The ‘Oi No Ka ‘Oi Steel Guitars - Part III

December 12, 2022 • Kevin Gillies • Instruments and Luthiers

Ed note: Kevin Gillies is Sole Proprietor and Luthier at ‘Oi Acoustics. He is a residential architect, land use planner, woodworker, and luthier. In his words, he "strives to achieve elegance through simplicity, reducing the number of visual elements to the minimum required. The focus is on ergonomics and tone to make an instrument that speaks for itself and you won't want to put down." This multi-part series delves into Kevin's design philosophy and what he considers important elements when it comes to instrument architecting and selection. This is Part III of a multi-part series. © 2022 www.oiacoustics.com.

Author note: Much of the following information is reasonably objective, but my opinions are solely mine. It doesn't mean I'm right, and some will disagree. Feel free to draw your own conclusions or send a note via www.oiacoustics.com.

PART III: WHY INTONATION MATTERS

The strings are suspended by the nut at the peghead and by the saddle at the pickup end. You can view these components as that little tension structure from last time:

The scale is the distance from the start of the string at the nut to the 12^th fret, times two, which is slightly less than the total distance from nut to saddle. It is chosen to accommodate different instrument body lengths, the player's arm length (and muscle memory), and string manufacturing. If the scale is set incorrectly, it throws off the tuning.

Steel strings are made of steel (or bronze); wound strings have an additional nickel or phosphor bronze wrapping over the same core. String makers build their guitar strings to match the scale and tonal output of the vast majority of the guitar market, which are designed around the 25.4" Martin scale.

The tension of strings is critical to their performance. Steel strings behave in a certain manner according to their ideal designed "speaking length." You can easily calculate the theoretical tensions for any string based on the unit weight, core size, scale and frequency. For an acoustic you can only work outside that parameter by a maximum of about 6% and might be able to get away with as short a scale as about 24" using standard strings of the appropriate weight to create the tension you want, but that's the limit.

After that, to maintain tension, the strings just get too heavy and unresponsive to vibrate correctly. As the scale gets longer, the strings get thinner to maintain the same tension, or deflection, under your fingers or bar pressure.

Electric guitars use the pickup to sense the string vibration and offer more latitude for string sizing as they may be lighter, but there is still a limit and they have to be calculated according to the scale.

ACTION

Action is the distance the string bends until it stops. In this case of a guitar, that's to the fretboard. An electric guitar is played lightly and the action is very small. An acoustic is played harder to create volume and the action is larger to let the strings vibrate. Steel strings are stiffer with more tension than classical, which require more action.

On a steel guitar the action is the amount of deflection, which is dependent on the tension of the strings and how hard you press down with the bar.

I use 23" for shorter scale lap guitars and 24¼" for longer legged models. 25½" as a super long scale is possible for those accustomed to playing an acoustic Weissenborn, but many lap players prefer to minimize the steel body overhang off their legs.

COMPENSATION

As the strings on any instrument are fretted or barred, they are being stretched, which raises the note. Depending on the amount of bend, extra length or "compensation" is added to all the strings so as you play higher, it maintains the correct relative pitch, and "sweetens" the sound from flat.

At 25# of string tension, the string deflection under bar pressure on a steel is about .05" and results in about 1/16" of overall compensation, making the overall distance from the nut to saddle a bit longer than the "scale." Lighter strings have less tension and more deflection, requiring more intonation.

Since there aren't any frets, and the bar can be placed anywhere, it doesn't matter much if you are playing single notes, in the sense that you are playing by ear regardless. In addition, many players use vibrato which both sweetens the sound and masks an off tone.

If you are using your markers to visually guide your playing it matters a bit more, and as you get higher, you are going to have to manually compensate. This is not critical until you get to the next aspect: intonation.

