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| Compensation measurements for short scales http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=40032 |
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| Author: | PeterF [ Thu Apr 11, 2013 3:16 pm ] |
| Post subject: | Compensation measurements for short scales |
I'm currently building a small guitar for the new builders challenge, but I've just realized I have no idea how much I should compensate the saddle for such a short scale (19in). Do you need more or less compensation for shorter scales? (Assuming the action stays the same) I will be using a thick saddle, so there will be a bit more room to adjust it. |
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| Author: | Ken Lewis [ Thu Apr 11, 2013 3:31 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
Hey Peter. Check the Stu. Mac. fret calculator. It'll figure for ya. Ken Lewis |
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| Author: | PeterF [ Thu Apr 11, 2013 4:06 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
Thanks
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| Author: | Eric Reid [ Fri Apr 12, 2013 9:53 am ] |
| Post subject: | Re: Compensation measurements for short scales |
I wouldn't rely on the Stew Mac calculator for compensation on such a short scale. Stew Mac's formula uses a set percentage of scale length for the compensation. According to their system, a 12" scale needs exactly half the compensation that 24" needs. I think most uke builders could tell you that's not the case. You can prove it to yourself. Tune your low E down to B and capo at the fifth fret. (This will be in the ballpark of your 19" scale) Check the fretted e at the 17th fret vs the octave harmonic there. Stew Mac's calculator would tell you that you have too much compensation. I think you'll find you have too little. Try Don Teeter's trick: break 1/2" off the end of a round toothpick, and wedge it under the string just past the fifth fret. Move it back and forth until you find the spot where the string plays in tune. The compensation you need to add is 1/2 the distance from the fret to the toothpick. |
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| Author: | Alan Carruth [ Fri Apr 12, 2013 12:45 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
A little theory: When you displace a string, it gets tighter, and the pitch rises. For a string of a given length and material, displaced in a given spot by a given amount, the rise in tension will be the same no matter how tight the string was to begin with (within reason, of course). If pushing your high E string down to the 12th fret increases the tension by one pound, then that's going to be the same no matter what pitch the string was at. If the string is tuned down to the same pitch as your open G string, which is about as low as a plain string can usefully go, then the one pound of tension change will alter the pitch a lot, since the string is so slack to begin with. Tuned up to the G above the usual E, which is pushing it, the pitch will hardly rise at all, since the tension is so high to begin with, and another pound isn't all that much to add. If you use a normal set of strings on your short scale guitar you'll probably find that they will need a lot of compensation. Even shifting everything up a string (using an 'E' for the 'A' and so on) might not work all that well. I made one with a 20" scale once, and even with a lot of compensation it was still a bear to play in tune. Any alteration in left hand pressure could change a chord from sounding OK to less than wonderful. It took me a while to get the hang of it. No, I didn't keep a record of how much compensation I used. I'd do it differently now anyway, since I've learned about nut compensation. |
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| Author: | PeterF [ Fri Apr 12, 2013 4:50 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
Thanks for the replies. I'm sorry, I should have mentioned before that the string tension will stay the same as a normal guitar - essentially, It's just going to be the same as capoing the fifth fret, tuned ADGCEA. Seeing as you can play a standard guitar with a capo without changing the nut, does that mean I can use the same measurements? |
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| Author: | profchris [ Fri Apr 12, 2013 5:10 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
PeterF wrote: Seeing as you can play a standard guitar with a capo without changing the nut, does that mean I can use the same measurements? My guess would be no, but it's only a guess. A short string of the same diameter is stiffer than a longer string when tuned as you described. Recently I made a pair of 7-inch scale ukes (just to see how small was playable; the answer is 7 inches, just!). This is a fraction over half the "standard" soprano uke scale of around 13.5 inches, so my theory was I would tune them an octave higher. Tuned that way the string tension was far too high for playing comfort. With the same strings as for the 13.5 inch scale, I found that tuning the small ukes up only a fifth gave me an acceptable tension. This was dGBE (up from gCEA). The compensation was greater, in both relative and absolute terms. For a soprano uke I build in around 3mm compensation. For these baby ukes I needed around 5mm. Luckily I used a floating bridge, so this was easy. (apologies for the mixed units, 3mm is about 1/8 inch) If this translates, your guitar might be happiest tuned GCFBbDG and need around 25% more compensation than a full-scale guitar. If I were building it I'd assume that it will need at least as much compensation as a full scale, and probably a fair bit more, and so use a wide saddle initially placed so the front edge was at standard full scale compensation. Then I could file the saddle back as needed.  IMAG0316 by profchrisreed, on Flickr |
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| Author: | LarryH [ Sat Apr 13, 2013 9:11 am ] |
| Post subject: | Re: Compensation measurements for short scales |
PeterF wrote: ... the string tension will stay the same as a normal guitar - essentially Jumping in here with little knowledge but I thought a shorter scale meant less string tension to get to a given pitch as a general rule Is this not true? |
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| Author: | profchris [ Sat Apr 13, 2013 12:46 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
LarryH wrote: PeterF wrote: ... the string tension will stay the same as a normal guitar - essentially Jumping in here with little knowledge but I thought a shorter scale meant less string tension to get to a given pitch as a general rule Is this not true? This is true if you're using the same strings and tuning to the same pitch. But PeterF is talking about using a shorter scale, same strings, but tuning to a higher pitch which gives the same tension. What I found was that tuning to the same tension on a shorter scale made the strings too taut to play comfortably. The shorter string requires more force to depress it the same distance as the longer string. For this reason, short scale instruments tend to have lower string tension than longer scales - or at least, I know this to be true for ukes: soprano < concert < tenor, when all are tuned at the same pitch. |
