/Metheny guitars that this guy is good!

Wren Guitars Mark Kett

DW:   So Mark, tell us how you got started with guitar building.

MK:   My interest in guitar making stemmed from my desire for a "proper" guitar to play, and the lack of money to buy one. I started getting into the idea when I was sixteen (1996) and frequenting guitar builder Oskar Graf's Blue Skies Music Festival. The idea found its way a year later when I built two guitars during a nine week course with Sergei de Jonge in Oshawa Ontario.

DW:   A lot of great builders got their start at that course! I understand that the course led to a full apprenticeship with Mr. de Jonge? Can you tell us a bit about that?

MK:   Yeah. After my course, I expressed my desire to continue working with Sergei. I was addicted. He agreed after much poking, prodding and not so subtle hints. I felt that he was a bit reluctant, as the de Jonge compound is always pretty crowded. It was not a full apprenticeship by the standard definition. I worked for room and board for about a year, then off and on for the next three years. At first, I was making parts, building boxes (unbound guitar bodies), sanding lacquered guitars, and sanding more lacquered guitars! I graduated, one process at a time, until I was building guitars start to finish. I can't present words that describe how much my time with the de Jonge family means to me.

DW:   Your career's come a long way since those days. Can you bring us up to date with what you're up to these days?

MK:   Currently I'm focused on my inlay work. I love making guitars and will continue to build. For now though, inlay is where I'm at. Recently I've been fortunate enough to create custom inlays for some amazing guitar makers: Jeff Bamburg, Dana Bourgeois, Sergei de Jonge, Greg Furan, Michel Pellerin, Linda Manzer, Alastair Miller, Pete Swanson (Dagmar), David Wren and Joseph Yanuziello. My work ranges from simple fret markers to fully inlayed headstocks and fingerboards. On my bench right now are a few 100 hour inlay projects. This is roughly the time it takes me to build and complete three guitars. I'm also working on creating a new pallet of inlay materials that will suit my evolving needs. They will debute at the 2012 Montreal Guitar Show.

Link to Mark Kett Guitars

Link to Mark Kett Inlay

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Building Blog Entry Dec. 20/10

Deflection Testing

  Stiffness as it relates to the guitar builder, is basically how much a component deflects or moves under a set weight or load.  Deflection testing the components of a guitar to determine stiffness, can be a very powerful tool in the pursuit of making the instrument energy efficient with regards to converting more string energy into sound.

   In a way, the top of your guitar is much like a trampoline … if the surface is too stiff and the person jumping very light, you don’t get much bounce.  Conversely, if the trampoline is tensioned loosely and a heavy person jumps,  you still don’t experience the optimum bounce.  In this analogy, the weight of the person is the gauge and pull of the strings … a known quantity.  One of the jobs of the luthier, is to create a trampoline that is tensioned perfectly for the weight of the person jumping … in other words, create a top that has the perfect stiffness to maximize the energy transfer of a particular gauge of strings.

   Anyone who has built more than a few instruments, has experienced the fact that very little reduction in top thickness or a small difference in the amount of strut scalloping, can dramatically affect the stiffness of the component involved and therefore affect the response and tone of the guitar.  This is a phenomenon that is realized intuitively over decades of guitar building. The more guitars I build, the more this instinct of optimum stiffness is fine tuned.

   One of the basic principals of physics that helps quantify deflection properties, is called the Cube Rule of Stiffness. This formula states that the stiffness of a structural component is a linear function of it’s width, but a cubed function of its height or thickness (say what?).  All this means is that if you double the width of a strut, the stiffness doubles, but if you double the height of the strut, the stiffness goes WAY up.   As an example; when a top brace starts out with a stiffness coefficient of 2, if you double its width, it doubles in stiffness to a coefficient of 4 (linear function) … BUT if you double the height of the same top brace,  its stiffness coefficient is cubed i.e. goes up to 2x2x2 = 8 (cubed function)!

   To look at it from another angle ... steel string guitar tops are typically from 2.6mm to 3.0mm thick, the differential between these two extremeties being a mere .4 millimeters.  All things being equal, the stiffness coefficient between these two dimensions is quite a bit more dramatic than some would initially think.  Doing the math,  the thinner top has a stiffness coefficient of 2.6 x 2.6 x 2.6 = 17.57 while the thicker top works out to be 3x3x3 = 27 … an increase in stiffness of over 150%!

