Acoustic Guitar Construction

Hi all.

Seems like we need a guitar design section for questions like this but here goes.

I prefer the looser sound of the Gibson scale so I was considering building another l-00/nls with a martin scale length but with the intention being to have tuned down half a step and using 12 gauge strings. Would the longer scale but lower pitch have basically the same string tension and do you guys consider the likely string tension when bracing tops? My l-00 is braced light as anything and I'm sure it would suffer with anything heavier. Any thoughts on this?
Thanks,
Mike.

String tension has a huge impact relative to the sound produced by any guitar -- even changing one string in a set.

Here's a calculator:

http://www.mcdonaldstrings.com/stringxxiii.html


Special thanks goes out to the author of the article, Rick Turner, for this hard-to-find resource!

There are three things to observe in calculating your string set:

Pitch: The note you wish to apply.
Gauge: The string gauge/size to use.
Tension: The pounds of pressure being applied to the bridge with this pitch and guage.

WARNING!!: A standard set of light gauge strings will apply a total of 179.2 pounds of pressure on the bridge ... if your guitar was built for light gauge strings, you may do damage to it by applying a heavier gauge - so apply these gauges with care, eh? The idea is to take the Tension amount of each string in your tuning ... add up the total ... and arrive somewhere around the following figures:

156.2 pounds for extra-lights
179.2 pounds for lights
197.5 pounds for mediums

NOTE: The common scale length on a guitar, from nut to bridge, is 25.5 inches ... if your instrument has a shorter scale length, the tension will be lower ... and, naturally, if the scale length is longer, the tensions will be higher.

EXAMPLE A:
Using my Open G tuning, which is (low to high) G G D G B D ... I find the following gauges and tensions from the chart below ... using the center column for Lights.

D: .014 ... 25.2
B: .016 ... 23.3
G: .025 ... 32.8
D: .034 ... 34.7
G: .047 ... 29.3
G: .047 ... 29.3

Totaling the tension amounts of each string, I arrive at 174.6 ... a "safe" number for light guage set ups.


EXAMPLE B:
If I wanted to apply an Open D tuning, which is (low to high) D.A.D.F#.A.D ... I find the following gauges and tensions from the chart below ... using the center column for Lights.

D: .014 ... 25.2
A: .020 ... 28.9
F#: .026 ... 31.4
D: .034 ... 34.7
A: .045 ... 34.0
D: .059 ... 25.6

Totaling the tension amounts of each string, I arrive at 179.8 ... again, a "safe" number for light guage set ups.

Now I know the correct gauges to order ... I order those gauges (see link below chart) ... put 'em on and twist 'em up, and I have the altered tuning I want without creating uneven pressure or causing damage to my instrument.

Pitch Extra-Lights ||| Tension Lights ||| Tension Mediums ||| Tension heavy
F# - 370.0Hz .010 ||| 20.4 .0105 ||| 22.5 .011 ||| 24.7
F - 349.2Hz .0105 ||| 20.0 .011 ||| 22.0 .012 ||| 26.2
E - 329.6Hz .011 ||| 19.6 .012 ||| 23.3 .013 ||| 27.4
D# - 311.1Hz .012 ||| 20.8 .013 ||| 24.4 .014 ||| 28.3
D - 293.7Hz .013 ||| 21.7 .014 ||| 25.2 .015 ||| 28.9
C# - 277.2Hz .014 ||| 22.5 .015 ||| 25.8 .016 ||| 29.3
C - 261.6Hz .015 ||| 23.0 .016 ||| 26.1 .017 ||| 29.5
B - 246.9Hz .015 ||| 20.5 .016 ||| 23.3 .017 ||| 26.3
A# - 233.1Hz .016 ||| 20.7 .017 ||| 23.4 .018 ||| 26.3
A - 220.0Hz .017 ||| 20.9 .020 ||| 28.9 .022 ||| 34.9
G# - 207.7Hz .020 ||| 23.5 .022 ||| 28.7 .025 ||| 36.8
G - 196.0Hz .023 ||| 27.9 .025 ||| 32.8 .027 ||| 38.4
F# - 185.0Hz .025 ||| 29.2 .026 ||| 31.4 .027 ||| 38.4
F - 174.6Hz .026 ||| 28.0 .029 ||| 35.8 .030 ||| 38.3
E - 164.8Hz .027 ||| 27.1 .030 ||| 34.1 .032 ||| 38.4
D# - 155.6Hz .030 ||| 30.4 .032 ||| 34.3 .034 ||| 38.9
D - 146.8Hz .032 ||| 30.5 .034 ||| 34.7 .036 ||| 38.9
C# - 138.6Hz .034 ||| 30.9 .036 ||| 34.7 .036 ||| 34.7
C - 130.8Hz .036 ||| 30.9 .036 ||| 35.8 .039 ||| 35.8
B - 123.5Hz .036 ||| 27.5 .039 ||| 32.0 .042 ||| 37.7
A# - 116.5Hz .042 ||| 33.5 .042 ||| 33.5 .045 ||| 38.1
A - 110.0Hz .042 ||| 29.9 .045 ||| 34.0 .047 ||| 36.9
G# - 103.8Hz .042 ||| 26.6 .045 ||| 30.3 .049 ||| 35.6
G - 98.0Hz .045 ||| 27.0 .047 ||| 29.3 .052 ||| 35.7
F# - 92.5Hz .047 ||| 26.1 .049 ||| 28.3 .053 ||| 32.8
F - 87.3Hz .049 ||| 25.2 .052 ||| 28.3 .053 ||| 29.2
E - 82.4Hz .049 ||| 22.5 .052 ||| 25.2 .056 ||| 29.0
D# - 77.8Hz .053 ||| 23.1 .056 ||| 25.9 .059 ||| 28.7
D - 73.4Hz .056 ||| 23.0 .059 ||| 25.6 .062 ||| 28.2
C# - 69.3Hz .059 ||| 22.8 .060 ||| 23.6 .064 ||| 26.8
C - 65.5Hz .060 ||| 21.0 .062 ||| 22.4 .066 ||| 25.3
B - 61.7Hz .062 ||| 19.9 .064 ||| 21.2 .070 ||| 24.8

Excellent resource, thanks.

That's exactly what I needed, thanks Ken. If anyone's interested or has any advice here are my numbers, in metric I'm afraid.

Strings. Scale. Tuning. Tension

12. 63cm E flat. 64kg
12. 65. E flat. 68kg
12. 65. E. 76kg
12. 63. E. 71
13. 63. E. 75
13. 65. E. 80

Seems like my plan of longer scale lower tuning should work and have slightly lower tension than long scale standard tuning. To my ear most longer scale guitars sound better down a semitone.

A 66.5cm scale (26.2") gives about the same tension in e flat as 63cm scale in e natural, maybe I should try that but I wouldn't be able to use my kmg bridge positioning tool. I think that's pretty close to the scale on Paul ho setters h13 13 fret nick Lucas style. I was also lucky enough to play john Arnold's nls and he very kindly gave me a plan he had made from that instrument and the bracing is heavier than the grellier plan which I followed previously.

Although the tension when tuned up to pitch would be pretty high (80kg) if you forgot or didnt know it was designed to tune lower.

Yes, scale length seem to matter.
I built two quite similar Jumbo's. One with a 25" scale and one with 25.4". The setup on both is almost similar. Everyone who played them, mentioned that the 25"-guitar played easier. And the sound of the guitars were not very different. Maybe they were more sensitive than me, 'cause I did not feel it. But hey, I'm from the rural part of the land. Big hands you know.
Herman