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String height above top?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=19149		 Page 1 of 1
Author:  CraigSz [ Sat Oct 11, 2008 4:40 am ]
Post subject:  String height above top?
I have thought about this issue quite a lot of late as I fiddled about with the bridge on No 2 . I remember reading some time ago that there was a small optimal window for this measurement (½inch to the bottom of the low E). Any significant variation above or below this was detrimental to the tone. Too high necessitated heavier bracing to counter the added rotational forces on the top as well as the added stresses on the bridge itself . Too low results in reduced energy input from the strings. Less volume .
I know some use around 6mm for the bridge height + 1/8 to 3/16 saddle height = approx 9-10 mm total . Others use 10mm bridge + 1/8 -3/16 = 13-14mm total .
Just wondering if the different string heights would cause the top to respond (move) differently and accentuate certain frequencies while subduing others?
The main reason I have been deliberating over this is that I thought that I could probably remove another 10 or more grams of weight off my 38g bridge by dropping the height from 9.5mm down to 6- 6.5mm or so .
Of course then I would have to shave the equivalent amount from the fretboard. Also the saddle slot depth would probably need redoing.
In the end I’ve decided just to leave it as is, but curious now as to why others use the dimensions they do. I would love to hear your thoughts /ideas regarding this.

Regards
Craig.
Author:  Mike Collins [ Sat Oct 11, 2008 1:49 pm ]
Post subject:  Re: String height above top?
I always use a 10mm saddle height!
Bridge is 7mm-slots for the strings and a curved pin set-up!
I make my guitars for light guage -unless ordered differently!

I've played tops with more-13mm but I do not like the tone or the results of the torque on the top after a few years!!!!
I brace my tops for this set-up (10mm)so the tone and tension are just where I want them!

Mike
Author:  Alan Carruth [ Sat Oct 11, 2008 6:26 pm ]
Post subject:  Re: String height above top?
I like to keep the string height off the top below 12mm (1/2"), but don't like to go much lower than 10 on 'normal' guitars. When it's to high the static torque gets to be aproble, over the long term, and when it's too low people ding up the tops too much.

When the string is displaced the tension is higher than it is when it's not. Since the tension rises both when the string is displaced 'up' or 'down' (or when it's to the left or the right when it's vibrating 'side to side') there is a peak in the tension twice for every cycle of vibration. This octave doubled signal pulls the top of the bridge tward the neck, and the higher the strings are off the top, the more of this there will be in the sound. Note that the tension change force is normally less then the transverse force, and that the top is built to resist bridge torque, which limits how much power can be developed from the tension change signal. Still, I found that when I increased the height of the strings off the top on one guitar by 10%, the even order partials in the output increased by something like 25%, iirc. IMO this is most likely the main mechanism for tone change when the saddle is raised.

Of course, the transverse string force only pushes on the top to the extent that the string moves 'up and down', but the tension change is effective no matter which direction the string vibrates. I checked that out once, setting up a rig that could drive the string in only one plane, and measuring the output of the guitar with a dB meter. I can't find the numbers offhand, but I believe the instrument was at least 10dB (ten times the power) louder when the string was moving 'up and down' relative to the soundboard as it was when the string moved crosswise. Again, the tension change isn't all that effective at driving the top.
Author:  Hesh [ Sat Oct 11, 2008 8:31 pm ]
Post subject:  Re: String height above top?
I use 1/2" for my guitars too. This number came from John Hall and everything I have ever learned from John has been spot on. If I recall correctly this is Martin's number too.

Right or wrong it is a number that I stick to with decent results, no cracked bridges etc., and yet another variable in my building that has been eliminated.
Author:  CraigSz [ Sun Oct 12, 2008 3:04 am ]
Post subject:  Re: String height above top?
Thanks Mike, Al and Hesh for your responses.

Mike can you tell me how you perceive the difference in tone that you mentioned . I know this is all vey subjective but I would like it if you could describe it generally for me.

Al the "octave doubled signal" component you mentioned ,is that the same as the "even order partials"?? Sorry trying to understand idunno .

Hesh thanks for the post. Its good to know for future reference. Obviously great guitars are built with different string heights. It's also obviously important to match the bracing to both the gauge of string and the saddle height.

Regards

Craig.
Author:  Alan Carruth [ Sun Oct 12, 2008 4:14 pm ]
Post subject:  Re: String height above top?
Craig asked:
"Al the "octave doubled signal" component you mentioned ,is that the same as the "even order partials"?? Sorry trying to understand "

Yup. A 'perfect' open A string would have energy in the signal at 110 Hz, 220, 330, 440, 550, 660, and so on. The A an octave above that has energy at 220, 440, 660, etc. Those are the even order partials of the lower note, and that's what the tension change signal of the open A string is putting into the top. So the tension change signal is 'octave doubled'. Nothing to apologize for; it's a little confusing until you get used to it.
Author:  CraigSz [ Mon Oct 13, 2008 6:20 am ]
Post subject:  Re: String height above top?
ToddStock wrote:
As a secondary issue, some bridge designs are already too tubby to execute in ebony or african blackwood with acceptable mass, and raising the string height above 1/2" exacerbates the problem

I have to agree as I proved this to be true only recently. I plan on redesigning my next bridge to reduce the height and the depth( more even width right across)
Al thanks for the explanation . I really marvel at the way you are able to make sense of the myriad of things going on inside and outside the instrument . I never thought I'd be interested in the scientific side but I was wrong . [:Y:]

Regards

Craig
Author:  Frank Cousins [ Mon Oct 13, 2008 6:29 am ]
Post subject:  Re: String height above top?
I can only really comment from a playing perspective, since I have way too little experience from a building point of view. It is still a mystery to me, but I have found generally that I prefer a set up with a steeper string break angle between saddle and bridge - which mostly, (mysteriously not always) tends to give a tighter feel to the strings and for me playing wise a 'crisper attack' - and these generally have a slightly higher string height above the 'board - which IMHO gives a better volume and tone - I only tend to have a low action set up and thus reduced height above the board on a couple of guitars that have fitted pick ups and tend to only be used live and amplified.

I would be interested if there is any concensus amongst teh more experienced builders on how best to achieve that 'tight' and crisp attack if possible at lower saddle heights?
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