Compensated bridges

[John Bertotti]

I am curious what exactly makes it necessary to compensate a bridge?
Is it the material in the strings is it the type of tuning why prevalently use? I mean why can't we just tune up intonate and be done with it? Thanks all John

[Luthier Vandross]

Huh? The compensation IS the intonation.

It's string gauge - vs -string length -vs- fret height.

You can't really be in-tune, until you are slightly out of tune. ;)


M
-Feiten Tech-

[John Bertotti]

Apparently I wasn't clear. What makes it needed other than the bridge placement? Even if it was string diameter I would think it would be more progressive from the E to the G. Perhaps it has something to do with the strings tension. I am curious> John

[sunburst]

A bigger string pulls sharp more as you fret it. That means bigger strings note sharper and have to be compensated.
The wound strings behave as tho they were the diameter of the string core without the windings. They act like smaller strings when fretted, so don't need as much compensation as they would if they were unwound and that big.

[greg boyd]

Hi,

Just a blurb on Compensation of bridge saddles...

It can easily be shown that a straight-across mandolin bridge will play out of tune as you fret. This is because the different string gauges, coupled with how tight they are tuned up to reach desired pitch, work to cause differing degrees of stretch when pushing them down to the frets.

It seems that most modern mandolin makers still follow the convention set by Gibson 80 years ago as far as "where" the various strings are moved forward or backward of the "centerline".

It is my opinion that this conventional compensation is not accurate at all...
I am referring to the standard compensation of:
E-string - all the way "forward"
A-string - almost to opposite backward position
D-string - "forward" but not all the way
G-string - "back", but not all the way

I think this needs to be changed in convention to work better with today's string gauges in a manner like this:
E-forward
A-forward part way
D-back part way
G-back(2/3???maybe)

I can't say how many times I have spent time finding the perfect intonation of a mandolin for E & G strings, only to have the A & D sound like their respective compensation placement should be "swapped"...

- Greg Boyd
House of Fine Instruments
www.gregboyd.com

[John Bertotti]

The way I see it I'll be making my own bridges and compensating them as needed for the strings I use. I am currently working on an A and an F style. I am gathering wood for some bowls also. I currently only have a bowlback and that is what started my intonation wonderings. Mainly because it doesn't use a compensated bridge. I am assuming this is because of the shorter scale and lighter strings as eluded to earlier. Thanks all for your comments they are always helpful. John http://www.mandolincafe.net/iB_html/non-cgi/emoticons/cool.gif

[Luthier Vandross]

Intonation, or not, you will never play -in tune- without a tempered tuning system.


M

[John Bertotti]

tempered tuning system? sounds like a new post to me. I can't begin to explain how far that just zoomed over my head. No seriously I've heard of it but never heard it. I've read of a lot of different tunings but haven't heard anything but standard A440hz. I imagine we could always go back to movable frets. John

[WaywardFiddler]

Intonation, or not, you will never play -in tune- without a tempered tuning system.

Ya gotta 'splain that one. I'm not sure what you are trying to say... with equal temperament tuning, the notes are spaced by a fractional power-of-two series, so they are almost always out of tune a wee bit with each other (fundamental and 5th excepted, but pick your fundamental...) but out of tune always by only a small amount, so that you don't notice. Or are you referring to something else? I don't get your drift.

But then, hey, I'm studying violin, and we use vibrato to cover our tracks :-)

-dave

[Luthier Vandross]

Violin! The big one, or the little one? ;)

The Feiten system.

The problem is your 3rd if your 5th, and 5th if your 3rd.. tail chasing, forever, trying to find the magic flat... it's not there.

The strings all have to have their own scale, and the frets must be dead on, with a very proper crown, nothing else sounds good.

The Feiten system is a shelved nut, when you have a correctly intonated saddle, and a nice peak on those teeny frets, ho my cod. Yah, I gotta get certified for the acoustic stuff, and soon. Man.


M

[Lee]

The general rule of thumb is to place the bridge so that the note produced when the 12th fret is played is the same note produced when the harmonic of the string is played by lightly touching the string at its mid-point at the 12th fret. The person who started this post has asked why does the bridge need compensating. Obviously the 12th fret goes straight across so by compensating the bridge the 12th fret won't necessarily be at each strings' exact midpoint. Here's why: First, a string vibrates three ways. The first is called Transverse vibration. It's the sideways motion visible to the eye. There is also Longitudinal vibration which is the action of areas of higher or lower tension moving up and down the length of the string. And there is torsional vibration which is the twisting motion caused in the string by the plucking action of your finger or pick; as opposed to the hammer type of action produced by a piano. I won't discuss longitudinal or torsional vibration as it has little relevance to this discussion. The fundamental tone of a vibrating string is mostly due to the tranverse vibrations occuring over the entire length of the string. The first harmonic (an octave higher than the fundamental) is due to the string's transverse vibrations occuring over each half of its overall length. You've probably seen an exagerated picture of this; it looks like a sine wave and the mid-point of the string has no transverse vibration. This is called a node point. The first harmonic of the string is easily played by lightly touching the mid-point node. There are other node points as well. A string's second harmonic is produced when the string vibrates over one-third of it's length. The harmonic series diminishes in volume but are generally audible past the seventh harmonic. We should know that a note an octave higher than another is vibrating at twice the cycles/second (hertz). Middle A is accepted to be at 440Hz. By definition the one an octave higher is at 880Hz. Unfortunately the mathematics don't describe music. The frequency of a vibrating string is largely a function of it's mass, tension, material, and diameter. Change one, and something else will too. A string has a certain length to thickness ratio, and this is commonly called stiffness. Because the first harmonic of a string is only vibrating over half the length of the fundamental, their stiffnesses are not the same which produces an interesting phenomena: The first harmonic vibrates slightly quicker than twice the fundamental. So now we have a problem; if we adjust the bridge position so that (on one string) the 12th fretted note and the mid-point harmonic are exactly pure and "beatless" with each other, then we will have "stretched" the octave from the open string up to the 12th fretted note. Now, because each of the strings' masses, tensions, materials, diameters and frequencies all don't form the same ratio, then the bridge must be compensated to account for different amounts of octave stretch. So, in short, compensation is made necessary because of string stiffness. And this also explains why the bridge may need adjustment when you try a different set of strings.

[Luthier Vandross]

That's what I said!

M http://www.mandolincafe.net/iB_html/non-cgi/emoticons/wink.gif

[John Bertotti]

Thanks everyone for your replies, lee957 you broke it down into what I was looking for much gracious. John

[Lee]

Thanks Reesaber. It's not unusual for us aural piano tuners (no electronic devices) to have a bizzarre fascination with string physics and scale theory. It's Friday night and I'm going home now to practice the Marpurg temperament. Is that sick or what?

[pickinpox]

Excellent explaination Mr. Knot.

I'll add that the phenomenon that Lee describes is often referred to as "inharmonicity." For this reason piano tuners "stretch the octaves" in other words, tune the upper registers sharp and leave the bass slightly flat. Better pianos generally need less stretching and sound less muddy.