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| Author: | PeterF [ Sat Apr 13, 2013 1:24 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
LarryH wrote: PeterF wrote: ... the string tension will stay the same as a normal guitar - essentially Jumping in here with little knowledge but I thought a shorter scale meant less string tension to get to a given pitch as a general rule Is this not true? I probably wasn't using the term in it's technical sense. I thought that just meant a guitar that has shorter strings than normal.  profchris wrote: What I found was that tuning to the same tension on a shorter scale made the strings too taut to play comfortably. The shorter string requires more force to depress it the same distance as the longer string. For this reason, short scale instruments tend to have lower string tension than longer scales - or at least, I know this to be true for ukes: soprano < concert < tenor, when all are tuned at the same pitch. I don't understand why it would feel tighter. Surely it would be the same as playing with a capo, wouldn't it? If you put a clamp across the neck at fret 5, then cut everything off above that point, the tuning, tension and pitch would stay the same. Or am I missing something? |
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| Author: | profchris [ Sun Apr 14, 2013 11:07 am ] |
| Post subject: | Re: Compensation measurements for short scales |
Mine was an extreme example, the equivalent of capoing at the 12th fret! Maybe the answer is simply to take a guitar, capo it at the 5th, and play it for a while. It's years since I played guitar, but I seem to recall that capoing up there made the strings feel harder to fret. But again, I can barely remember what happened yesterday, so .... The other thing you could do at the same time is check out the intonation with a capo on the 5th. Does the current compensation give you a 12th fret harmonic which matches the new 12th? If it's sharp, as I guess it might be, then that tells you that you need more compensation. I think I remember a previous thread which gave a formula for calculating compensation based on the cents sharp/flat at the 12th - perhaps the author will chip in here. |
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| Author: | PeterF [ Sun Apr 14, 2013 11:46 am ] |
| Post subject: | Re: Compensation measurements for short scales |
OK, thanks. I just tried it and it seems the intonation stays pretty much the same with a capo, give or take a cent or two. Looks like that answers my question!  There's so much going on with string theory (the guitar one, not the physics one!) it's hard to keep track of all the variables.
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| Author: | Alan Carruth [ Sun Apr 14, 2013 6:37 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
Yeah, strings are not simple, and keep in mind that they're the simplest part of the system! |
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| Author: | Trevor Gore [ Sun Apr 14, 2013 7:06 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
PeterF wrote: I don't understand why it would feel tighter. Surely it would be the same as playing with a capo, wouldn't it? If you put a clamp across the neck at fret 5, then cut everything off above that point, the tuning, tension and pitch would stay the same. Or am I missing something? You're missing something. The raise in pitch you get when fretting is related to the longitudinal stiffness of the string, which is related to the string length. If you apply a capo, you still have the long string length, as the string will not be "locked" by a capo over a fret. If you apply a true clamp (locking nut type) you have a shorter effective length and therefore a stiffer string that requires more compensation. The string is actually stiffer (longitudianlly) which can be felt, but most people describe it as feeling tighter. You get the same issue at nuts. Is the piece of string between the nut and the tuner part of the stretchable length or not? because it affects the amount of compensation you need and the feel. On most guitars, sometimes it is and sometime it isn't. Not what you want. If you do a straight-through string pull like this, it seems that the piece of string between the nut and tuner is part of the stretchable length, so you get a more stable intonation solution and a lighter feel. |
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| Author: | PeterF [ Mon Apr 15, 2013 2:14 am ] |
| Post subject: | Re: Compensation measurements for short scales |
 I knew there must be something...What sort of amount are we talking about here? A couple mm extra compensation or more than that? I'm using a 6mm wide saddle, so is it OK to put it in it's normal place and then file it back until it plays in tune? Or do I have to work out a new bridge location? THe trouble is, I've already closed the box, so the bridge plate can't be changed. |
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| Author: | Trevor Gore [ Mon Apr 15, 2013 3:12 am ] |
| Post subject: | Re: Compensation measurements for short scales |
PeterF wrote: What sort of amount are we talking about here? Well, for saddle only compensation* the first part, at least, is linear (for once). For a given string, halve the string length and it is twice as stiff longitudinally. So for the same amount of depression onto a fret, it will have double the tension increase. For the next part, the increase in frequency you get is proportional to the square root of the increase in tension. To knock the frequency increase back to zero, you add string length, and frequency is inversely proportional to string length. So if you know what the compensation is on a "standard" guitar, a few simultaneous equations should get you to the answer on whatever it is you're building. Unless you've got a really small bridge plate** (rarely a smart move) you should be OK. *All bets are off if you're doing nut compensation as well. ** If you happen to have a really small bridge plate, nut compensation might get you out of jail |
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| Author: | PeterF [ Mon Apr 15, 2013 4:35 am ] |
| Post subject: | Re: Compensation measurements for short scales |
Thank you very much. I'll try and work that through. I do have a fairly large bridge plate, so hopefully it should be OK.
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| Author: | Alain Lambert [ Sat Apr 20, 2013 2:36 pm ] |
| Post subject: | Re: Compensation measurements for short scales |
There is an elaborated Excell spreadsheet at MIMF called Fretcomp, that can possibly calculate that compensation, as it consider scale length, string gauge , string height, tension, etc. Can be woorth a try. It is in the MIMF library section under programs and spreadsheets. |
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