Intonation difference between strings

[funnyfingers]

Why can the effective length of the E string be different from the G string yet all of the notes are right on for both strings?

[Sni77]

The different thickness of the string usually determines its stiffness. A stiffer string will vibrate less close to the nut and the bridge, which is why you need to compensate. Makes sense?

[Bruce Johnson]

To clarify the terms: Intonation means making the adjustment so that the pitch at the 12th fret matches the pitch at the open fret (or however you are setting it). The intonation is adjusted by moving the bridge saddle forward or backward a small amount.

Compensation is the small distance that the saddle ends up beyond where it should theoretically be for the scale length of the bass, after you've adjusted the Intonation. The Compensation distance will be different on each string, varying from about 1/16" on a low-action high string to about 1/4" on a high-action low string.

The reason that you need that Compensation is that, when you push down on a string, you are slightly increasing its tension, because you are bending it just a bit. Because of that, if the saddle is exactly at the scale length distance (no Compensation), the fretted note will go slightly sharp. So, you add Compensation, moving the saddle back just enough to bring the octave note down to the right pitch.

How much Compensation you will need for each string will depend on three things:

The type of string: Usually, springier strings like roundwounds will need less Compensation than stiffer strings like flatwounds.

The gauge of the string: The larger the string, the more Compensation is needed. That's why you see more on the E than on the G.

The Action height: The higher the action, the more Compensation is needed, because you have to push the string down further.

That's about it. All strings will need some amount of Compensation to be Intonated correctly. The Compensation will always be a small amount of extra length, moving the saddle back away from the neck. That's why, when you locate a bridge on a new instrument, you need to first move the saddles up to the forward end of their travel, and then locate that to the theoretical scale length point. The saddles need to have room to move back, not forward.

[funnyfingers]

Appreciate it!

[megafiddle]

Both of the effects described above are involved.

There is a bridge called the "Perfect Tune Bridge" which maintains constant tension on
the strings. So much so that you can't bend a string - you can slide it along the fret,
but the pitch doesn't change. (a limit or stop has to be set right at the point where
bending begins to allow bending).

The strings with this bridge should show no increase in tension due to fretting. Yet the
saddles are still compensated in the typical pattern. I haven't used this bridge, but have
inquired about it and apparently this is indeed the case. The strings still need intonation
compensation at the saddles.

I call it "end effect", some call it "end tension". A "perfect" string would vibrate as if the
ends of the string were attached to the nut and saddle with tiny swivel joints. It would
be able to bend sharply right at the take-off point at the nut and saddle. But real strings
are too stiff to bend right there. The string tries to remain straight for a short distance
where it comes off the nut or saddle. The heavier the string, the longer this distance is.
The result is that the string acts as if it were shorter than it actually is. So the saddle is
moved back to compensate.

[walterw]

yup; here's a crude visual of the effect:

riis ("zooberwerx") hipped me to this effect when he pointed how taper-core B strings intonate very differently, needing much less compensation due to the skinny section of string over the saddle beginning its flex much closer to the actual contact point.