View Full Version : close to topic- fret spacing

08-26-2007, 03:51 AM
At long last I've gotten back to one old project, the alumitar- aluminum electic guitar. I found the figures I had calculated for fret spacing and wanted to check them for accuracy. Then I started reading about stretch compensation which counteracts the increase in tension of a string when you bend it to touch a fret. I was surprised at the relatively large difference in fret spacing when the compensation factor was put into the equation. For example, at the tenth fret, there's about a .070 difference in the position where the fret should be. That sure seems like a lot when you consider that the fret positions are calculated to four decimal places- not that I know of anyone who can saw a fret slot that closely. My questions, how close is good enough, say for a trained ear, and are there any frets which should be very finely positioned because an error there would be most noticable? I can imagine that at a harmonic, that fret position would be more critical.

I've decided to go with the compensated figures since from one simple test using a keyboard and beat tones, the positions given seem to be right on the money. What I have read about equal temperment, etc, suggests that many of the notes could be altered slightly in their frequency (by positioning the frets differently than the standard formula suggests) to achieve a more pleasing sound. After all, hearing a guitar being played is supposed to be pleasant- not getting into the relative merits of differing types of music-.

I'm aware of pretty much all the parameters regarding intonation and adjustments, including the effects of different brands of strings, the height of the strings, the curve of the fretboard under the strings, etc, but the rather broad range of possible fret positions seems like an anomaly to me. If a guitar is properly in tune, what kinds of changes in fret position might make it sound even better that it normally might? Does anybody ever add a compensation factor for average finger pressure?- that alone seems to me to be a much larger factor than small deviations in fret position.

Comments, anyone?

08-26-2007, 08:11 AM
My questions, how close is good enough, say for a trained ear, and are there any frets which should be very finely positioned because an error there would be most noticable? I can imagine that at a harmonic, that fret position would be more critical.

I haven't tested my frequency resolution in a long time but I do have perfect relative pitch. What that means is that if you give me a note I can then give back any other note accurately relative to the tone supplied. When I tested that back in high school I was good to about 1/4 cycle per second at 440 hz (American Standard A). Since the next semitone, B flat, is the 12th root of two higher at 466.16372 hz that corresponds to a sensitivity of about 1 percent.

I don't know how my tone discrimination compares with others but it doesn't take much off key for me to notice. This is particularly apparent in the beat frequencies. In fact, I don't really like the equally tempered scale as it is not a perfect tuning.

08-26-2007, 08:23 AM
darryl, you might consider sending a private message to Frank Ford on this as I believe it would be right up his alley. Incidently, I'm may need to pick your brain as I'm pondering a lap steel project for the winter and since I'm a total newbie, I'm going to need some serious guidance.

08-26-2007, 08:27 AM
Never heard of it done at the frets. At the bridge, yes.

The tuning of a guitar and even a piano are a comprimise. The human ear actually likes a tiny bit of what seems to be out of tune.

Is this going to be an "all" aluminum guitar? Seems like it would feel cold. And prone to bad vibrations.

08-26-2007, 08:46 AM
The human ear actually likes a tiny bit of what seems to be out of tune.

Speak for yourself. :) I like perfect tuning. It does mean playing in only one key though unless you retune, a somewhat awkward proposition for a piano.

08-26-2007, 08:51 AM
Actually, there are a lot of aluminum guitars around. I suppose they are used to play light metal music...

Your Old Dog
08-26-2007, 09:30 AM
The tuning of a guitar and even a piano are a comprimise. The human ear actually likes a tiny bit of what seems to be out of tune.

In the visual art world it's called "evidence of hand". If it exist there, no reason it shouldn't exist in music. It's one of the reasons I maintain we live in a analog universe and not digital.

As for the OPers question, I'm betting it ain't that critical. If it is, explain to me how they made guitars hundreds of years ago.

08-26-2007, 10:50 AM
The tuning of a guitar and even a piano are a comprimise. The human ear actually likes a tiny bit of what seems to be out of tune.

Temper is actually a little more complex than that. It stems from the oddity that if you set perfect intervals going up the scale, then perfect intervals coming down, you wind up with a slightly different note from where you started. Tempering is an attempt to deal with this remainder. If the error is distributed among all the intervening notes it's equal tempering. However, this seems to just make any key sound bad. The more common strategy is even tempering which distributes the error unevenly, assigning more of the error to keys that are used less frequently. That's why transposing a piece of music to a different key can make it sound different. If all things were equal, transposing a piece would make it just the same except at a higher or lower pitch. Because of the uneven distribution in even tempering the color of a piece can change in a different key. In modern times we've become accustomed enough to even temper that we hardly notice, but in the Baroque era they were much more sensitive and struggled with how to solve the problem. There some instruments made with multiple keys for the "same" note tuned for diffent keys so you could actually play the correct note in that key. Didn't turn out to be a very viable solution.

