Post by twostring on Dec 13, 2006 2:30:56 GMT -5
The title of this post is rhetorical. I'm going to answer the question here as best I can. If I leave anything out or have anything wrong, please respond.
Here's the rhetorical question again with more detail:
Q.
What's with adjustable bridges and why are they usually adjusted on a slant with the second string a bit offset from the others? This isn't about height adjustment, its about the other direction.
A.
Actually, it may look like only the second string bridge is out of line but look closer. Adjustable bridges are usually set up like two parallel slanting saddles (bridges), One is for the first two strings and the other is for the 3rd through the 6th.
Before I go into more detail, I'm pretty sure that configuration was set up to accomodate the first and second strings being a single wire and the rest of the strings being wound. If you get a set of light guage strings with the 3rd string unwound, you may want to adjust its bridge back in line with the first two strings.
More detail:
Theoretically, the 12th fret should be at the mid point between the bridge and the nut. That's because when you shrink a vibrating string to half its length, the pitch doubles (increases one octave). The distance from the nut to the 12th fret is the same for all six strings but the bridge location for each is in a different place. Something is fishy, right?
There are two reasons for the bridge not being at the theoretical true position. The first is called "stretch compensation". (You can read about this one on several web sites.) When you press a string down to a fret, you stretch it. That increases its pitch beyond what you would get by simply dividing its length in two. So, the bridge is moved back a bit to compensate. Usually the 1st string is located closest to the theoretical true location, the rest are moved back away, making the string longer.
Q.
"But", you ask, "Don't each of the strings get stretched the same amount?" "Couldn't you just keep the bridge parallel with the frets and move it back slightly?"
A.
No. The second factor affecting the bridge location (You might only read this here. I've never seen it anywhere else.) has to do with the stiffness of the strings. The end points of a vibrating string are called "nodes". Theoretically, those nodes are dead center on top of a bridge, a nut, or on top of fret you have your finger behind. The theory assumes the attachment of the string to those points is like a hinged or pinned point--like a jump rope coming out of a wooden handle. In actuality, the string crosses over those points "constrained" to them (to use an engineering term). The string doesn't bend freely from that point but must bend like a fishing rod you hold in your hand. The stiffness of the string passing over the fret moves the effective node away from the fret a bit which has the effect of the string acting a bit shorter than it actually is. The string has the same stifness regardless of which fret or nut or bridge it goes over. So as you fret higher on the neck, the error caused by this factor becomes an increasingly larger persentage of the vibrating length. Thicker strings are stiffer and cause node displacement more than thinner strings.
Ok, now you know mostly everything.
One more question:
Q. "How do they know where to put frets on a neck?"
A. The location of frets is based on the 12th root of 2. The ratio of the distance between any two adjacent frets and the bridge equals the 12th root of 2. Its actually pretty easy to set up an Excel Spreadsheet to calculate where frets would go on any length neck you want to make. You can also download "fret calculators" from the internet that can calculate actual fret locations and also add in compensation factors for each string.
-This is my first post.
Here's the rhetorical question again with more detail:
Q.
What's with adjustable bridges and why are they usually adjusted on a slant with the second string a bit offset from the others? This isn't about height adjustment, its about the other direction.
A.
Actually, it may look like only the second string bridge is out of line but look closer. Adjustable bridges are usually set up like two parallel slanting saddles (bridges), One is for the first two strings and the other is for the 3rd through the 6th.
Before I go into more detail, I'm pretty sure that configuration was set up to accomodate the first and second strings being a single wire and the rest of the strings being wound. If you get a set of light guage strings with the 3rd string unwound, you may want to adjust its bridge back in line with the first two strings.
More detail:
Theoretically, the 12th fret should be at the mid point between the bridge and the nut. That's because when you shrink a vibrating string to half its length, the pitch doubles (increases one octave). The distance from the nut to the 12th fret is the same for all six strings but the bridge location for each is in a different place. Something is fishy, right?
There are two reasons for the bridge not being at the theoretical true position. The first is called "stretch compensation". (You can read about this one on several web sites.) When you press a string down to a fret, you stretch it. That increases its pitch beyond what you would get by simply dividing its length in two. So, the bridge is moved back a bit to compensate. Usually the 1st string is located closest to the theoretical true location, the rest are moved back away, making the string longer.
Q.
"But", you ask, "Don't each of the strings get stretched the same amount?" "Couldn't you just keep the bridge parallel with the frets and move it back slightly?"
A.
No. The second factor affecting the bridge location (You might only read this here. I've never seen it anywhere else.) has to do with the stiffness of the strings. The end points of a vibrating string are called "nodes". Theoretically, those nodes are dead center on top of a bridge, a nut, or on top of fret you have your finger behind. The theory assumes the attachment of the string to those points is like a hinged or pinned point--like a jump rope coming out of a wooden handle. In actuality, the string crosses over those points "constrained" to them (to use an engineering term). The string doesn't bend freely from that point but must bend like a fishing rod you hold in your hand. The stiffness of the string passing over the fret moves the effective node away from the fret a bit which has the effect of the string acting a bit shorter than it actually is. The string has the same stifness regardless of which fret or nut or bridge it goes over. So as you fret higher on the neck, the error caused by this factor becomes an increasingly larger persentage of the vibrating length. Thicker strings are stiffer and cause node displacement more than thinner strings.
Ok, now you know mostly everything.
One more question:
Q. "How do they know where to put frets on a neck?"
A. The location of frets is based on the 12th root of 2. The ratio of the distance between any two adjacent frets and the bridge equals the 12th root of 2. Its actually pretty easy to set up an Excel Spreadsheet to calculate where frets would go on any length neck you want to make. You can also download "fret calculators" from the internet that can calculate actual fret locations and also add in compensation factors for each string.
-This is my first post.