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   On reading over what I’ve written so far, it almost seems like I’m trying to indicate that the point of this exercise is to find the dimensions of the top thickness and bracing that gives the stiffness that results in a responsive instrument … and for many medium to large production builders, this may be the way they craft their instruments. They find a good combination of specifications, such as a 2.8mm thick top with struts scalloped down to 11mm, and make all of their guitars to these dimensions.   My point though is … every top and brace is made from wood which can and does vary quite dramatically in stiffness characteristics. If you made them all the same dimension, they would react quite differently to the load of the strings.  The advantage that an individual luthier has over the big boys, is that we can individually deflection test each component to fine tune its dimensions to get the exact right deflection for the load that will be applied … producing consistently maximized energy transfers from the strings to the top … to your ears.  The key is to keep meticulous notes on all of the instruments you build and to correlate this data with the final sound of the instrument.

   In practical terms, we are never dealing with two pieces of wood that exhibit the same stiffness characteristics, which makes deflection testing even more critical to achieving consistently exceptional response from a musical instrument.  Having said this, I’d also like to emphasize that determining the optimum stiffness of any component in my guitars is just the starting point to fine tuning the sonic characteristics of my sound, not the final destination.  I always deflection test in tandem with the actual tap tuning of the top, air and back of the guitar (see my blog on resonant frequencies).

Wren Guitar

    To conclude, I’d like to emphasize that even though in my mind, I often temporarily differentiate between the act of controlling the deflection of any of the components of the guitar and the corresponding resonant frequency of the same component (for instance the deflection and resonant frequency of the top) … these two specifications are inseparable ... and both crucial to controlling the response of the finished instrument.

   Until my next blog … keep on picking!

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Building Blog Entry October 14/10

The Guitar Bridge

   The bridge of a guitar is often thought of as simply the place that you anchor the strings and adjust the string height, which of course is part of the story.  To my mind though, the bridge is an extremely important tone bar … which just happens to be positioned on the outside of the instrument, rather than the inside.  After the musician’s fingers, the bridge and saddle are really where all tone production starts.  This often misunderstood component is certainly where all of the modes of vibration of the instrument originate.

   The modes of vibration of a guitar bridge and top are extremely complex to say the least.  For our purposes today though, I will simplify the bridge/top modes to three main movements:  a pumping up and down (technically termed the “Monopole” mode);  an almost teeter totter side-to-side movement (Cross Dipole) and a torqueing movement in line with the strings (Long Dipole). As a guitar builder, getting a handle on how to maximize or subdue specific vibrational modes will help you to shape your signature sound.

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    For instance, on my instruments, the Monopole mode affects bass response and overall fullness of the guitar. This mode can be enhanced by thinning the perimeter of the top around the vibrating area,  reducing the radius of the top (or making it flat),  shifting the X-brace and/or scalloping braces immediately below bridge. The Cross Dipole mode seems to affect subtle tone nuances as well as the complexity and prominence of the overtone series and can be enhanced by thinning top only on treble and bass edge extremeties,  shortening bridge length and/or scalloping the braces beneath the bridge. The projection on my guitars is partially dependant on the Long Dipole mode, which can be enhanced by narrowing the width of the bridge, increasing the break angle at the saddle and/or shifting the X-brace.

   There are many factors that come into play when designing a bridge for a specific model of guitar, a few of which are the actual shape,  the mass,  the saddle slot depth and the break angle of the string.

   Determining the proper depth of the saddle slot is quite straightforward and is determined by the initial height of the saddle above the surface of the bridge i.e. the higher the saddle exposure,  the deeper it should sit within the bridge.

   The actual footprint or physical outline of the bridge has a very noticeable affect on the tone of a guitar. A narrow pyramid or simple rectangular bridge will trigger a stronger Long Dipole vibration, while a belly bridge will slightly attenuate this same mode. In the case of my asymmetrical "frequency compensated" bridge shape that I use on the Concert model ... the narrow treble side and wide bass side seems to offer a wider range of frequency lengths on the top as well as a more complex and layered tonality to the final sound.

   The term “downbearing” or “break angle” is used to describe the angle at which the strings hit the saddle as they exit their pin-holes in the bridge.  A low saddle that barely protrudes from the bridge is said to have little downbearing or a low break angle. In extreme cases this can lessen the leverage the strings have on the top and impact the projection of the sound negatively.  A higher saddle will provide good downbearing and often produces a slightly more powerful sounding instrument from a projection standpoint … mainly because the higher saddle promotes  the torqueing mode of vibration (Long Dipole mode)

   An experienced luthier can alter the length, footprint and mass of the bridge to be appropriate for the instrument it is going on (as opposed to putting the same belly bridge on all models, as some production shops do).  It often does not make sense to put the same bridge on a smaller instrument that you would put on a large guitar. For instance, a bridge that is disproportionately long in relationship to the width of a specific guitar body can often produce a very one dimensional sound with low volume and an inhibited overall response.

