string height at bridge.

Construction and repair of Classical Guitar and related instruments
Alan Clark
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string height at bridge.

Post by Alan Clark » Wed Mar 16, 2016 7:19 pm

I am currently drawing out my plan for a Classical guitar, and need to settle on a measurement for the height of the strings above the front, at the bridge, which would seem to be a critical measurement. Cumpiano gives this as half an inch. Romanillios gives it as 10.5mm at the treble and 11.5mm at the bass.
Is there an optimum height?

Alan

John higgon
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Re: string height at bridge.

Post by John higgon » Wed Mar 16, 2016 7:55 pm

In reality the string height at the saddle will have to be adjusted to gain the required action at the twelfth fret. This is a more critical measurement than the height of the string at the bridge, as I understand it. As a rule of thumb, extra height at the saddle translates into additional volume. Many makers ensure that the bass strings are a little higher than the trebles, but again, this is largely to do with the action. It's usual to find saddle heights anywhere from 9mm to 13 or 14mm. Less than 9mm means that there is insufficient back-and-forward rocking of the saddle, hence reduced volume. Over 14mm makes for a high break angle, which might contribute early failure of the strings. Break angle is another important consideration: too shallow an angle will risk the string vibrating all the way to the tie block, rather than the vibrating length of the string stopping at the saddle. These are the thoughts of an inexperienced and amateur guitar maker, so take more notice of other posts from more experienced makers! :desole:

Alan Clark
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Re: string height at bridge.

Post by Alan Clark » Wed Mar 16, 2016 8:11 pm

John, for an inexperienced amateur you seem to have come up with a pretty comprehensive answer to my question. Thank you.
One small point. I would have thought you could have your height of choice at the twelfth fret ,and at the bridge, by adjusting the neck angle and fingerboard thickness....

Alan

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Chris Sobel
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Re: string height at bridge.

Post by Chris Sobel » Thu Mar 17, 2016 5:07 am

Alan,

Plan on 10-12mm, but remember that when the neck first pulls up under tension and the bridge rotates, whatever you plan for will lessen by a little bit. How much is dependent on neck and top stifness, but I usually find I have to add .5mm to my unstrung numbers. So if you draw out 11mm, it will actually end up 10.5 or so. You can further plan that after 10-15 years that might become 10mm... So I like to start out around 11-11.5. The Romanillos numbers are what I do exactly.

The volume does not really increase or decrease with the saddle going up or down, but it can sound like it because the higher the saddle, the more 'pop' you get to the note. But I think the sound output is the same, it just sounds different.

You're exactly right that you can have your height of choice by adjusting the neck angle and fingerboard plane. The fine adjustment comes with lowering or raising the saddle once complete, but look at it like a fine adjustment not a gross adjustment. The neck angle and fingerboard plane are going to put you in the rough ballpark plus or minus 1mm. It's very hard to make gross action adjustments via the saddle; even a well designed bridge can't adjust the action much more than 2mm at the 12th fret because it equates to a 4mm difference at the saddle.

Chris
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Alan Clark
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Re: string height at bridge.

Post by Alan Clark » Thu Mar 17, 2016 10:16 am

Chris,
I am very grateful for your full and clear answer to my question. I can now go ahead with confidence. I have built a couple of Classical guitars, 45 years ago; the first two instruments I ever built. I got the neck angle wrong on the second and had to rescue things with a bridge saddle which is too high, and this has thrown too much strain on the top. I still have the guitar, and it has just about held up, but I don't want to repeat my mistake. Hence my advance planning and question about string height.

Alan

Stephen Eden
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Re: string height at bridge.

Post by Stephen Eden » Thu Mar 17, 2016 2:05 pm

String height is something that should be considered in the overall design of the instrument. There is no one magic point at which it works optimally for all guitars. I know it's no help saying that though. My guitars have the heights of around 9mm trebles and 10mm basses. It works well with my guitars.

Also although raising the string height can have the effect of raising the volume it probably does this because the guitar needed it. Some very LOUD flamenco guitars have their string height as low as 6-7mm.

Alan Clark
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Re: string height at bridge.

Post by Alan Clark » Thu Mar 17, 2016 3:07 pm

Thanks Stephen. Would it be true to say that a low string height at the bridge is suited to a lightly constructed guitar with, perhaps, fairly low tension strings? And a higher string height more suited to a stiffer top? I have no experience in guitar making, but am just trying to come to some intuative understanding based on my experience as a former lute maker.

Alan

simonm
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Re: string height at bridge.

Post by simonm » Thu Mar 17, 2016 3:54 pm

Chris Sobel wrote:... So if you draw out 11mm, it will actually end up 10.5 or so. You can further plan that after 10-15 years that might become 10mm…
Surprises me. You must be expecting the top to cave in quite a bit to counteract the neck pulling up. This seems like a lot. I have never measured it long term. My intuition would have said the distance would normally get bigger rather than smaller. Curious.

Stephen Eden
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Re: string height at bridge.

Post by Stephen Eden » Thu Mar 17, 2016 4:25 pm

Yeah I think that would be fairly true to say. The easy way to think about it is that the higher the string height from the sound board the more torque you put through it. The bracing would need to help counter that so would need to beefed up as well. It will also effect how stiff the strings feel in your hands

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Chris Sobel
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Re: string height at bridge.