08-26-2007, 04:20 PM
Darryl, did you happen to see this?


08-26-2007, 06:29 PM
Yep, I've played with people that say they have perfect pitch and they are constantly tuning the instrument because it never sounds right to them. When they get it right in one tune and we play another tune they start tuning again to get the strings right. We no longer stop playing for them to tune.

Having perfect pitch is a curse if you want to enjoy the music. They even complain that the music is out of tune on a recording, which I am sure it is to their ears.

I just tune it and play it and enjoy it. I'm glad I'm not afflicted with perfect pitch.

Most guitar finger boards have the fret slots cut with a CNC machine. They can program the spacing for the length of the strings from the nut to the bridge and have it sound ok but it will never be perfect and always sound off to those with perfect pitch.

I would suggest you buy a ready made finger board rather than cutting your own.

08-26-2007, 07:33 PM

My first reaction to that fanned fretting was positive, but then common sense set in. I can see how, let's call it second degree adjustments, like the odd angled fret, could be useful in compensating for differences in fretting lengths required for different string thicknesses etc, but how can it possibly be necessary for different strings to have different lengths ?

If a string is too long, open (unfretted), you just tune it - you tighten it or loosen it. Maybe to cope with some fret positioning errors, like the fretting compensation this thread is about, you might wish (theoretically) to have frets that aren't straight across, but lengthening the string isn't going to do anything.


Sorry about the OT. Have you inquired what the fretboards/fingerboards from the luthier suppliers are set to ? I've had this kind of thing on my project list for a while, but unlike the gifted few, I would have to have more recourse to calculation than to the ear to feel I had got it right.

08-26-2007, 07:58 PM
Ok, well, first of all, the guitar in not all aluminum. There's some laminate in there, brass screws, and considerable stainless. There will be some maple laid in alongside the aluminum neck, and also some maple around the body. None of that is structural, so my thinking is that its main contribution is esthetic and mass. The nut on this guitar is an abomination of brass rollers and stainless pins. The strings are captive within this part so they can't get out of the groove, so to speak. This is important as will be shown later.

The bridge is a fairly understandable piece with individual height and length adjustments. Behind the bridge is the piece which constrains the ends of the strings, also in such a way that they can't leave the 'groove'. This piece has a sort of hinge action, and can be adjusted to tune all the strings at once, or taken off completely and easily. Again this is important.

The neck is held to the body using one knob, no tools required. This guitar comes apart in two major pieces, and can go back together again quickly and be in tune. Individual string adjustments put direct tension on the strings, so there's no unwinding going on when the guitar is apart. There are no tuning pegs to wind the strings onto.

The fretboard mounting area is flat, which is a compromise. I would have preferred to have a slight curve across the top of the strings, but this is how it will be. It's not a big deal to me.

The fretboard itself- this is going to be a build-up of sections with frets held captive between sections. As of this point tiday, I have about a third of these sections machined to individual sizes, and done according to the stretch compensated program to determine fret spacing. I'm exactly at the point where I need to make sure the fifth fret is exactly where I want it, and basically this is what prompted my questions.

Because finger action changes the tone produced more than about any other parameter, I wanted to include a corrective factor for this. If I find that a typical finger pressure against a fret results in the exact tone desired, then I will have suceeded in placing that fret correctly, regardless if the program says it should be somewhere else. As I suggested earlier, there is a considerable variation in fret positions when only one corrective factor is considered- and I want to include at least three factors, one ot those being 'does the note sound right'. The one compromise anyone must make is because every individual fret must work for all strings, so the result is that few notes will be exactly what they should be. For the sake of actual buildability, the equal tempered scale is used, and as suggested, it's not quite right- close, but not exact.

It's been interesting for me to educate myself on all this, and relate it to actual experience, that is, of having listened to the guitar being played, and pondering the 'imperfections', even though the guitar could be in 'perfect tune'. I've had decades to consider this, but only recently have I explored ways to improve on this situation.

Interesting read on the Charlie Hunter site. That's the first radical change in fretting and scale length I've seen to address some of the problems. It would appear that some practice would be needed in order to play one of those guitars. Maybe it's actually easier- hmm.

Anyway, it's back to the project now. Thanks for all your inputs.

John and Richard- I'm sure that by the end of this project I'll have some more insights into making it tick, and I'll share that for whatever it's worth. Others are certainly more knowledgeable than me on this topic, but maybe I can add something.

Last point- I guess that since this guitar is constructed of aluminum parts with substantial thickness, I can't really lay claim to having invented a new type of music, but light metal sounds like a welcome change from heavy metal. Had I gone thinner, I could have invented sheet metal music :)

08-27-2007, 12:18 AM
No, I think that the "chink-a-chink" music of those that were sent way south by the English were a few years earlier.