   The bridge can’t be fully understood in isolation though.  You can only really get a good grasp of this component’s function by thinking of it as an integral part of the bridge/top/tone-bar assembly. In this respect, the bridge’s position in relation to the main vibrating area of the top is important as a tone shaping element.  Some builders define the predominant vibrating area of the top as everything below the large transverse brace above the soundhole.  My experience with my own models tells me that the most important tone shaping real estate on my tops are the three lowest quadrants defined by the X-brace (i.e. everything except the top quadrant in the soundhole/fingerboard extension area).  As a matter of fact, I find that if the area around the soundhole vibrates too much, I hear enharmonic or dissonant harmonics in the overtone series that are strident and unmusical sounding.

   Defining your top’s main vibrational area becomes important when designing a new guitar with regards to determining where the bridge should sit.  In very general terms, the closer your bridge is to the centre of your area of vibration,  the more prominent your note fundamental will be and the more your bass/mid/treble balance will tip toward the bass end of the spectrum.  Conversely, as your bridge gets off centre (usually towards the soundhole),  your attack transient will tighten up and get more clearly defined,  overall crispness is enhanced and the balance will shift toward the upper midrange and treble side of things.

   I guess what I'm really trying to say is, that it is very seldom that I think of the bridge as a component on its own . I almost always think in terms of the bridge/top/tone-bar assembly ... and always ... I think of it as an extremely important part of the "tone drivetrain" of the guitar. Also ... there are many more modes of vibration associated with the guitar bridge and top assembly ... so again, this article is meant to be just a brief general overview of this particular topic.

   As soon as time permits, I'll blog-on about another random topic that comes to mind ... so until then ... happy picking!

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Building Blog Entry August 28/10

Wood Terminology

    In this month’s blog entry I’d like to talk a little bit about wood in general.  A lot of this you will probably already know, but I’d like to explain some of the tonewood terminology in reference to the actual log … terms like quartersawn, medullary ray, grain count,  spring growth, strength-to-weight ratio, grain runout etc.

    Just like we all learned back in school, trees grow by adding cells in the outermost part of the trunk known as the cambium layer.  The outer part of the trunk is also where the sapwood is found.  This is where a lot of the action happens with regards to the transfer of nutrients and the actual tree growth.  We of course see nice bookmatched traces of light coloured sapwood on some back & sides sets … notably Cocobolo and Ziricote. 

     When I think of the structure of wood, I envision a collection of drinking-straw-like tubes made up largely of cellulose.  When the wood is freshly cut, these tubes are filled with moisture and sap.  Before an instrument builder can use this wood, it has to be dried (ie. resawn to rough dimension and fanned to slowly remove the water content) and seasoned (ie. left for a period of time until the sap crystallizes on the inside walls of the cells).  This is why guitar builders always have a few years worth of wood on hand at all times … so that it can be drying and seasoning. Both of these processes also reduce the weight of the wood and increase its strength.

     We have all noticed the alternating narrow dark lines and wider light lines on a guitar top. The wider light lines represent the initial fast growth of the tree in the spring and early summer and are made up of fairly large diameter cells.  When seasoned and dry, these cells become very light and thin-walled ... an almost styrafoam-like material.

    The narrow dark lines in softwoods represent the slower growth period of late summer and early fall.  These small diameter cells have thick cellulose cell walls which gives the wood the majority of its strength and stiffness. 

    Strength-to-weight ratio is extremely important when choosing a top plate.  The wood has to be strong enough to withstand the torque exerted by the constant pull of the strings, but light enough to be musically responsive.  The light spring growth in conjunction with the strong fall growth gives spruce, cedar and redwood the ideal strength-to-weight ratio for a luthier’s purposes.

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    Again referring to a guitar top, we often notice a change in the grain width or grain count ... a gradual change from wider grain to a tighter grain pattern.  This phenomenon is explained by the fact that as the tree gets larger over the decades, the annual growth cells are spread over a larger diameter trunk and are therefore closer together as you approach the outer extremities of the trunk. Grain count is often gauged in “grains per inch”.  Some used to equate a higher grain count (ie. approx 23 grains per inch or higher) with a better quality top, but this notion has largely been discredited.

