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Thread: String angle over bridge

  1. #1

    Default String angle over bridge

    What sort of angle is conventional over the bridge? I've drawn up my lockdown project with about 12 degrees, same as over the nut, and the bridge is coming out spectacularly high off the body. The only model I have here is my antique Washburn bowlback, which has nothing like that, but even if that instrument was best practice then, is it now?

  2. #2
    Registered User sunburst's Avatar
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    Default Re: String angle over bridge

    There are various schools of thought on this.
    One is:
    A sharper breakover angle applies more static downward bridge pressure to the top so string motion drives the top better.
    I don't subscribe to that notion but instead adhere more closely to the school of thought that says:
    Only enough static downward pressure on the bridge to hold it in place is needed.

    Static downward pressure does nothing to drive the top. The top is driven by string motion and thus it is the dynamic pressures that drive the top. Why would we want to limit top motion by pressing a bridge hard against the top damping some of it's motion?

    In practice, there is probably an optimal breakover angle and static downward pressure for each instrument depending on type of construction, materials used, desired sound and perhaps all sorts of other things. I don't know what standard angles are for various types of mandolins, but I'm not sure standard angles are optimal in all cases.

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  4. #3
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    Default Re: String angle over bridge

    Quote Originally Posted by sunburst View Post
    There are various schools of thought on this.
    One is:
    A sharper breakover angle applies more static downward bridge pressure to the top so string motion drives the top better.
    I don't subscribe to that notion but instead adhere more closely to the school of thought that says:
    Only enough static downward pressure on the bridge to hold it in place is needed.

    Static downward pressure does nothing to drive the top. The top is driven by string motion and thus it is the dynamic pressures that drive the top. Why would we want to limit top motion by pressing a bridge hard against the top damping some of it's motion?

    In practice, there is probably an optimal breakover angle and static downward pressure for each instrument depending on type of construction, materials used, desired sound and perhaps all sorts of other things. I don't know what standard angles are for various types of mandolins, but I'm not sure standard angles are optimal in all cases.
    I would b an interesting experiment to make a tailpiece that let you vary the angle and see what the results were.

  5. #4

    Default Re: String angle over bridge

    Or even just something to change the downforce at constant break angle, plus an appropriate laser vibration map. I’m guessing a really minor effect. Just looking over my little collection, they’re all visibly different, even between the bowlbacks where this angle defines the whole instrument shape. Now in something like a banjolin, a bit different - an unsupported edge tensioned membrane.

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    Registered User sunburst's Avatar
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    Default Re: String angle over bridge

    Quote Originally Posted by Nevin View Post
    I would b an interesting experiment to make a tailpiece that let you vary the angle and see what the results were.
    If you can think of something, rest assured that it has been tried.

    FWIW, banjo players frequently change tailpiece height and/or angles to raise or lower downward force on the bridge, and sound differences are quite noticeable. It is tempting to think that the same would be true for mandolins, but it is not. Pressing down harder on a banjo bridge effectively tightens the head, essentially stiffening the top of the instrument. Pressing down harder on a mandolin bridge does not change the stiffness of the top, though it can deflect the top downward a little.

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  8. #6

    Default Re: String angle over bridge

    Quote Originally Posted by sunburst View Post
    I don't subscribe to that notion but instead adhere more closely to the school of thought that says:
    Only enough static downward pressure on the bridge to hold it in place is needed.
    I suppose logically there must be enough static pressure to ensure that the bridge is making some positive pressure on the body right through the vibration cycle. Beyond that who knows.

    This is an interesting digression, but I would be grateful for some actual numbers, although at the current rate of progress converting timber to the thickness I want with the limited tools (and skill/confidence to use them) I have here it may be a day or so before I need them.

  9. #7
    Registered User sunburst's Avatar
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    Default Re: String angle over bridge

    Quote Originally Posted by JimCh View Post
    I suppose logically there must be enough static pressure to ensure that the bridge is making some positive pressure on the body right through the vibration cycle. Beyond that who knows...
    I'm not sure what you mean by the 'vibrating cycle', but the bridge "feels" straight tugs from the vibrating strings. Square pulses, as I've heard them described. The bridge and the top respond as a unit (and in fact, the whole instrument), with the top vibrating in it's normal modes of motion. It is hard for me to infer an optimal bridge pressure for that, just as a thought experiment.
    The bridge needs to be held in place firmly enough so that the player does not move it with aggressive playing, and according to some that amount and no more is optimal.
    If there is a measurable difference in sound because of bridge pressure, then it becomes subjective since sound is subjective, and in that case there can be no optimal bridge pressure.

  10. #8

    Default Re: String angle over bridge

    Jim, your post a month ago was about a “semi hollow” project, which I suppose means electric. Break angle would be of even less importance. Bridge height would relate to pickup placement if the sound relates to disturbances in a magnetic field instead of wiggling wood. Interesting question though!

  11. #9

    Default Re: String angle over bridge

    John, correct, but I’d guess that anyone who hasn’t had a reason to think about it supposes that the bridge moves up and down, which is what Jim probably meant. Strings vibrate transversely, in any plane, but by definition, the nut and bridge are being ‘tugged’ along the string direction. They are nodes. The bridge therefore really rocks fore and aft and that replicates the sound wave in a complicated way at the instrument top. But the force still has to be a sinusoid because that’s what the string motion is. The square wave idea may relate to the fact that the string is set in motion by being plucked, which is an impulse, with a fast rise time, and that impulse is transmitted through the bridge, but the string itself then is in ‘simple harmonic motion’ with a single fundamental and its harmonics.
    On an electric the bridge can also be the tailpiece, and effectively immobile.

