Innards - reinforcements, struts and bass bars?

Replies to This Discussion

Permalink Reply by bairfoot cajun on June 22, 2009 at 6:54pm

i have thought of this before too Diane and thought of the sound post idea also fer a cigar box guitar but have not tried either idea yet but i will real soon as i am building kinda regular so i,ll tell ya what happens within a week or two cause its on my build schedule. thanks fer the discussion post cause it gives me a more urgent reason to do these experiments sooner than later. i have built 3 fiddles before and i understand how the sound bar and sound post works and why they work so it will be a transferable procedure i,m sure. i will post results to the procedures soon as i can.

Permalink Reply by Doc Oakroot on June 22, 2009 at 8:55pm

The sound post and bass bar in a fiddle are part of a lever system involving the bridge to translate side to side string vibration to vertical motion of the top (and to some extent bottom) of the fiddle. For it to work, the bridge has to be fairly tall - like on a fiddle. There have been some experiments on regular guitars using sound posts and bass bars without much success.

(But don't let any of that stop you from experimenting with it)

Permalink Reply by Diane in Chicago on June 23, 2009 at 12:11am

Wow, Doc, that was really interesting! Does the bridge have to also be very flat, like the fiddle variety?

Doc Oakroot said:

The sound post and bass bar in a fiddle are part of a lever system involving the bridge to translate side to side string vibration to vertical motion of the top (and to some extent bottom) of the fiddle. For it to work, the bridge has to be fairly tall - like on a fiddle. There have been some experiments on regular guitars using sound posts and bass bars without much success.

(But don't let any of that stop you from experimenting with it)

Permalink Reply by Mark Werner on June 23, 2009 at 10:33am

I'm dredging up old memories here from 10 years of Guitar Player subscriptions....I know some arch-top players use soundposts as well. There was even a technical article in the repair column on fitting the things.

Of course, arch-tops are made pretty much like a big fiddle....

On most acoustics, the top is constructed and braced so that a bass bar wouldn't be practical. Certainly worth a box to try though; most CBGs are pretty trebly.

Permalink Reply by Wade on June 23, 2009 at 10:37am

From my short building experience and research, a thinner top with some sort of bass bar or reinforcement will improve the tone(if nice tone is what you want!). Not having the carry through inside the box should also improve the tone(also more volume inside)-There is a builder out there that makes his with a carry through that goes behind the body. I'm not sure if he uses a bass bar and sound post- I have his plans around here somewhere.

Last thing -I started building an upright bass and was'nt happy with the low bridge. low bridge=low tension and volume.

Permalink Reply by Diane in Chicago on June 23, 2009 at 11:40am

Sorry, but what is a carry-through? Dunno my lutherie terms . . . .

Wade said:

From my short building experience and research, a thinner top with some sort of bass bar or reinforcement will improve the tone(if nice tone is what you want!). Not having the carry through inside the box should also improve the tone(also more volume inside)-There is a builder out there that makes his with a carry through that goes behind the body. I'm not sure if he uses a bass bar and sound post- I have his plans around here somewhere.

Last thing -I started building an upright bass and was'nt happy with the low bridge. low bridge=low tension and volume.

Permalink Reply by steve (Roots) on June 23, 2009 at 4:07pm

A carry through is the neck going thru the box in one piece.(traditional cbg construction.) A bolt on neck would leave more air space inside the box.

Permalink Reply by Diane in Chicago on June 23, 2009 at 9:59pm

I sometimes bolt the neck to a piece of wood going lenghtwise along the bottom. Is that better or worse than the carry through/thru neck? Or not much difference?

steve (Roots) said:

A carry through is the neck going thru the box in one piece.(traditional cbg construction.) A bolt on neck would leave more air space inside the box.

Permalink Reply by steve (Roots) on June 23, 2009 at 11:26pm

I like that design so if u need to put a bridge post under bridge for bracing u can come off of that. Also on the bolt on design u can shim under bolts if u want your neck to angle down. That will help in string clearance. I dont know if this design is better than any but it's good to experiment.

Permalink Reply by Diane in Chicago on June 24, 2009 at 8:04am

When you put in your bridge post, do you center it up under the bridge or stick in under the treble foot/end like a violin? I didn't know posts were a matter of course for some builders.

steve (Roots) said:

I like that design so if u need to put a bridge post under bridge for bracing u can come off of that. Also on the bolt on design u can shim under bolts if u want your neck to angle down. That will help in string clearance. I dont know if this design is better than any but it's good to experiment.

Permalink Reply by Steven Miller on July 19, 2009 at 11:32am

Doc Oakroot is correct is describing the roles of the soundpost and bass bar in a violin or other bowed instrument. You definitely do not want a sound post in a plucked instrument. The soundpost is a fulcrum point around which the bow drives energy into the instrument top. You don't need that in a plucked instrument, and it would only stop vibrations in the top.

Think about energy inputs.

With a bowed instrument like a violin or bass the player can easily control the attack, volume, timber and release of the sound by using the bow. The instrument receives a relatively large, continuous and controllable energy input.

A plucked instrument, like a guitar, banjo or mandolin has a very limited energy input. You can control the attack and initial power of the pluck, but you cannot add additional energy. The energy decays immediately as the pick leaves the string.

A bowed acoustic instrument will always be louder and have more sustain than a plucked acoustic instrument of the same size. Simply based on the method of energy input.

That is why many of you guitar builders are concerned about resonators and electric pickups. You are trying to take that small initial energy input and multiply it.

Regarding thin tops, think about the thickness of a banjo top compared to a guitar top. If you have a guitar and a banjo top of equal size, which is louder? The banjo. Why? The banjo top has less mass to be driven by the energy input. So more of your input energy comes out in sound, and less is lost in moving the mass of the top.

So yes, thin tops are louder. Thin tops are also fragile, so (in a guitar) you have to design a spidery bracing system that allows the top to withstand the static loading of the string tension and the up or down pressure of the bridge, but also offers the least possible resistance to sympathetic vibration with the string. Top thickness and bracing systems are half magic and half science. Here is the place to play.

Volume is also dependant on the internal volume of the box. Bigger is better up to the point at which the available energy input is overwhelmed by the mass of air you are trying to move. At that point the sound just gets lost somewhere inside the box.

Volume is also dependant on the relation of internal volume to the area of the opening. The vibrating top pumps the air in and out of the sound hole, energizing the air that strikes your ear. If the sound hole is too small, the energy never gets out of the box. If the sound hole is too large, the energy is not focused, and is dissapated.

Bass vibrations have longer wavelength than treble ones. They need a bigger top surface and larger internal volume to propagate. Because Cigar Boxes are by nature smaller than ordinary acoustic guitars, you will never achieve the bassiness in a CB that you can get in a standard instrument.

How to use this information in a CB context?

Look carefully at traditional acoustic instruments that work well. Notice the various dimensions mentioned above in relation to each other. Try to emulate these proportions in the instrument you are building. It is the relationships of these factors that count, not the exageration of any one of them.

For a plucked instrument, start with a relatively large box that has a thin, solid wood top with no paper on it. And go from there.

Hope this helps, Best Regards, Steven (CarolinaFiddle)

Permalink Reply by Diane in Chicago on July 19, 2009 at 4:17pm

Well, thanks! That's very helpful. So how does a really wide bridge affect things? Lately I've got bridges several inches wide and am thinking that is making things louder. However, again, since I have never built the same thing twice, apples to oranges . . .

(My fiddle is coming along, slowly, but coming along . . .)

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