08-27-2007, 12:28 AM
Having perfect pitch is a curse if you want to enjoy the music. They even complain that the music is out of tune on a recording, which I am sure it is to their ears.

It can be a curse. I don't listen to music at all any more. That is one of the reasons.

Forrest Addy
08-27-2007, 02:15 AM
I build an accoustic guitar from fiber glass years ago. That's all fiberglass, no wood. Don't ask me; I was proving something. Anyway I built my fret saw machine strictly to 12th root of two fret spacing and once the bridge was set to produce true octaves the rest of the frets produced almost perfect tempered pitching.

No fiddle factors no nothing. Adjustable bridges used by electric guitar players and jazz musicians are seldom tweaked from string change to string change. From this I conclude that special compensation factors are more mystic refinement than anything else.

I suggest a test. Make a crude sample neck and stretch a string above some cobbled up frets. Tension the string with a precision scale on it. Fret the string and note the tension change. If the tension change is down in the third decimal place it's too subtle a change for listeners to note.

I still have the fret sawing machine. Anyone interested?

08-27-2007, 04:43 AM
Forrest, that's an interesting test- measuring actual tension and whatever small changes happen as the string is bent. I think it's unlikely that such small changes would show up on what might be a typical gauge. In any event, it's probably true that the average listener wouldn't detect such small variances.

Unfortunately, for some, like Evan, (and myself it would seem) the variations from what the 'proper' notes should be are easily heard, and can be annoying. The musician who cannot stop retuning is a similar case. I find that I have to interpret a beat cycle over 3 or 4 seconds in order for the tone to sound similar to the test note from the keyboard. The notes on the keyboard don't all sound right to me either, but I have no choice but to accept what they are.

From my testing so far, I'm finding the tension compensated fret program is giving fret positions which make for a more accurate note, at least as compared to notes played on a keyboard. I'm also finding that the three higher strings will have to be slightly shorter than the three lower strings in order to work well enough with the frets all at right angles to the strings. This was expected, and I have more than enough adjustment range to take care of this. I'm also finding that to please my ear, I'll have to deviate slightly from the stretch compensated figures as the frets get closer to the bridge. I'm not sure, but I think this is because of the light strings I'm using. That or the compensation factor should have a slight non-linearity built in to it. These are all very subtle differences, but I figure if I get it as close as I can, then my playing won't sound so bad :)

Interestingly, the harmonic nodes on the strings don't correspond exactly to the position of the fret that should produce the same note. I have to attribute this to the increased string tension when fretted, and therefore the fret is somewhat behind where 'logic' says it should be. I compared this with my classical guitar, and it's harmonic nodes do coincide with the fret positions. However, fretting that note results in a tone that is sharp. If I could I'd move the bridge back, but that's not possible on that axe.

I continue to be amazed at the repeatability of the tuning as the strings are taken off and put back on as a group. It takes all of about five seconds to take them off, and about ten to put them back on. Turn one knob and all strings come into tune at once. One thing it can't do though is be tuned to a different key and have all strings come into proper tune for that key. For that you have to make individual adjustments.

This must be awfully boring for those of you who aren't musically inclined. Feel free to stop reading any time :) For those who have read to this point, I hope there has been at least something of interest. For my part, I'm glad to be back on this project again.

08-27-2007, 10:05 AM
I put the link up to show how far someone has gone to compensate for string size, length, whatever.

It apparently hasn't caught on to well as I have yet to see one, other than on that site. An idea like that can't be patented, so no one else seems to be making them. And two guitar players I know of, with at least a hundred years of playing between them, think it's a joke.

Darryl, when you get this done, some pictures would be great.

08-27-2007, 01:32 PM
A properly set up guitar - string height above frets - relief or no - will help to reduce the need for compensation. The fanned fret system is a solution to a problem that doesn't exist. The human ear is a tricky beast!

08-27-2007, 02:45 PM
I can sympathize with Evan's, well ,plight. I used to get really irritated in church on Sunday when the singer would be just a smidge off. My mother couldn't understand what was wrong.
That being said, have you thought of going fretless?

08-27-2007, 05:08 PM
That being said, have you thought of going fretless?

Or use the lute construction of tied-on gut frets. That way you can move the frets a little when you change key.

"Does anybody here have a key?"

"No, but I've got a lid out in the car."
(60's comedy group. I forget who)


08-27-2007, 09:54 PM
There will be pictures, but probably not until I'm back from holidays. I hope I can get the fretwork done before I go, but I don't want to rush it. The concept is simple- get it right the first time. The process is tedious, and I knew it would be considering how I wanted to approach doing it. This is probably what put me off this project in the first place.