  12. #10

    Default Re: String angle over bridge

    Interesting, I was trying to figure out how the bridge could be a node and still feel the sound earlier, but that it feels changes in string tension as the string vibrates makes more sense.

    So does the sound just come from the bridge rocking fore and aft, or does deflection of the bridge/soundboard assembly under the varying string tension come into it?

  13. #11

    Default Re: String angle over bridge

    Anyway, my takeaway from that delightfully nerdy and very informative discussion is that its probably not too critical, so if I aim for a break angle a tad in excess of that of my vintage Washburn I should be pretty safe.

  14. #12
    Registered User sunburst's Avatar
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    Default Re: String angle over bridge

    Quote Originally Posted by JimCh View Post
    ...So does the sound just come from the bridge rocking fore and aft, or does deflection of the bridge/soundboard assembly under the varying string tension come into it?
    The pulses of energy from the strings set the whole instrument in motion, and the laws of physics tell us that all parts of the instrument move in their respective normal modes of motion, and that they can do nothing else. Anything that sets the instrument in motion accomplishes the same thing; exciting the normal modes of motion. That is why we feel an instrument "vibrate" is response to music, or any loud-enough sound.
    The pulses that the bridge "feels" are two per cycle. When the string is plucked it is deflected from it's straight position and then released to go back to it's straight position. Because of inertia, it overshoots the straight position and keeps moving until it's energy is overcome by tension, then it reverses and the cycle repeats. Each time the string stops and then goes the other way the string is at maximum tension for that cycle. Those are the tugs that the bridge "feels". Whether the bridge moves up and down, side to side, or fore and aft, it excites the whole instrument into moving in it's normal modes of motion.

    That is a simplified and not completely accurate explanation, but not only can things become awfully complicated-sounding, but I can get in over my head pretty quickly when trying to explain things further.

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  16. #13
    Registered User sblock's Avatar
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    Default Re: String angle over bridge

    The topic of string break angle over the bridge has been fairly extensively discussed in the MC Forum before. I would suggest checking out this thread right away:

    https://www.mandolincafe.com/forum/t...string-tension

    and see especially Post #68 in that thread, for the correct formula to use to compute the downbearing force on the top exerted by the bridge, as a function of (1) total string tension and (2) string break angle over the bridge. (Unfortunately, several folks derived or used the wrong formula, so take care!)

    You can think of the downbearing force as a "displacement offset", i.e., a "pre-loading" of the mandolin top, before the vibrating bridge -- driven by vibrating string motions -- produces its own additional, up-and-down displacements.

    Now, here's some important physics associated with pre-loading the top: If the top acted like a purely linear spring for up-down motion (definition: a linear spring is one where the displacement is linearly proportional to the force applied, and that constant of proportionality is the stiffness), then the amount of any initial downbearing force would NOT affect any subsequent motion of the top arising from the vibrations imparted by the bridge. That's because the stiffness is CONSTANT.

    However, most mandolin tops tend to act more like nonlinear springs, not like linear ones. As the top gets pre-stressed more and more, it also tends to get a little bit stiffer. Therefore, the downbearing force applied via the resting string tension can, and does, affect the amount of up-and-down motion subsequently imparted to the top by the bridge, via the strings. And that will affect both the volume and the tone. In addition to all that, the degree of pre-stressing on the top can affect its damping (vibrational loss of energy per cycle), which is a whole 'nuther can of worms. But the bottom line is that the string break angle can most assuredly affect the sound of a mandolin, provided that it is sufficient to bring the top into its nonlinear stiffness regime. So, for small break angles and lower string tensions, there is virtually no effect. In this regime, the top is acting like a linear elastic element. The effect kicks in at larger a tensions and break angles, however! In this regime, the top stiffens and starts to behave like a nonlinear elastic element.

    Please see Post #73 in that thread for a longer discussion of break angle and tone.

    I hope this simplified explanation is understandable to all.
    Last edited by sblock; May-14-2020 at 3:29pm.

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  18. #14
    formerly Philphool Phil Goodson's Avatar
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    Default Re: String angle over bridge

    I know that we have discussed node patterns and such of mandolin tops and these observations are important in knowing what building points are important.

    I also remember seeing oscilloscopic pictures comparing a trumpet to a French horn, showing the difference of many spikes at many frequencies which characterized the sound of a trumpet, vs the 'almost pure sine wave' appearance of the French horn pattern.

    I'm wondering if such tracings of various mandolins could ever be used to "oscilloscope tune" (instead of tap tune) the mandolin to sound with a specific tone and timbre. Has anyone ever made such an attempt?

    Merely looking at nodal patterns seems a little like looking for potholes in the roads by studying a satellite image of the entire country.
    Phil

    “Sharps/Flats” “Accidentals”

  19. #15

    Default Re: String angle over bridge

    thanks for that thread, I dunno, I tried several search strings but never turned up that one. Especially as at the end there's mention of a typical break angle...

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