Replies to This Discussion

Permalink Reply by Korrigan on January 30, 2018 at 10:44am

I accidentally made a sound post in a uke. It was a 1/4" square oak dowel just meant to be a support but acts like a sound post, vibrating the back of the box. The problem is that in normal playing position my belly is touching the back of the instrument so dampening the effect of the sound post.

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Permalink Reply by Grandpa on January 30, 2018 at 10:56am

I don't know, either, but many of us have sound posts like those in Korrigan's pic built into all or most of our gits. Most of us have a stomach muting the back, also. Maybe I'll try some over-the shoulder, behind-the neck git playin' for comparison. Jimi did it, Stevie did it, Robert Cray does it...you know, a lot of players have done it. Perhaps there is a clue to tonal improvement in there! We'll see.

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Permalink Reply by Taffy Evans on January 30, 2018 at 11:44am

Hi, it will make a difference but the questions to ask is why use it, what's its function, will it be beneficial or will it restrict top movement.

If the CBG was built like a violin, but they are not, then a post would work the way it is supposed to.  In fact CBG's are not even played like a violin.

The soundpost  in the violin family of instruments is not there to support the top, the carved shape of the top plate does that very well. It is there to couple the top and back plates together so that they will work or harmonise together in producing a more powerful and tonefull sound.

Even the size and placement of the post is important.

The post also works in conjunction with a "bass bar. This bass bar [brace] is under the bass leg of the bridge, whilst the post is located behind the treble leg. This set up assists the bridge to transfer string vibration to the back. Its a far more intricate system than I have outlined here, but may help you to understand the "why". But some claim it makes a difference in their CBG build.

Also remember that the bowing action of these instruments creates the need for different construction considerations.

When I build my acoustic guitars I pay a lot of attention to "tuning" the top and the back so that they work together, I suppose the air in the body does what a soundpost does in a violin.

Not all guitars have an "active" back though. 

Oh! and I never play an acoustic guitar with the back against my body, it will restict volume in some guitars.

For what it worth Taff

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Permalink Reply by darryl kernaghan on January 30, 2018 at 1:31pm

All that Taff said is correct, plus he almost said, the violin type of instruments get their sustain by the continuity of the bow across the strings, when picked or plucked they have very reduced sustain due to the top and bottom being less flexible

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Permalink Reply by Br Harold F Stubbs III on January 30, 2018 at 5:06pm

You can get around the stomach muting with a resonator back, like some banjos have... but you probably knew that, Grandpa..

My vote for acoustic / mechanical amplification is still some sort of stroh / grammaphone type deal... 

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Permalink Reply by JL on January 31, 2018 at 2:07am

No, a soundpost does not make bowed instruments louder.  because it ties the belly and the back together they vibrate in the same direction, there is no "pumping" of air in/out with the back moving the same direction as the belly.

Instead it does 2 things

1: the soundpost braces the trebble side of the belly/soundboard so that it can be thinner and move more.  The bass bar on the bass side of the belly is just like a brace on an acoustic guitar soundboard, strengthens it so it can be thinner. without a soundpost a violin/viola/cello/bass is at risk of the string/bridge pressure deforming the belly over time.

2: it acts as a fulcrum.  bowed instruments only vibrate the string sideways as the bow drags across the string, the elevated bridge acts as a lever to change that side-to-side string vibration to an up/down vibration on the bass side of the belly with the soundpost acting as the fulcrum.

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Permalink Reply by jawbone on January 31, 2018 at 4:26am

This is getting tooooo complicated - I always thought "the fulcum" was that complete chunk you finally got coughed up when you inhaled while eating toast !!!! Am I right??? AmI right????

;-)  ;-)   - I need emoticons!!!

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Permalink Reply by Taffy Evans on January 31, 2018 at 2:40pm

Hi jawbone, It was too complicated before the explanations.

Here's the simple bottom line:

If the string vibrations are continuous [as in a bowed instrument] then the soundpost plays an important role in producing sound.

If the string vibrations are instantaneous [plucked as in a guitar] the soundpost has a dampening effect on sound.

Simple example: plucked notes on a violin produce a lower volume.....remove the sound post and the volume is increased markedly.

Cheers Taff

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Permalink Reply by jawbone on January 31, 2018 at 4:24pm

Thanx Taff - I was just joking - sometimes it just jumps out - sometimes words just strike me funny - today it was "fulcrum" - probably the same reason that in my shop when I talk to myself I use cartoon voices - makes me seem more interesting !!!  If I didn't have my cartoon voices the only person I may talk to is my wife, and she is tired of listening !!!

I need emoticons !!!

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Permalink Reply by Hans on February 5, 2018 at 12:01am

This sounds very convincing, Taffy, and I'm sure you're right in theory. In practice it seems to be more complicated though and there seem to be other factors than continuous vs. instantaneous vibrations playing into this: I installed a soundpost in my 8 string mandolin and it not only considerably helped with sustain but also with volume. But in my 3 and 4 string guitars I could not replicate that effect. Quite the contrary, the effect was detrimental to sustain and volume. How can this be explained? String vibration is instantaneous on both guitars and mandolin and both have square cigar box bodies, but only in the mandolin the sound post helps... And also: If this is just about continuous vs. instantaneous vibration, why do some archtop guitars have soundposts? Just to support the top?

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Permalink Reply by Taffy Evans on February 5, 2018 at 12:38pm

Hi Hans, as mentioned earlier it is a very complicated subject, and sound physics at this level is not my thing.

Just to clarify, I do not build violins, but have tried to learn as much as I can over the years about it, to help me with repairs and the  set up of orchestral stringed instruments. 

I try to answer questions on here from my experiences in building and repair, and not google someone else's answers. Having said that I have a crap memory and have refreshed my thoughts prior to responding at times either from my many reference books or, yes, google. Where would we be without google.

So you can see I'm not an expert so I try/have to keep things simple.

To answer your question why you got a better response from your mandolin, for me ,is only guesswork as I don't have the mandolin to evaluate. But this is what comes to mind from my experience.

What I have found in the past with guitars that have a too flexible a top, is that they lack volume and tone. For example if a brace breaks or becomes unglued from the inside of the top. When I reglue or repair the braces, this stiffens the top and it will react better to string vibrations and brings about better volume and a more defined tone. So..........

It could be that with 8 strings on a top that may have been underbraced you may have been getting less than optimum output. By fitting the post you stiffened up the top to get the result as per the guitar story above.

With the 3-4 stringers, they may have been working efficiently to start with and fitting the post had a dampening effect on the top. Three strings trying to move the top and back is a big ask, you have 8 strings doing that job on the mandolin.

Re soundposts in guitars. My dozen's of luthiery books make no mention of them. However I did find some info that suggested to me that they may be there for support or to control the top to limit feedback, by some makers.

As a matter of interest I have posts in the full resonator guitars I build, but I don't call them soundposts as their job is for support.

Just my thoughts. Taff  

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Permalink Reply by JL on February 6, 2018 at 2:49am

Taff nailed it.  it adds support, so a too-soft soundboard gets better, and already firm enough one gets too stiff.

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