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Lefty's Tutorial on 

Stringed Instrument Acoustics

There are many aspects to controlling the sound of stringed instruments, some scientific and some artistic. The whole idea is to convert the attacked string’s energy into sound waves that are pleasing to the ear. All stringed instruments are an assembly of many diverse components that must be properly configured to achieve satisfactory performance. Change one component, even slightly, and sound quality can be drastically altered. Only through building many instruments, rigorously evaluating even slight changes and paying close attention to hundreds of details can one expect predictable results. I have listed some of these details in order of importance based on my experience.

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01Accurate Fret Position. If the frets are not properly positioned, fully inserted in the fretboard grooves, and all at the same height, the instrument will never sound right. It is vital that the distance between the nut’s string contact point and the 12th fret be EXACTLY one-half the fret scale length. In order to have minimum finger pressure, the string/fret clearance must be maintained very low, generally below of 0.020 inch at the first fret and no more than 0.030 at the 12th fret. If the frets are not dead level this is impossible to achieve without buzz. The industry standard fret material is nickel silver and they work great; a similar material, EVO, is gold in color. Stainless steel however is more durable. Unfortunately stainless steel is far more difficult to install in a radius fretboard and very difficult to level.  If there are any imperfections in the board contours or slots there will be buzz so more precision work is necessary. The choice between an arched and flat fretboard is strictly personal.

02Soundboard mechanical properties. Spruce (Picea) and Alaskan Yellow Cedar/Canadian Cypress (Chamaecyparis nootkatensis) have the greatest strength to weight ratio of any woods except Balsa. They permit more of the string’s energy to be converted to air movement. I also use Western Red Cedar (Thuja plicata) which, when properly selected, sawn, or preferably split, and shaped, is excellent as well. The number of growth rings per inch, their uniformity and orientation as well as stiffness are critical factors. All of my soundboard woods are either pie sawed or hand split and have in excess of 14 growth rings per inch and in the case of Red Cedar, in excess of 20. Russian or Jezo Spruce (Picea Jezoensis) is the strongest spruce species that I have ever used, it has very narrow growth rings (occasionally in excess of 30 per inch) because it comes from a region with a short growing season, unfortunately it frequently has knots due to a smaller diameter trunk and as a result has more wastage and thus a higher cost; it is otherwise indistinguishable from Sitka Spruce (Pica stichensis). Carpathian Spruce (picea abies) is another fine tone wood that seems to require less break-in time and has the volume and tone of Russian Spruce. It is particularly desirable in flamenco guitars and violins. Yellow Cedar, Englemann Spruce (Picea Engelmannii) and Adirondack Red Spruce (Picea rubens) combine high strength and great beauty with excellent sound quality. A typical 24-ring  Carpathian, Russian Spruce or Yellow Cedar F5 soundboard weighs between 90 and 100 grams when properly tuned whereas one constructed from the other spruces (typically having 16-rings) will weigh 125 grams or more. That difference may not sound like much but believe me the difference in inertial resistance to string excitation is substantial, resulting in additional sound amplification for a given string set and break angle.

03Soundboard shaping. The classical shaping of archtop soundboard contours is called a curtate cycloid, which in plain English translates into selecting a spot on the radius of a circle and tracing its’ path while the circle rolls. Using this method for both the inner and outer surfaces generates a contour that gradually thickens towards the center. For the Spruces and Yellow Cedar a thickness of 0.120 at the rim graduated to 0.200 at the center is an optimal starting point on the mandolin. Red Cedar needs to be a bit thicker. The mandocello and archtop guitar require a different contour that I establish as a balance between string load and resonant tone based on deflection tests of the individual top. The real key to good soundboard vibratory modal response is uniformity. Small thickness variations significantly alter the vibratory mode shape and thus affect sound quality. To achieve good results, careful shaping and visual inspection with a very bright backlight is required. Once the soundboard is correctly and uniformly contoured, tuning adjustments with very precise electronic measurements can commence.

The bracing of guitars plays a similar role to the graduated contouring of archtop instruments. Starting with a soundboard thickness of from 0.080 to 0.100 and adding both longitudinal and cross reinforcement at the correct locations provides control of both string tension deflection and resonant frequency. For steel string guitars there are an infinate number of bracing patterns but I have found none better than the X-brace Martin format with slightly modified tone bar location. I use a 15 ft. radius arch curvature to the bracing whose apex is just forward of the bridge. On classical and flamenco guitars the fan brace format developed in Spain is preferred.

04Tone bar and bracing placement and geometry. Every soundboard, even two carved from the same board to identical contours, has a unique frequency response and resonant pattern. To adjust this variability on archtop instruments, small bars of wood are glued to the inner surface on either side of the soundboard just towards the center from the F-holes. More elaborate bracing is used on the flatter top guitars.  After the outer soundboard surface has been shaped to specification, its’ deflection is set by carefully shaping the inner surface as the first step in producing the proper resonant response of the soundboard; then the tone bars or braces are glued in place and by tap tuning, the exact frequency response is set by carefully removing small amounts of tone bar/brace wood.

