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bridges - materials - setup etc.

A lot of creativity is shown regarding bridges on the CB instruments I've seen. The quintessential eye bolt seems to show up a lot, dog bones, ball point pens, bottle caps... all kinds of cool stuff. These obviously works great for plugged in CBs, [& for aesthetic "funk factor"] but what about acoustically?

In most non amplified instruments some general rules of thumb I've observed:
1. Most are made of pretty dense material like bone, ebony, rosewood, maple, corian etc.
2. sometimes the body of the bridge is made with dense material like, ebony, rosewood or maple and capped with even harder materials like for banjos ebony atop maple and guitars bone on ebony.
3. Softer materials tend to absorb highs and denser materials accentuate them. I've used this to advantage on instruments like hammered dulcimers by capping the bridges with Delrin [a sort of self lubricating synthetic - I think it's generic name is acytal] this not only helped reduce friction to make tuning easier - it filtered out some of the high frequencies that HDs often have too much of anyway.

One of the ways you lower the volume of traditional stringed instruments is by using a "mute." [And in spite of what one of my T-shirts says a banjo mute is NOT a 9 lb sledge hammer...;~) ]
These are weighted clips that fasten to the bridge of say a fiddle or a banjo. This is helpful if we're trying to balance their volume with other acoustic instruments. But this also tells us that all else being equal, if you merely add weight to a bridge is reduces it's ability to vibrate freely and pass that vibration on to the sound board, and so the instrument gets quieter.

With all that said, I'm guessing large eye bolts work against us if we're trying for optimum sound unplugged. An easy experiment someone might want to try is make a few bridges of different materials, but all the same height, and test them on the same instrument. If you have a cohort who's got the patience a blind listening test might be even a little bit scientific. Any takers? Recording the results and comparing that way might be cool too, but keeping all variables the same can be tricky.

How about other bridge observations?

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Replies to This Discussion

Permalink Reply by Mark Werner on April 27, 2009 at 6:03pm

I agree, the bolt is quick and serviceable, but likely not the best for sound transfer. Most guitar bridges are firmly glued to the top, or in the case of other stringed instruments or arch-top guitars, very carefully fitted for full contact.

Bridge material has to transmit string vibration without damping it.

Red Henry has a very good page on bridge construction for mandolin-family instruments; I used one of his designs for my mandola:
http://www.murphymethod.com/redbridge.html

His idea is that relatively light, hard maple transmits vibrations with minimal loss.

Permalink Reply by David Beede on April 27, 2009 at 6:31pm

Mark,
Thanks for that link to Red's bridge research page. Some great observations there.
[I love Red, we're actually playing sets back to back at the Gamble Roger's Festival this weekend in St. Augustine. I might have to pick his brain a little.] Your point about transmitting vibration without damping is an important one. I know on arched top instruments sand paper is placed on the box grit up and the base of the bridge is rubbed on it to get a tight fit. Not as tricky with flat top CBs. But tail piece/neck angle also influences the downward pressure of the bridge against the sound board which also effects "fit." But that whole neck issue probably needs its own discussion area.
The gluing on of bridges mostly happens on instruments like guitars and ukes that are also terminated in the middle of the top, and so essentially trying to pull the top off - hence all the bracing and arching to prevent that big sproing!! If folks start getting that fancy in their CB building they might want to go ahead and try "real" guitars. I'm seeking that balance between simplicity and optimum sound.

Permalink Reply by Jim Mitchell on April 27, 2009 at 10:25pm

Hey Mark - thanks for the link,very interesting.... Do you guys have any thoughts regarding the thickness of the bridge? (particularly where the strings come across) I have 10 builds so far and have used Oak, Maple, Mahogany and a plastic material that is used in turning pens. At the same time I have been playing around with using cake pans for a very low tech reso sound. If you take a look at my photos you can see what I have been doing. I can say that since I have added a bolt tying the drain and cake pan together I have had improved volume... I will try to pay better attention on upcoming builds and post my findings. Thanks again great information... Jim

Permalink Reply by Mark Werner on April 28, 2009 at 5:46pm

The Red Henry design is quite thick compared to traditional bridges, but because they use light-hard woods and have a lot of cutouts and holes, they remain very light.
I've used the design on my mandola and on my just-completed reso; both are a good 3/8" thick. You have to make sure to cut the slots properly, tapered from string-anchor to break point for a nice, clean break.

All that material allows you to relieve the string slot if necessary for intonation.

Permalink Reply by David Beede on April 29, 2009 at 11:17am

Hey Randy,
Thanks for your take on bridges and for that great jpg.
I've noticed that a lot of the methods or materials I gravitate towards over time, ends up being a variation of some traditional approach that has proven itself over and over again across centuries of building a wide variety of instruments. I think bone is probably one of those materials - tho the "pay off" from it varies widely depending on the kind of instruments we're making.

I noticed that the shim you used to raise the action on the bridge in your jpeg is tapered. It reminded me of a jig/gizmo I use to assist in setup. [I should probably post this somewhere with a photo, but I'll describe it here.]
I've made a long tapering temporary bridge that I use to determine the finished height. Since it tapers I can put it at the correct location with the string tuned to pitch, then I pull the wedge out gradually - slowly lowering the action of the string while plucking it. I do this until the string buzzes and then push it back in, raising the action until the buzzing goes away. Then I know I've found the optimum height for that string. I measure the string height with calipers and make the bridge that tall for that string. Often the wound bass strings need to be a bit higher off the fretboard than the plain strings.
Thanks again for the cool pic.