   We often hear a cut of wood referred to as either quartersawn or flatsawn.  Most of the wood utilized in the construction of a guitar is quartersawn, which is mainly a consideration dictated by stability with regards to shrinkage and expansion.  If you consider that wood shrinks considerably more along its anual growth ring than across the growth ring, you can see how a top or back would be more dimensionally stable in its quartersawn form. In some cases, a quartersawn grain orientation can contribute to cross grain stiffness as well.

   We hear the term “medullary rays”, “quarter flower” or even “silking” used in reference to guitar tops. These rays radiate at right angles to the grain and are the structures that help tie the growth rings together and add additional structural stability to wood.  Looking at my illustration of the trunk of a tree, you can see how a strong showing of medullary rays would indicate that the wood was perfectly quartersawn.  A good display of silking can also be an indicator of enhanced cross grain stiffness in a top.

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   The occurrence of “grain runout” is quite common, especially in the European varieties of spruce. This phenomenon is explained by the fact that trees, like small plants, follow the sun on its travel across the sky. Of course a large tree doesn’t mimic the dramatic daily movement of a small plant, but the fact that the trunk of a tree often exhibits a lengthwise twisting of its grain in the direction of the sun’s travel proves that even large trees seek the sun and are affected by its movement. The fact that trees north of the equator twist clockwise and those south of the equator twist counter clockwise is further proof that runout is caused by this phenominon.

   In severe cases, this lengthwise twisting of the grain, or runout, not only creates a different light refraction when viewing the guitar (ie. one half of the top looks darker than the other), but it can also effect the top stiffness and the overall guitar’s response.

   An interesting side note is the fact that tropical mahogany trees that grow in proximity to the equator exhibit alternating clockwise and counter clockwise grain twisting as the earth’s axis allows the sun to seasonally cross the equator … making this one of the most structurally stable woods for guitar necks!

    Well … that just about ends my primer about wood terminology as it applies to guitars.  Keep on picking everyone!

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Building Blog Entry: July21/10:

Resonant Frequencies

   Every object has a resonant frequency … several in fact.  So what is a resonant frequency and why do you hear this phrase in connection with musical instruments?  Simply put, everything …your computer desk,  the building you live in and yes, your guitar, has different frequencies that it will resonate in sympathy with. In the case of a guitar, the main resonant frequencies that we are dealing with both as a builder and ultimately as a player, are the resonant frequencies of the air inside the box, the top tap tone, and the back tap tone.

    The air has a resonant frequency?  Prove it to yourself.  Blow over the mouth of an empty glass juice bottle.  You hear a note right?  Put a couple of inches of water in the bottom of the bottle (reducing the air volume) and you get a higher note.  The air inside a guitar box is subject to the same laws of physics and similarly, varies its resonant frequency with its air volume i.e. the larger the box … the larger the air volume … the lower the resonant frequency note.

    The tap tone or main resonant frequency of the guitar top can be determined in many different ways, the simplest being the “voice excite” method.  As a matter of fact, all of the resonant frequencies of the instrument can be determined in this fashion.  Lay your guitar horizontally with the top facing up and something soft at either end that will suspend the instrument off the table surface. Sing into the soundhole area, from the lowest note you can manage, to the highest.  As you are doing this, lay a hand gently on the top and under the back.  You will feel the instrument vibrate noticeably more aggressively at the resonant frequency points as you sweep your voice from lower to higher registers.  You are experiencing the many different resonant frequencies of your guitar.

   There are certainly much more scientific ways of determining the exact resonant frequencies of an instrument. When I worked with Jean Larrivee in the ‘70s, we used to get invited down to the R&D department of Martin Guitars by Don Thomson.  He would ask Jean to bring a guitar with him so that they could analyze it and track its response. They would do this by hanging the guitar vertically by its tuners, attaching the voice coil of a speaker to the bridge with a dab of hot wax, then prop a magnet within the voice coil. With the use of a frequency generator, Don would then drive the top of the guitar like it was a speaker … through a wide range of frequencies (my memory is from around 40 Hz to approx. 20,000 Hz).  Around ten feet from the instrument he positioned a microphone that was attached to a Brush Chart Recorder (think lie detector test and you’ll get the idea of the look of this device and the graph of resonant frequencies that it produced).  Myself? For years I used my Conn Strobotuner with an external mic … and had a system of tapping that tells me the frequencies that I like to keep track of. These days I use software that more accurately tracks all of the resonant frequencies in my guitars.