05Backboard acoustic reflectivity and flexure. A very stiff backboard reflects more acoustic energy, particularly at higher frequencies but in order to get the correct air chamber acoustic modes, it must respond to vibratory excitation much like the soundboard. While not quite as critical as the soundboard, correct contouring greatly enhances modal shaping. By carefully thinning the center of the backboard, the resonant frequency of the air chamber can be slightly lowered during final tuning. Red Maple (Acer rubrum) is best for nearly every application though Black Walnut (Juglans nigra) and Honduran Mahogany (Swietenia macrophylla), which have a bit more internal damping, is also excellent for mellow tone. I have found no woods more suitable for mandolin and mandola backboards than Red Maple and Rosewood on guitars. My favorite for the mandolin family is Curly or Birdseye Red Maple from Ontario but for true beauty, it is hard to surpass burl Walnut with a French polish finish. My favorite for guitars is Brazilian Rosewood though Curly Maple is also great. The Flamenco guitar traditionally has a Mediterranian Cypress back and the finest violins always have European Maple backs.

06Sound hole sizing and placement. Increasing the size of the sound holes increases the resonant frequency of the air chamber and to a limited extent raises acoustic output, though if one goes too far, sound quality suddenly drops. One must be very careful here because it is hard to put wood back on the F-holes. The F-holes of archtop instruments should be located so that the bridge sits at their widest point and above the apex of crossed braces on flat top instruments.

07Bridge rigidity, weight, and perfect contact with the soundboard. The bridge is what drives the string’s energy into the soundboard. Ebony is the standard on archtop instruments but Rosewood is also very good, particularly on guitars. For maximum bark and on most mellow tuned archtop instruments, I strive for a minimum Ebony bridge weight within the limits of rigidity. On mellow sound instruments that have a Virzi, I generally use an Ebony bridge that has a wider foot under the bass tone bar but occasionally use a similar Rosewood base with a bone or ivory insert in the saddle if testing indicates weak treble. On the mandola and mandocello, a non-adjustable bridge is preferable due to the greater string loads. I generally use the  unique design developed by Red Henry, which gives exceptional acoustic response. That said; I custom build each bridge to specifically benefit the individual instrument after taking a series of electronic measurements of air chamber response using standard bridges reserved for this purpose. I have developed a proprietary process that assures 100% contact between the archtop soundboard and bridge typically resulting in a 10% increase in soundboard amplitude compared to the average mass production mandolin. Intimate contact between the bridge feet and top cannot be overemphasized,

08String nut rigidity and perfect contact with the neck. A poorly installed and notched nut will negate an otherwise perfectly constructed mandolin. Bone is the standard and works very well but ivory is a bit more dense and pearl is denser yet and hence transmits high frequencies into the neck better and will generally last the life of the mandolin. An old wife’s tail occasionally circulates that pearl wears the strings out; this is totally false if the nut notches are properly shaped and smoothed. The new synthetic ivory “trade named TUSQ” is an acceptable substitute for bone for the larger instruments but lacks the high frequency transmissibility of pearl so desirable in the mandolin. It may sound trivial but the width of the string notches, their smoothness, and alignment, are major factors in accurate tuning. It is best to have the nut notch align tuner post and bridge with smooth rounded bottoms exactly the diameter of the string, which makes binding less likely while preventing the string from slipping sideways.

09Virzi Tone Producer. This small disk is mounted to the underside of the archtop soundboard between but not touching the tone bars. Its’ purpose is to enhance the higher frequency partials and increase sustain. Shaping and installation of the Virzi is a very exacting process that belies its’ simple appearance. For A and F5 mandolins, the improvement in sound quality is, in my opinion, marginal due to the damping effect of added weight on the soundboard, for A+ mandolins it depends on the individual’s sound specification but on mandolas there is a noticeable improvement and the mandocello would sound weak without a Virzi. I install the Virzi only on mandolas and mandocellos unless specifically ordered as an option on mandolins.

10Proper tuning machine installation. If the pegs don’t turn smoothly, you will find it very difficult to properly tune the instrument. Some tuners (Waverly & Alessi) are better than others but the real key is proper installation. The back and front surfaces of the peghead are never perfectly parallel, thus the peg holes must be drilled square to the back to prevent peg binding. If the knobs don’t turn easily, something is wrong, even with very inexpensive tuners.

