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Thread: Optimum Bridge Height

  1. #1
    Registered User rosewoodmusic's Avatar
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    Default Optimum Bridge Height

    I was reading some threads about neck angles and John Hamlett wrote in a neck resetting thread that the optimum neck angle depends upon the desired bridge height. What factors into that decision?
    Does it vary much with carved top mandolins?
    Do you pretty much want to be in the middle adjustment range of a standard bridge?

  2. #2
    Registered User sunburst's Avatar
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    Default Re: Optimum Bridge Height

    I don't know what the optimum bridge height is.
    Some think a taller bridge produces more downward pressure on the top so the strings can "drive" the top better. I don't think there is much to that. All the strings need to "drive" the top is enough pressure to hold the bridge in place, and that is another school of thought; that the bridge need only be held in place, and that is preferable to downward pressure. I notice you mentioned that I said "desired bridge height", and I'm glad you caught that, because obviously, from the two schools of though mentioned above, desired bridge height can be different things for different builders, so neck angle can be quite different too. There's a little more to it than simply desired bridge height though.
    What factors into the decision? The downward pressure desired (see above), the height of the arch of a carved top, the height of the overstand (the height of the fingerboard above the top where the neck joins the body), desired action height, fret height, and other things. A mandolin with a high arch, a low overstand, and a short bridge can have a similar string breakover angle at the bridge as will a mandolin with a low arch, a high overstand and a tall bridge. Those mandolins will both have about the same string pressure at the bridge, but the height of the arch being different might mean that each top is affected differently by the same pressure. There is no simple route to figuring the optimal bridge height.
    If we are using a standard adjustable bridge, then yes, we need the height to be in the range of adjustment of the bridge. Bridges can also be taller of shorter than standard and still be adjustable.
    For carved top mandolins, my personal preference is a bridge around 5/8" to 11/16", and not over 3/4".

    Bridge height probably affects the sound of a mandolin somewhat, so the optimum bridge height might have to do with the sound the player likes to hear from a specific mandolin.

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  4. #3
    Registered User Tavy's Avatar
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    Default Re: Optimum Bridge Height

    Quote Originally Posted by sunburst View Post
    Bridge height probably affects the sound of a mandolin somewhat, so the optimum bridge height might have to do with the sound the player likes to hear from a specific mandolin.
    +1

    The greatest changes occur at the extremes - when the bridge break angle is too low for example everything can sound thin and "stringy" and the strings will want to pop out of their slots if you play hard, in the middle there's probably quite a range over which the sound will change, but not necessarily better or worse, just different. A greater break angle may also help overcome the difficiencies of a poor instrument. Or not

    So I would say dial in a decent action and concentrate on playing the thing....

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  6. #4
    Registered User rosewoodmusic's Avatar
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    Default Re: Optimum Bridge Height

    Thanks. That helps a lot.

  7. #5

    Default Re: Optimum Bridge Height

    I think to understand the effects of string pressure on the bridge the banjo might be a case in point. The strings on a b----o are much looser and there are only one string instead of a pair of strings pressing down. Normally on a b----o the bridge has so little pressure on it that the bridge is in danger of tipping over, even to the point where some bridges have four feet to keep them upright. Too great a pressure on the banjo head will dampen the sound just as too much bridge pressure on a mando will will likely dampen the sound.

    There is an optimum pressure for sound transmission on both the mando and the b----o. The major difference is that the b---o (because of its skin head) is made to be adjustable and the mando is not.

    Hope this makes sense...
    Last edited by bart mcneil; Nov-01-2014 at 8:34am.

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  9. #6
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    Default Re: Optimum Bridge Height

    To achieve the ideal action for any given setup, the optimum bridge height is the result of neck pitch, crown height of the soundboard, and location (height) of the tailpiece. If the neck pitch increases, the bridge height will likewise need to be increased. If the soundboard crown is higher than normal then the bridge height will need to be lowered, etc. In each case, the bridge height is adjusted to achieve the proper action (for the performer's preference).

    A change of 1° of neck pitch will require a .210˝ change in bridge height.

    HOWEVER, what is more important than the physical bridge height is the string break angle over the bridge; the angle the strings make as they pass over the bridge. As the string break angle increases, the down pressure on the soundboard likewise increases. A set of J73 strings will exert a down pressure on the soundboard of 37 pounds at 14°, 45.5 pounds at 16°, and 53.5 pounds at 18°. Above 18° the down pressure increases exponentially. (As a point of reference, a 16-count red spruce soundboard properly graduated can withstand a download of about 75-80 pounds at the bridge before it implodes.) As the string break angle increases above 16°, the soundboard is burdened to the point where it is restricted from functioning efficiently. If you are interested in the subject, I have a down pressure loads chart and more about string break angle loads here: http://siminoff.net/string-break-angle-loads/

    String break angle is very important, and from testing, it appears that a string break angle somewhere between 15° and 16° is ideal to drive a properly graduated soundboard and produce good timbre, amplitude, and sustain. Above 17° the increased download on the soundboard restricts its movement and will kill the voice of the instrument by diminishing timbre, amplitude, and sustain.

    So, while bridge height and string break angle go hand in hand, the actual height of the bridge alone only tells one small part of the story.

    .R

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