question about back graduations

[Ken McKay]

Originally Posted by Arnold Schnitzer

... there are makers who believe that the back table should be as stiff as possible and not vibrate, therefore allowing the top to vibrate more. Then there are makers who believe the back should behave as a secondary soundboard and vibrate like crazy. Whether you rework a back table's thickness (and by how much) will depend on which camp you are in. I've seen back thicknesses (round-backs) of 6mm all the way to 14mm.

Interesting comment. Put me in the back-needs-to-vibrate camp.

As far as back plate stiffness goes my understanding and belief is that the top and back plate should match in stiffness somewhat. A double bass resonates similar to big violin when it has a carved back plate. Customarily the plates are usually tap-tuned so that they are about a tone or semi-tone apart or thereabout. This seems to be traditional and results in a normal sounding and responding instrument (violin, viola or cello and roundback basses).

If the instrument sounds stiff and non-resonant it might be a candidate for a regrad. After measuring the thickness of the top, if excessively thick, I might consider taking it off for some thinning. Any structural concern would trump any thinning for resonant purposes.

While the top is off, I would assess the top and back thickness and get the frequencies of their modes, Taking into consideration the type of wood but regardless, I might try to match the back to the top by thinning one or both while the top is off. One tone to a semitone is what I would shoot for depending on where they are to begin with. There would no advantage to removing the back for this. The tap tone of the back on the ribs will be close enough.

If it were a flat back bass, the individual braces might be too thick or thin and can be thinned or rebraced to achieve a normal stiffness. there is no tap tone of a flat back, it acts more like individual sections with their own resonances in each section. The platform that the soundpost sits on I think of as a separate resonating body. But it shouldn't be too floppy or stiff. I have no tuning scheme for the back braces but here is my belief. Flatter, thinner back braces act more like a carved back, smoothing out the resonances with higher damping to spread the peaks of the individual resonances of the braces tones therby making it more like a carved back which has a more damped tones. Flat backs are peaky with closely spaced resonances, while carved backs have less peaks but are much wider with higher damping. They vibrate in a different ways, if you have lazor and a camera you can see it clearly, or better yet, just read about it.

[Arnold Schnitzer]

Ken M., it's my belief that when backs and tops are "voiced" too closely to a pitch, the result can be a wicked wolf tone. Also, I think most flat backs are way over-braced.

[Ken McKay]

Some interesting inside baseball for ya...

On my cornerless bass that I just finished I made the top first and then wanted a hybrid bracing for the flat back. I designed my own to spread out the stiffness with an X-brace with the intersection at the soundpost. This is not anything new, many makers have done an X but mine was different because it is braced similar to a flat top guitar that CF Martin designed. An X with lower fan type braces. The reason was for the braces to hold the back into a slight dome shape with minimal weight. I thought this might make it behave similar to a carved back. And not have the cross grain situation of the flat back with its cracking problems.

Since I was shooting in the dark I tap tuned it, removing wood from the braces to make its stiffness similar to the top. The X back did have nice ringing tap tone that was close to the top's. It was about a tone higher. After stringing, it did have a pretty strong wolf at open A. I was kind of expecting that and was able to move it a little as to not interfere with bowing by using a brass weight on the afterlength.

The bass seems to have a pretty nice smooth round sound that is a little different from the typical flat back to my ear. As for long term stability, I am certain that I have done a positive thing by not gluing 3 or 4 braces straight across the full 27 inch width of the back. So now the wood can expand and contract across the grain freely as well as flatten the dome when it gets dry. I will know if a few seasons just how beneficial this was but have already noticed the back going flat during dry spells. I also notice the post getting looser and tighter with this.

[Ken Smith]

Originally Posted by Ken McKay

Some interesting inside baseball for ya...

On my cornerless bass that I just finished I made the top first and then wanted a hybrid bracing for the flat back. I designed my own to spread out the stiffness with an X-brace with the intersection at the soundpost. This is not anything new, many makers have done an X but mine was different because it is braced similar to a flat top guitar that CF Martin designed. An X with lower fan type braces. The reason was for the braces to hold the back into a slight dome shape with minimal weight. I thought this might make it behave similar to a carved back. And not have the cross grain situation of the flat back with its cracking problems.

Since I was shooting in the dark I tap tuned it, removing wood from the braces to make its stiffness similar to the top. The X back did have nice ringing tap tone that was close to the top's. It was about a tone higher. After stringing, it did have a pretty strong wolf at open A. I was kind of expecting that and was able to move it a little as to not interfere with bowing by using a brass weight on the afterlength.

The bass seems to have a pretty nice smooth round sound that is a little different from the typical flat back to my ear. As for long term stability, I am certain that I have done a positive thing by not gluing 3 or 4 braces straight across the full 27 inch width of the back. So now the wood can expand and contract across the grain freely as well as flatten the dome when it gets dry. I will know if a few seasons just how beneficial this was but have already noticed the back going flat during dry spells. I also notice the post getting looser and tighter with this.

Ken, unless you make two identical basses it's hard to attribute the sound to the bracing. The Cornerless design in itself is a huge difference in sound due to the fact the fibers run end to end with no blocks stiffening them up in the middle. I think you have too many variables here to consider the bracing itself a contributing factor.

[Ken McKay]

Well, I kinda agree with you there Ken. But a flat back is a viol and a roundback is violin. Each type has a characteristic timbre regardless of the shape of the instrument. Psychoacoustics and playing style also have an effect on the sound of a given instrument but the mental exercises transfer to reality. For example ask yourself what if this bass had that back or what if this roundback bass had a flat back? Armed with all of the available knowledge we have, as well as tradition and a good ear, the puzzle comes together.

In other words expected results are born from understanding.

Attached is Andrew Brown's double bass dissertation in pdf.

[Ken Smith]

Originally Posted by Ken McKay

Well, I kinda agree with you there Ken. But a flat back is a viol and a roundback is violin. Each type has a characteristic timbre regardless of the shape of the instrument. Psychoacoustics and playing style also have an effect on the sound of a given instrument but the mental exercises transfer to reality. For example ask yourself what if this bass had that back or what if this roundback bass had a flat back? Armed with all of the available knowledge we have, as well as tradition and a good ear, the puzzle comes together.

In other words expected results are born from understanding.

Attached is William Browns double bass dissertation in pdf.

Ok, you expect me or someone to read this 136 page study and make something out of it? From this study, what advancement on the making or repairing the DB has come about?

I think the Wood and The Varnish has a lot to do with the sound of a bass as well. The Player not so much. Maybe a good player can get a better sound than the Plummer can but even if my wife pulls the stings on my Storioni it will sound like the Storioni.

[Ken McKay]

Oh common! A little light reading...Paul reads it in the bathroom.

Truthfully, I don't expect anyone to read it or be interested in this unless they are a geek like me. But there is a lot of good stuff in there with some very careful and thorough research and investigation. I see you read some of it by your comments...

There is a lot of stuff to talk about if anyone is interested. This might be a good topic of its own.