11Neck stiffness and installation integrity. String energy absorbed by the neck does not excite the soundboard through the bridge. The best neck is useless unless mated with a solid dovetail joint to a properly fabricated head block for the smaller archtop instruments. For the larger instruments,  particularly for archtop and 000 guitars, a Spanish style neck integral with the body is preferable. It is impossible to accurately transmit vibrations through a wide glue joint shimmed and packed with filler. My joints and those of any competent Luthier are so tight that only a film of glue is required to assure rigid attachment. On all guitars, I use the Spanish construction method where the neck is integral with the body unless directed otherwise.

12Correct string break angle. The standard string break angle for my mandolin family instruments is from 16 to 18 degrees for the Spruces and Yellow Cedar and 14 to 16 degrees for Red Cedar, all with a neck/headblock angle of 5.5 to 6.0 degrees.  On special order F5s, a string break angle of 20 degrees is possible but otherwise imposes too great a load on the soundboard. Decreasing the string break angle over the bridge lowers the force on the soundboard and, when combined with more limber tone bars provides a smooth mellow tone. Conversely, increasing the string to soundboard angle combined with stiffer tone bars produces a sharp bark desirable for Bluegrass music. Keep in mind that I am talking about very small changes (+/- 2.0 deg max) in string break angle to make major changes in acoustic response.

13Proper installation and mass of tailpiece. Lightweight tailpieces (Gibson, Bill James) attached to a minimum tailblock will slightly improve high frequency response on the mandolin family of instruments but the heavier ones (Custom Lefty, Montalone, Orrico, in that order) attached to a dense tailblock have better overall mechanical coupling and improve instrument balance. There is an acoustic place for each of these tailpieces as part of an integrated sound system that is more important than visual pleasure. All of the tailpieces that I offer are beautiful. If you desire a sharp bark, the Bill James is best providing your budget can stand the cost; otherwise the Gibson though the thin covers are suseptable to coming loose and causing buzz. If you desire a mellow tone, either the Montalone or Custom Lefty are generally best. I recommend the Orrico only on mandolas or mandocellos where heavy strings and a Virzi are used. On archtop guitars, I prefer the Sacconi style tail made from wood matching the fretboard.

14String mechanical properties. The length, mass and tension of the strings determine their vibrating frequency. The only variables are the string’s stretch stability, diametric uniformity, and sectional density. On the mandolin family, the best C, G, and D strings for mellow tone and some bluegrass tuned instruments are phosphor-bronze wire wound over a steel core. For maximum bark and bright tone with bluegrass tuning, nickel-steel strings are optional. There are many excellent makes on the market but I prefer those made by D’Addario because of their consistent quality. On my F5 mandolins, I use and recommend the medium-heavy J74 or EXP77 string sets for mellow tone and medium-light J62, J67 or FW 74 string sets for bluegrass tuning. My A+ mandolin performs best with their J74 or J75 string sets and I generally use the J75 or J76 string sets on my H5 mandolas and J78 on the MC4 mandocello. For the 000 guitars, the D'Addario EJ series of strings are preferred. Only on the archtop guitar do I prefer E&O Mari "La Bella" strings, there are none better for that ringing jazz sound. Material and diametric uniformity make a huge difference in the life and performance of the smaller diameter solid wire A and E strings. For the professional musician, I make custom A & E strings for the mandolin from chrome-molybdenum steel wire that generally last far longer and permit more stable tuning since they don’t stretch as easily as standard steel. Regardless of E string material, careful tightening in gradual steps is essential to prevent breakage.

15Intonation. Once a string set has been selected, the string contact points on the Bridge saddle must be individually adjusted by alternating between an open string and the 12th fret. The bridge contact point is moved forward if flat and to the rear if sharp. This is very critical on the shorter scaled mandolin and mandola but it is still important to get as close as possible on the longer scaled guitars and mandocellos.  Generally axial adjustments of about 0.005 inches on a mandolin will shift the frequency 2 cents, about as repeatable as can be measured reliably. I generally start at a point where all but one string is sharp and cut back on the saddle contacts as required. Shifting the fret crown with a special diamond file enables slight intonation adjustment on nickel silver or EVO frets 1 thru 11 but this is not possible with stainless steel. Caution, changing string gauge or manufacturer can drastically alter your instrument’s sound and increasing the string weights could cause string buzz on the frets and soundboard collapse in an extreme case.

16Peghead mass has a significant effect on balance and thus playability. With standard tuning machines the F5 mandolin and H5 mandola peghead is sufficiently heavy to cause the instrument to be peghead heavy. To minimize this effect I laminate a thin piece of Maple transverse to the Ebony or Rosewood top veneer to strengthen the narrow area of the scroll while keeping overall thickness to a minimum.  I also generally use a Rosewood tail block and cast bronze tailpiece as counterweights. This is less critical on guitars since the balance is much nearer its' center.