INTONATION & STEEL SADDLE DESIGN

Intonation in steel playing is simply the accuracy of pitch, or "in tune," which as you know isn't all that easy, and is critical. For our purposes we'll define intonation in guitar building as the individual difference in compensation between each string. This occurs because solid strings or the cores of wound strings stretch more as the diameter increases; thicker strings require more compensation. The cores of wound strings, which determine their tensions, start out close to the treble strings, so after the lowest solid string, the wound intonation begins at close to the start of the solids, and again gets larger. This applies to most fretted or unfretted steel stringed instruments. (Noting that classical strings require very little compensation or intonation due to different physical characteristics of the strings.)

A well-made custom acoustic steel string guitar typically has a Z-shaped or lightning bolt saddle, which shifts at the change between the solid and wound strings. Electric guitars and basses frequently have a "hard tail" bridge with individually adjustable saddles for each string.

Some maintain that because a steel has no hard frets, intonation isn't required, but that is incorrect according to the science. The same compensation and intonation principles apply to a steel guitar; just think of it as upside down from a fretted guitar; as with "bending" a note, bar pressure causes the string frequency to rise.

This is an example of this calculation for a 10 string steel with 5 solid strings (with an 024 at #5) and 5 wound strings in the lower register:

Note that #1 to 5 require progressively longer intonation, and then it repeats but slightly differently for #6 to 10. Due to the physics of strings, as they get lower they get larger and results in different tensions; it just isn't possible to have exactly the same tension for all strings at all frequencies. Larger strings are subject to more inharmony in the third column below, which is critical:

In this example, the #5 solid string would require the most intonation and would have the most inharmony at over ½ of a cent (1/100 of a tone). That doesn't sound like much, but the first chart requires about .16 inch of intonation. If not, you have to adjust your bar by .08 inch at the 12^th marker, a bit over 1/16, to be in tune, and confirms steel guitarist John Ely's position that anything over an 022 solid string is too unstable. Once this was calculated, #5 switched to a wound string, which shifted the jump in the Z saddle from 5 solid & 5 wound to 4 solid & 6 wound.

If you are playing single notes you can adjust by ear, but if you are playing a chord of some sort, your bar has to incline to an angle to make up for the lack of intonation. But it can't because the intonations are unequal between string types, and most significant is the shift point between solid and wound strings which would show the most off tuning if you happened to be playing both strings, such as an A and F# in A6. An intonated saddle allows placing the bar consistently perpendicular to the strings. Note on the chart that it's not exactly a linear equation, but an average for each section is quite close and helps to play in tune. The "Oi 5/16" thick compensated & intonated saddle for the 10 string steel with 4 solid and 6 wound strings looks like this:

The greater the frequency range of tuning, as with C13 from C2 to G4, the more important this is. A tighter range as with E13 from B2 to G#4 results in shallower angles, with the same jog between solid and wound strings.

Here's a quick experiment anyone can do on their own: tune your upper string to the right note, in C6 it's usually either E or G, with a digital tuner. Now find the same exact note, an octave higher, at the 12^th marker and note where you are. If you're not exactly above the marker, you are not compensated. You can double check that with a tape measure: the length from the nut to the crown of the saddle should be a little more than twice the length from the nut to the 12^th marker. If it's exactly double, it's uncompensated.

That really doesn't matter so much as you are finding the notes by ear with your digital assistant. Just note the relative position of the bar to the marker. Now do the same on the lowest string. If your octave has moved towards the nut, and that is likely if your saddle goes straight across, perpendicular to the strings, you are not intonated. You can also check to see if there is a difference at the 12^th marker between the thickest solid string and thinnest wound string.

TUNINGS

Taking into account tuning and scales, the appropriate calculations resulted in the following steel guitar tuning chart, using one set of strings for each scale (23" & 24¼") to maintain tension, to offer some different possibilities with 6, 8 and 10 string setups. Each of the teachers noted below utilizes different tunings in their material based on C or A; all of the tunings can be accessed with the 10 string with only a ½ step retuning of half or fewer of the strings. The transition between the solids and wounds — the Z breakpoint — always occurs below A regardless of the instrument:

Now you know why an intonated saddle matters, and why the ‘Oi No Ka ‘Oi is made only with an intonated brass saddle. It is more accurate in pitch and is mo' betta.

Next time we'll look at a few other design considerations.