String height above bridge: How tall is too tall?

[Howard Klepper]

If your guitar has never been strung, you can expect the top to come up and that will bring your saddle down to a safer and more standard height.

[Talldad]

I think about it like this

Strings vibrate, this means they get shorter and then relax, this pulls the top of the saddle forwards by an amount (L) , which rotates the bridge through an angle (A). This oscillating pattern makes the soundboard active in many different ways.

The saddle acts as a lever on the bridge affecting the rotation.

Saddle Height Very Low
Same pull, D, leading to larger angle of rotation, A, bigger movement of the soundboard
Greater resistive force of the soundboard against large rotation, A, leading to lower sustain
More energy required to rotate bridge. The higher strings have more energy and the guitar will have a treble sound. A plinky plonky guitar

Saddle Height Very High
Small angle of rotation, A = smaller movement of the soundboard
Less resistive force of the soundboard against the bridge rotation, higher sustain
Less energy required to rotate bridge, the lower strings will have a much greater say, a too much bass sound

Saddle Height just right
Correct level of rotation and movement of top
Correct level of sustain and balance of different tones

There is no standard optimum height, the correct height depends on the engineering of your soundboard. If the top is over-engineered you need a higher saddle height to overcome the resistive forces. Vice-versa, if your top is under engineered a lower saddle height could be used.

And that is why guitar building is an art more than a skill.

Most Luthiers, I imagine, decide on their 2º neck angle, 13mm saddle height and they keep working on their soundboards until they master building one with just the right amount of flex and tonal qualities. When they have it right the guitar simply comes alive. This is why we are tapping and bending and listening to the boards, recording notes about its behaviour, one day we will know how a particular soundboard will sound before we finish the build.

This isn't to say that 13mm is the best height to design to but it does mean that if you have a 13mm saddle height, the reason your guitar sounds underwhelming is not the bridge.

[LouieAtienza]

Everything is relative. You could have a vey high string height, and a bridge that extends about an inch toward the soundhole, and greatly reduce the tosional effect of the string pull on the bridge. And you can further reduce that by extending the bridge toward the lower bout. And then there is the relationship of bridge to bracing and bridge plate. So a stiffly made (longitudally) top could likely handle more string height than one that's looser.

Which brings me to a theorem. If your strings are at a standard height above the soundboard, and you feel or find you need to add extra height to increase volume, the top may be too stiff...

[Talldad]

Agree with your idea that if you need to vary from your standard height it’s only as a compensation factor for building an over/under engineered soundboard.

If you take into account the curvature of the saddle on a steel string acoustic which favours the middle strings then calculating a perfect saddle height is close to impossible.

[ruby50]

At the factory, Martin installs one of 3 different thickness bridges depending on the needs of a particular body/neck. Do they vary by 1/32" in thickness??

If they are installing varying bridge thicknesses, that must mean they are going after a certain saddle height above the bridge.

So does ending up with a super-sounding Martin depend at all on that bridge thickness?

Ed

[LouieAtienza]

Originally Posted by Talldad

Agree with your idea that if you need to vary from your standard height it’s only as a compensation factor for building an over/under engineered soundboard.

If you take into account the curvature of the saddle on a steel string acoustic which favours the middle strings then calculating a perfect saddle height is close to impossible.

Yes... an 18" saddle radius would equate to about a 6% change in string height from middle to outer strings. I do wonder if the strings' volumes are affected individually due to the saddle curvature, or does it "average out" since the bridge moves as a unit? The strings also don't have the same exact amount of tension as well.

At the factory, Martin installs one of 3 different thickness bridges depending on the needs of a particular body/neck. Do they vary by 1/32" in thickness??

If they are installing varying bridge thicknesses, that must mean they are going after a certain saddle height above the bridge.

So does ending up with a super-sounding Martin depend at all on that bridge thickness?