Post by Chris Sobel » Thu Mar 17, 2016 5:04 pm

simonm wrote:
Chris Sobel wrote:... So if you draw out 11mm, it will actually end up 10.5 or so. You can further plan that after 10-15 years that might become 10mm…
Surprises me. You must be expecting the top to cave in quite a bit to counteract the neck pulling up. This seems like a lot. I have never measured it long term. My intuition would have said the distance would normally get bigger rather than smaller. Curious.
Think about it though--if the neck pulls up it raises the action at the 12th feet. In order to compensate, you have to lower the saddle. Thus you end up with a lower saddle height than if you drew it out.

Chris
CE Sobel Guitars

Alan Carruth
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Re: string height at bridge.

Post by Alan Carruth » Thu Mar 17, 2016 5:07 pm

Chris got it right, if my measurements are to be believed: there's a change in timbre, but not power.

Several years ago I got interested in the discussion about break angle. Folks would post on (mostly steel string) lists on line to say that they had increased the break angle over the saddle and the sound improved. When I asked them how they'd changed it, usually they'd say it was by putting in a taller saddle. Since this alters both the break angle and the string height off the top, I did an experiment to see which of the two variables was having the biggest impact.

I set up a 'test mule' classical guitar with an 18-hole tie block. This allowed for tying the strings in two ways: the usual one into the holes in front of the block, or by passing the string over the tie block and into the holes in the back. The usual tie gave a break angle of about 25 degrees, while the modified tie gave a minimal break of around 6 degrees. I also made a taller saddle so that the 'back' tie would give a 25 degree break. The regular saddle put the strings 11 mm off the top, while the tall saddle had them at 18 mm off the top (don't try this at home!).

I used a mechanical plucker based on the 'wire break' method to get uniform plucks of the open strings for each setup case. The sound was recorded on my computer using a microphone one meter out in front of the guitar in a 'semi-anechoic' closet. I compared six plucks on each string to make sure they were reasonably uniform. I also made up a 'synthetic strum' for each case to use in listening tests. These were randomly paired and played back through ear phones. Listeners were asked only if the sounds were 'the same' or 'different'.

Objective measurements showed that there was no significant difference in the maximum amplitude, or rise or fall time, for the plucks in any case: the overall power output of the guitar was the same as far as could be told. There was also no difference in the spectral content of the sound for the two cases where the string height off the top was 11mm: changing the break angle does not seem to alter the amount of sound the strings can put into the top. Raising the string height to 18 mm off the top resulted in two changes in the sound output:
1) there was more energy in the second partial, and, in some cases, in the fourth partial of the string, and
2) there was more energy at the frequency of the longitudinal 'zip tone'; a compression wave within the string that is normally up around the 7th or 8th partial.

String tension rises twice per cycle at it vibrates, and this pulls the top of the saddle toward the nut, rocking the bridge at even multiples of the fundamental pitch and producing some top motion. The longitudinal 'zip tone' wave acts in much the same way. A taller saddle gives both of these signals more leverage to produce sound. Presumably, energy fed into the system in this way reduces the power available to drive the bridge through the more effective 'transverse' force, which may explain why there's no more power available with the taller saddle.

In the listening tests people were basically guessing when they were called on to compare the two 'low' saddle cases with different break angles. Virtually everybody heard the difference between the 'low' and 'high' saddle cases. Thus it seems that the measured differences in harmonic content of the sound between the cases translated pretty reliably into perception.

Although I did not ask for impressions of timbre, it does seem as though the increase in output at the 'zip tone' pitch could well be perceived as a 'sharper' or 'more cutting' timbre. It comes in at relatively high frequencies, and is usually more or less dissonant, so it would tend to stand out .

Sorry for the long post, but I wanted to get a summary of the work out.

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Chris Sobel
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Re: string height at bridge.

Post by Chris Sobel » Thu Mar 17, 2016 5:15 pm

Thanks Alan, that was highly informative. Also thank you Stephen for your insights.

Chris
CE Sobel Guitars

Dave M
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Re: string height at bridge.

Post by Dave M » Thu Mar 17, 2016 5:38 pm

Alan that was a pretty serious experiment and an important result. Thanks for putting up the post.


Dave
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Alan Clark
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Re: string height at bridge.

Post by Alan Clark » Thu Mar 17, 2016 5:43 pm

I have learned a lot from the very generous replies to a basic question. Thanks again everyone.

Alan

simonm
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Re: string height at bridge.

Post by simonm » Thu Mar 17, 2016 7:18 pm

Chris Sobel wrote:
simonm wrote:
Chris Sobel wrote:... So if you draw out 11mm, it will actually end up 10.5 or so. You can further plan that after 10-15 years that might become 10mm…
Surprises me. You must be expecting the top to cave in quite a bit to counteract the neck pulling up. This seems like a lot. I have never measured it long term. My intuition would have said the distance would normally get bigger rather than smaller. Curious.
Think about it though--if the neck pulls up it raises the action at the 12th feet. In order to compensate, you have to lower the saddle. Thus you end up with a lower saddle height than if you drew it out.

Chris
Thanks Chris. I thought you were referring to geometry not to the result of the corrective action: it didn't occur to me that you meant the result after lowering the saddle. :-)

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