Permalink Reply by Old Lowe on April 29, 2009 at 5:57pm

I just wanted to say the I think the bolt bridges are working very hard against us.
I make a high bridge usually around an inch tall and try to get the angle of the strings coming down to the tail end as sharp as I can.
I have been able to get them really loud this way.
I use every bit of space the box allows to make this angle sharp so the strings are almost touching the edge of the box before they go to the tailpiece.
I make my bridges from whatever hardest wood I can find and I shape them to a sharp point right on top and after they have been strung up for a day or two I hold them up to a light and if I can see light coming under the bridge I sand it till it fits perfect on top of the box.

Permalink Reply by L.J. on May 1, 2009 at 9:14pm

I have used the large eyebolts quite frequently.And noticed some boxes are louder than others.On a build I did a few months back(romeo and Juliet distressed) I used a tensioner removing the eyebolts and a lil grinding and it made a particle board top much louder than some of my cedar tops.It was not surprising to me because I am a fan of old vintage gibsons.I learned a couple of years ago that the old les pauls came with a lightwieght aluminum tailpieces(stopbar) Its said that by trading the zinc piece with the lightweight aluminum you find more transfer into the body of the guitar.creating more woody sustain in the instrument.I replaced my tailpiecxe on my les paul and was impressed with the difference.I have used a few aluminum tensioners now.And am very pleased with the volume from them.The only problem I have with them is they are hard on the top of the box when readjusting.and you have to grind the top of the rear of it down or it buzzes.Roughing it up.Or like I like to say adding a lil cbg charachter.The real plus is the volume! and the fact you get 2 eyebolts to use on other builds.i try to stick with the flat alum tensioners with the 3/16 eyebolts.I have also been using some other lightweight aluminum pieces like nuts.and my next 4 builds are going to have alum rod bridges.No matter what bridge i use I always make sure its flush with the top by grinding thru the threads.I lay the eyebolt on a board and rock the eye and grind till its flat. more connection more transfer.I have also learned over time that too big of a soundhole can knock your volume down.I believe if it lets out too much vibration the box doesnt sing right.Gotta trap some buzz in the box to keep it hummin.Maybe one day ill find a square inch formula for that purpose.as for now im still learning as I go.

Permalink Reply by Brian Lemin on June 12, 2009 at 1:20am

From what i have gathered after trying to find what might make a "softer" sound is that Bridges which are "steep" from the string to the body produce sharper sounds than a bridge that graduates out. So a bridge that is on top of, or built into, an "Oval" (Or possibly rectangle) shape, and gradually slopes down to the board after raising the strings, then affixed to the sound board will produce a softer sound.

A look at some classical guitars seems to support this to some extent.

Then comes the issue of where on the soundboard the bridge should be placed. I have always worked roughly on a third (+) from the tail, but it seems that many makers are choosing to locate it somewhat closer to the center.

Of course this all "guitar" lore!

Permalink Reply by MichaelS Country Boy Guitars on February 10, 2010 at 10:33am

Question of bridge placement. I see alot of bridges on or near the edge of the box, I would think the bridge would work better further into the lid for better vibration and resonance, what do you think? Dose it matter?

Permalink Reply by Mark on March 3, 2010 at 4:32pm

I think much of the reason for placing bridges close to the edge of the box is down to practical construction factors rather than sound. The box top (or bottom) can take larger forces close to the edge (I'd have to go into some maths to explain this properly but essentially it comes down to being closer to the sides of the box, which is where the support is). If you have no way of knowing for sure what load the box will take before it bows or gives way (and no one wants to find out by destructive testing!) then you play safe - especially if you've got a lightly constructed box or you're going for mor ethan three strings..
Also, cigar boxes tend to be quite a bit smaller in area than regular guitar bodies, so it's more of a squeeze to fit in all the hardware and soundholes (if you're putting in a magnetic pickup and you want soundholes and your want control knobs and you want to have a clear area for strumming then the bridge can get pushed back to make room).

However, it's an interesting question as to where you'd choose to put the bridge for absolutely optimum sound quality. I don't think it's as simple as being better the further it is into the centre. My gut feeling is there will be many different answers because boxes come in different proportions and sizes and probably trial and error is the only practical way to find out what works best. On top of that, sound quality can be a subjective thing, so each builder might have his or her own idea of what sounds best.

So the best way to answer your question is to get building...

MichaelS said:

Question of bridge placement. I see alot of bridges on or near the edge of the box, I would think the bridge would work better further into the lid for better vibration and resonance, what do you think? Dose it matter?

Permalink Reply by Kris Robinson on March 31, 2010 at 1:05am

one of my bridges is small steel handle, prolly a garden fence or a drawer, I used a hack saw to cut grooves in it. It seems to work well. The guitar it's mounted on produces a fair amount of sound, my strings are mounted in a block of oak that's almost flush with the back end of the box, so the strings are also tight across the corner of the box before they hit the bridge. Do you think making contact with the box affects the sound?

Permalink Reply by Rob Ryan on October 18, 2010 at 10:41pm