Well if you think about it, if you went to a store or factory and played several Martins, or any other manufacturer's guitar, even among identically appointed and set-up (as much as possible) guitars, you will find one that you gravitate more toward. I'm not saying that the saddle or string height is the factor, but it probably is one of a few... Not saying that I've heard bad Martins (that were set up correctly with fresh strings) wither; it's just that some seem to stand out more than others.

[bausin]

>> Strings vibrate, this means they get shorter and then relax, this pulls the top of the saddle forwards by an amount (L) , which rotates the bridge through an angle (A). This oscillating pattern makes the soundboard active in many different ways.

I'm not following any of this. Why would a string get shorter when it vibrates? What does "makes the soundboard active in many different ways" mean?

[charles Tauber]

Variability.

Suppose we listed all of the factors that we could think of that influence the sound of an instrument. If we identified all of those that have "significant" variation in their properties, we'd have numerous simultaneously varying distributions. This gets into an area of discussion often called probabilistic design.

When using a probabilistic approach to design, the designer no longer thinks of each variable as a single value or number. Instead, each variable is viewed as a probability distribution. From this perspective, probabilistic design predicts the flow of variability (or distributions) through a system. By considering this flow, a designer can make adjustments to reduce the flow of random variability, and improve quality.

https://en.wikipedia.org/wiki/Probabilistic_design

When two probability distributions overlap, statistical interference exists. Knowledge of the distributions can be used to determine the likelihood that one parameter exceeds another, and by how much.

This technique can be used for dimensioning of mechanical parts, determining when an applied load exceeds the strength of a structure, and in many other situations.

https://en.wikipedia.org/wiki/Statistical_interference


When attempting to discuss a complex system that has many varying parameters, it is difficult to isolate a single parameter and examine the influence that variations in that single parameter have on the behaviour of the system. Thus, attempting to isolate the influence that the variation in bridge height or width has on a guitar's sound, while ignoring variations in bridge material and its stiffness, top thickness and its stiffness, brace locations, brace dimensions, brace stiffness, sound hole size... probably isn't a very fruitful exercise.

Originally Posted by ruby50

So does ending up with a super-sounding Martin depend at all on that bridge thickness?

When we acknowledge that there is little agreement on what a "super-sounding" Martin is - largely, it is a subjective assessment - it becomes largely speculation.

[Trees123]

Really appreciate your answer Talldad. Clears up a lot of mystery about how good sounding guitars work. Thanks.

[LouieAtienza]

Originally Posted by bausin

>> Strings vibrate, this means they get shorter and then relax, this pulls the top of the saddle forwards by an amount (L) , which rotates the bridge through an angle (A). This oscillating pattern makes the soundboard active in many different ways.

I'm not following any of this. Why would a string get shorter when it vibrates? What does "makes the soundboard active in many different ways" mean?

Saying the "strings get shorter" is something of a misnomer. The "speaking length" of the string varies in length as the bridge assembly rotates about an axis, somewhere around the front edge of the bridge. The string is like a heavy jumprope in a way - if you tied one end to a post and you held it at the other end and moved your hand up and down to create oscillations, the jumprope does not get significantly shorter/ If you looked carefully however under slow-motion video, you may see your hand moving slightly toward and away from the post. Unlike the jumprope, a guitar string does not move in perfect parabolic shapes. And it just doesn't move up and down (with the guitar in playing position.) It also moves toward and away from the guitar at the same time, in a three-dimensional pattern.

Also, because the soundboard is not evenly shaped, the braces are not exactly the same size and weight, and the strings are not exactly on the center of the soundboard, and the forces on the soundboard are not evenly distributed, and that soundboard does not have a perfectly even density or stiffness about it, etc., the soundboard will "oscillate" in many different patterns. One goal of many builders is the controlling of these "oscillations" and "nodes" on the plate, but that's a different story.