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   So what?  What is the big deal about resonant frequencies and why all of the chatter about them with regards to guitars? Why determine where they are in the first place? Because when you play the note on your guitar that corresponds to the frequency of one of its components, be it the top, back or air … the note you have played is noticeably affected. You can hear it.  That is what the big deal is.

    There is not unanimity with regards to how the resonant frequency of a guitar component affects the corresponding note when ultimately played by the musician. Some builders maintain for instance, that when the note corresponding to the main air resonance (typically on the low E string between low F and A) is plucked, it is reinforced and is an overall “stronger” note.  From a strictly scientific viewpoint this is true.  When you put a decibel meter in front of the instrument, it often registers louder on these notes. But from a musician’s standpoint, the note is often heard as weak. How are these two perceptions reconciled?  In my opinion, the note in question begins with an aggressive attack transient and a brief stronger-than-normal note fundamental, but then the overtone series initiates prematurely, losing the note fundamental within the context of a chord.  When one note fundamental does not sustain nearly as long as its harmonic counterparts within a chord, the player often perceives it as a slightly weaker note. This is how I hear it as well.    

    Whether the builder or manufacturer of a musical instrument pays attention to these resonant frequencies or not, they are inherent in every guitar and the player can hear the slightly uneven response even if he/she doesn’t know the cause. One of the jobs of a guitar builder is to produce as even a response as possible up and down the fingerboard. When a guitar is handmade, the opportunity arises for the luthier to manipulate these resonant frequencies or at the very least, keep track of the resonant frequencies of their instruments and make note of which ones produce the most even and musical response in their instruments.

    When I'm building a guitar, I keep track and manipulate the deflection characteristics and weight of the top and back throughout the build process ... before strutting, after strutting, after shaving the struts, etc. etc. I also at every stage tune the resonant frequencies of the top, back and air to maximize response characteristics and achieve an even response throughout the frequency range.

    This is of course a very oversimplified explanation of resonant frequencies and their effect on guitars.  Many other factors are involved … soundhole and sideport size,  the coupling effect between two or more different resonant frequencies,  the effect of the instrument’s sonic maturing on evenness of response etc. etc. But for our immediate purposes, it serves as food for thought … and a launching point for further blogs!

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Building Blog Entry: June 14/10:

French Polish Finishes

   The use of shellac dates back 3000 years and more,  although its widespread prominence as a wood finish seems to be between 1550 and 1650 when it went from a relative rarity utilized on exotic pieces only,  to being referred to in standard texts of that time period.  Certainly,  it almost completely replaced oil and wax furniture finishes in the 1800’s,  and was the dominant wood finish until the introduction of nitrocellulose lacquer in the 1920’s and 1930’s.

Shellac is actually a resin secreted by the lac bug and deposited on trees,  predominantly in India. The insects suck the sap of the tree and then constantly secrete this sticky substance,  the colour of which is determined by the patina of the sap of the particular tree they are parasitic upon.  This “stick-lac” secreted resin is then scraped from the trees,  deposited in canvas tubes and heated over fires.  In this way,  the substance liquefies and is filtered onto flat sheets, where it is broken up into smaller pieces called flakes. These flakes are eventually dissolved in alcohol to make liquid shellac.

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Shellac flakes and a fad (stored in airtight container)

   The viscosity or strength of a liquid shellac mixture is measured in the unit “pound cut”.  For example,  a one pound cut is created by dissolving one pound of shellac flakes in a gallon of alcohol,  with a two pound cut being two pounds of shellac in a gallon of alcohol etc.

The term “French Polish” refers not to an actual finishing material,  but rather to a specific way to apply a thin and refined shellac finish to an object.  Many individual luthiers as well as larger manufacturers such as C.F. Martin,  French Polished their instruments from the mid 1800’s until the 1920’s.  Many woodworkers switched to a sprayed nitrocellulose lacquer in the late 1920’s as it was much less labor intensive.

The actual process of applying shellac using the French Polish method involves loading very small amounts of liquid shellac, alcohol and a drop of oil,  on to a “fad” or “muneca”.  The fad is simply a wad of absorbent cotton material wrapped in a linen cloth.  The inner cotton (or wool) material is charged with liquid shellac which is then left in the open air to attain a gel-like consistency.  At this point, when fresh shellac and alcohol is applied to the outside of the fad,  it teases the thicker viscosity shellac from the inner cotton,  on to your instrument.  Sounds easy right?  Wrong!  To attain the correct balance of shellac, alcohol and oil can be very elusive … and if you don’t get the mixture right … it starts to remove any finish that you were lucky enough to apply in the first place!