17Stiffness and mass of the finish. There are four categories of finish; each has a unique effect on stringed instrument sound. I frequently use sprayed  lacquer because it is crystal clear and easily buffs to a mirror finish. A lacquer-finished soundboard occasionally gets minor crazing if a rapid change in temperature or humidity occurs but this in no way affects the sound quality. Spirit varnish applied in the French polish manner is equally beautiful and generally considered superior in sound quality but more fragile than lacquer. It is easy to repair a scratch in either finish.  Oil-based varnish is the classic finish for the violin family and works very well for all stringed instruments. The only complaint that I have about oil varnish is the long drying time which invites dust contamination. The fourth is polyeurethane, which I do not use or recommend. Poly finishes have a gummy texture that damps the natural vibration properties of the soundboard and on archtop instruments attenuates its' coupling of the bridge. Rubbed oil finishes such as "Tru-Oil" are used by some builders but I have never found them to give the quality of finish that is possible with the others. Also, one must never, ever, apply a silicone based polish, if you do the instrument cannot be properly refinished without sanding to bare wood; this voids my warranty.

18Instrument Setup. The finest instrument in the world will not sound its’ best unless set up properly. This includes but is not limited to: proper bridge, nut, tailpiece, and tuning machine installation, accurate string nut notches at the correct height, dead level frets that are properly spaced and crowned, correct string set, intonation adjustment, and action height.

19Dedamping the completed instrument. Exposing stringed instruments to sustained acoustic excitation helps blend the components into a single organic entity. This can be accomplished by playing for years or in a shorter period of time with any sort of vibrator such as a Tonerite, massager, or even a fish tank air pump, all of which operate at 60 Hz; but using a loudspeaker voice coil between the strings and bridge and driving it over the entire acoustic range of the instrument best achieve initial dedamping, which I do before delivery.

20Glue Myth. In many advertisements, you will see a claim that some glue is “acoustically transparent”. After extensive laboratory testing, I assure you that no adhesive has acoustic transmissibility identical to the woods it binds. The best glue joint is the one that has near perfect contact between the pieces being joined. This is technically called a wringing fit; such a fit cannot be achieved with only a saw cut. Both surfaces must be carefully smoothed and very flat. I generally use Titebond I cement for the simple reason that it requires no preparation time. Hot hide glue is stronger but no better acoustically than Titebond. I use hot hide glue if the customer asks though I charge extra due to the additional labor required. A good point for hot hide glue is that with damp heat, a joint can be safely opened or an old joint revitalized; it also accepts stain readily whereas Titebond penetrates and seals the wood and can sometimes be difficult to sand out should a drip get on a noticeable place. In precise laboratory tests of properly prepared Spruce and Maple samples, no acoustic difference was measured between Titebond and hot hide glue joints.

 

Instrument Specifications 

 

F5 & A

A+

H5

A-Style

Mandola

MC4 & MC5

Boomer

Rim Height (in) 1.37 1.50 1.55 2.00 1.75/2.00 1.80
Max Soundboard Diameter (in) 9.90 10.00 10.80 11.00 12.00/14.00 11.00
Neck Angle (deg) 5.5/6.0 5.5 5.5 5.5 5.5 5.0
Scale Length (in) 13.875 14.76 15.65/17.00 15.65/17.00 21.00/ 25.45
18.60 or 26.19
Nut Length (in) 1.05/1.25 1.05/1.25 1.15/1.20 1.15/1.20 1.25/1.50 1.20
String Spacing 1.00/1.15 1.00/1.15 1.08/1.15 1.08/1.15 1.15/1.30
1.08 (#5 @ 0.27)
Tone Bars A#3/G#3 F#3/F#3 G#3/F#3 G#3/G#3 F#2/B2 G#3/F#3
Chamber Tuning D#3 C#3 C3 C3 A#2 C3
String Break Angle (deg) 16-20 16-18 16-18 16-18 15-16 14-16
String Tuning G3-D4-A4-E5 F3-C4-G4-D5 C3-G3-D4-A4 C3-G3-D4-A4

G2-D3-A3-E4

C2-G2-D3-A3

(g4)-D3-G3-B3-D4

Alternate Tuning* None G3-D4-A4-E5 None None F2-C3-G3-D4 on MC4
(e5)-C3-G3-D4-A4
D'Addario String Set J62/J74 J74/J75 J72/J75/J76 J76 J76/J78 J 76/J55/EPS510
Acoustic Range at fret 12 (Hz) 196/1318 185/1244 130/880 130/880

98/659 on MC4

65/440 on MC5

131/1318

*When using alternate A+ tuning, the E-strings must be made from 0.011 Chrome-Moly steel to reliably withstand the tension.

Due to its' fretboard length, the mandocello has a full register greater range than the mandolin or mandola. The Boomer covers the full range of both mandolin and mandola or the 5-string banjo.

Lefty's Tailpieces

The three most popular tailpieces: Gold Montalone, Nickel Gibson and Bronze Lefty.