[Talldad]

Originally Posted by bausin

>> Strings vibrate, this means they get shorter and then relax, this pulls the top of the saddle forwards by an amount (L) , which rotates the bridge through an angle (A). This oscillating pattern makes the soundboard active in many different ways.

I'm not following any of this. Why would a string get shorter when it vibrates? What does "makes the soundboard active in many different ways" mean?

Apologies. Theres a big difference between knowing what you mean and writing it down! The strings don't get shorter. The distance between the Nut and Saddle gets shorter due to the pull of the string on the top of the saddle.

Darn it, I proof read that three times too!

[Talldad]

Originally Posted by LouieAtienza

Also, because the soundboard is not evenly shaped, the braces are not exactly the same size and weight, and the strings are not exactly on the center of the soundboard, and the forces on the soundboard are not evenly distributed, and that soundboard does not have a perfectly even density or stiffness about it, etc., the soundboard will "oscillate" in many different patterns. One goal of many builders is the controlling of these "oscillations" and "nodes" on the plate, but that's a different story.

I was starting to think that you were going to end this complicated sentence with "Theres no bloody way its going to make a sound!!!!!"

[LouieAtienza]

Originally Posted by Talldad

I was starting to think that you were going to end this complicated sentence with "Theres no bloody way its going to make a sound!!!!!"

I thought the same thing too LOLOL! Especially after I'm done with a pile of wood...

[Alan Carruth]

Talldad wrote:

"Strings vibrate, this means they get shorter and then relax, this pulls the top of the saddle forwards by an amount (L) , which rotates the bridge through an angle (A). This oscillating pattern makes the soundboard active in many different ways."

The strings do change tension as they vibrate: the tension rises twice per cycle. How much shorter they get depends on a whole bunch of factors.

"The saddle acts as a lever on the bridge affecting the rotation."

Yes: you got that right.

"Saddle Height Very Low
Same pull, D, leading to larger angle of rotation..."

Again, the distance between the nut and saddle is not getting shorter and longer by a set amount: that would presuppose an infinite Young's modulus in the string. The string pulls the saddle top toward the nut with a well defined force when it is displaced from a straight line, and that rocks the bridge forward.

"Saddle Height Very High
Small angle of rotation, A = smaller movement of the soundboard"


"Saddle Height just right
Correct level of rotation and movement of top
Correct level of sustain and balance of different tones"

Since we're dealing with a defined force rather than a defined motion the taller saddle actually produces more sound from the bridge rocking rather than less: there's more leverage rotating the bridge around it's axis.

At any rate, as I pointed out in a previous post, the tension change signal is not what makes most of the sound: that comes from vertical motion of the string relative to the soundboard which pulls the top up and down. This moves the top like a loudspeaker, which is 'way more effective at making sound than rocking the bridge.

I could say a lot more, but since I've already said it in this thread, and numerous other times in various groups, I don't want to repeat myself now. This all comes from experiments I've done, and fits with the well known physics of strings and guitar tops. Look up those other posts, read them through carefully, and get back to me if there's still something that needs explaining.

[lar]

I've learned a lot from this post. What a great exchange of information.

Speaking of experiments, I have some homework for all of you. This experiment should be video captured with your cell phone.

1) Cut a 2by4 1 foot long and drill a hole through it, in the end
(2by4 represents the saddle)

2) Tie a 6 ft long rope through the hole (rope = string).

3) Hold the wood vertically from the end opposite to where the rope is tied and have your wife on the other end of the rope apply tension and move the rope up-&-down VIGOROUSLY in a periodic motion (like jumping rope but purely in a vertical motion). To accentuate this motion, while moving the rope up&down with her arms, she should also jump up and down quickly.

4) Tell me what the dominant forces are that you feel while holding the wood (and the less dominant forces too). Are they in tension, bending, or vertical?

5) Oh - and while doing this, don't forget to stand on 1 leg and scream like a chicken.

5) Finally, to complete your assignment, pleeeeease send me the video.