So why has their been a resurgence in lutherie of the French Polish method of applying  shellac?  It certainly isn't because of its ease of application.  It is definitely one of the most time consuming and skill intensive methods of applying a finish that there is,  but the advantages it offers make the effort well worthwhile. 

To begin with,  there is simply no other finish that feels as great as shellac in my opinion.  It’s hard to describe,  but it definitely possesses a very non-plastic feel,  and its extremely low friction coefficient makes it perfect for a fast-feeling neck … no sticky gummy drag on your hand as you negotiate the fingerboard. This is not the main reason for expending all of this effort though. It all comes down to sound.

   If you are trying to squeeze every last drop of response out of a guitar, a thin finish is an absolute must. The weight and damping factor involved in some thick factory finishes that can be up to .010" thick, is quite substantial. By applying shellac using the French Polish method, you don't have to build up extra finish to account for all of the abrasive processes involved in some spray finisher's regime (ie. level sanding, wet sanding, buffing with different grades of abrasive compounds). There is minimal sanding involved with French Polishing, and to achieve the final gloss, there is minimal if any polishing compounds involved. You can lay on the finish as thin as you want without having to build up some "insurance" thickness to avoid sanding or buffing through.

   A thin French Polish applied finish has some definite drawbacks as well. It is more prone to marking and scratching than modern polymers. It eventually "telegraphs" every glue joint through the finish. It is also not the finish for players with an acidic body chemistry. If you kill strings rapidly, it is probably an indication that you have an acidic body chemistry and should not be ordering an instrument with a shellac finish. I use this finish despite it's limitations, because I hear an increase in overtone complexity and overall response! It is also beautiful to look at and has a nice low friction coefficient i.e. it feels good on the neck.

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Building Blog Entry:  April 27/10

Why Handmade?

   Why would anyone buy a handmade guitar?  The first word that pops into my mind is continuity.  To have the same craftsperson selecting and grading the tonewoods,  choosing the top and back species combinations,  thicknessing the plates,  adjusting the bracing pattern nuances,  manipulating the resonant frequencies of the various components,  gathering and interpreting deflection test data … that kind of continuity can be the difference between a good guitar and a great musical instrument.

    A large manufacturer by necessity,  to one degree or another,  has to formula-build.  All of the tops go through the thickness sander and come out the same dimension.  Ditto the backs and sides.  An individual luthier has the luxury of flexing and testing each piece of wood and determining whether that particular component is appropriate for the instrument they are about to create.  Does this mean that all handmade instruments are superior to more mass-produced guitars?  Absolutely not.  A small production or individual luthier does however start off with a huge advantage that this kind of attention to detail provides.

    So exactly how does one go about making a consistently responsive guitar?  The short answer is that you make a lot of them.  Collect as much information regarding the physical attributes of the components as you can … the weight of the different parts,  pay attention to how they flex with and across the grain,  the notes they emanate when tapped … anything you can think of that makes sense to you …  then relate that to the quality of the sound of the finished product.  This is an over simplification of course,  but not a bad place to start nonetheless.

    There are essentially two schools of guitar building,  the scientific approach and the more intuitive line of attack.  In reality, most builders walk a line somewhere between these extremes.  I must admit,  most of the great builders I admire will readily admit to a fairly non-scientific methodology.  They have made two guitars at a time for decades,  often utilizing tops from the same billet (i.e. almost identical for all intents and purposes) and make small changes in strutting patterns and top graduations … and compare the two finished instruments.  This pragmatic and evolutional development yields unarguable results albeit over a fairly long period of time. 

    A lot of builders also get excellent results from a variety of slightly more science-based methods.  Some luthiers manipulate the resonant frequencies of the top and back plates as well as the main air volume note (sometimes referred to as the Helmholtz resonance).  Some go as far as tuning each strut or tone bar to a specific pitch.  Others shape their tone by means of manipulating the Chladni patterns, in what is called “free plate tuning”.  A third method involves measuring the long and cross grain deflection patterns of the top plate, then duplicating these measurements to achieve consistency. I know some very fine builders who tap their tops and backs until they hear a certain gong-like decay response that for their instruments, produces good results. I’d like to go into each of these methods in more detail … perhaps in my next blog entry!

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