question about back graduations

[Rob Menapace]

I have a roundback bass that's had a top off re-graduation plus a new bass bar, the back also was re-graduated, but with the back still on. So the back is still the original thickness around the edges, where it connects to the ribs. How important is it that the back have it's graduations going all the way to the edges? I'm trying to figure out whether it's worth the extra expense to have the work done. The bass is sort of a pet project of mine, I've probably already exceeded the amount that it's actually worth, but I've gone this far, and I'm hearing conflicting ideas about how important the back is. There's the idea of "if it ain't broke, don't fix it", but I also know that the edges can be especially critical in the graduations. I'm just hoping for some general info on the importance of the back graduations from some of the esteemed experts that post here, thank you!

[Ken Smith]

Well, just for conversation, what were the thicknesses of the Back measuring in the center on thru to the edges by the Lining and what are the numbers 'after' you re-graduated the Back? Also, what were the numbers of the Top as well?

In my opinion, numbers by themselves do not tell the whole story. The density of the wood plays a huge role as do the dimensions of the bass as far as the length and width of the plates and combined.

Harder wood can possibly go thinner than softer variations of the various species. Wider and/or longer pieces might need to be thicker than smaller ones like the difference between a Cello and a Bass but in a gradual sense. Arching and Bracing also add to the equation or subtract from it at times. A bigger arched Top by design is stronger but not necessarily better.

I have one English Bass with a very dense Top that someone thought from looking from the F cuts was too thin and his comment to a potential buyer helped to 'kill' a sale for me. When it was later 'gauged' and its extra density evaluated it was deemed 'graduated just right within itself'. 'Within itself' is the key phrase here. If the wood is soft and you go too thin, it will be weak and fail in one or more ways. If the wood is hard and left too thick, the sound will be suppressed to the degree that the plates are not graduated within its personal range.

How do I know all of this? I don't, and I admit it. I just happen to have owned many many basses or all ages and schools of making and have had most of them on a bench at one time or another for examination as well as seeing many other basses that I didn't own. After examining more basses than I can remember, I do know that more often than not, the bass is too thick or too thin somewhere on the plates. If we are not sure by just looking at the Bass, then the graduations are left alone. Only when we are totally certain that something is way off to I approve an action in regards to graduation or adding back in. 'We' meaning the Luthier and myself discussing the work to be done.

What to do? Evaluate every project thoroughly before cutting anything. Taking away is easy. Putting back is not. Adding breast patches to re-thicken and reinforce a thinned, depressed or sunken Top is very difficult and not all the Luthiers do such a great job of it. Many basses done in 'professional' shops have been repaired for profit rather then for quality and 5, 10 or 20 years down the road the truth will be told when the Bass itself speaks out from its human imposed illness!

[Ken McKay]

Back grads are critical, edges are not. Where the plate is glued to the ribs/linings, thickness and stiffness are not important within normal limits. It would waste your money to regraduate the back plate again for that reason.

As a matter of fact I think the best possible way to graduate the back would be from the outside after the instrument is fully strung. But that has nothing to do with your question.

What exactly are you thinking you might want to achieve by this regrad?

Someone with a few basses could try this experiment of give and take. Tap the back at the point where the SP is situated with a thump of the finger. Listen for the sound level of the open strings. Now repeat with a couple other basses and relate this to their sound level, loudness, punchiness (not necessarily timbre) . A stiff back can prevent a bass with a perfectly good top from sounding loud and responding to the bow well. Now reverse this in your head as you imagine the sound starting at the string, through the bridge, rocking the top, causing the post to move the back.

[Rob Menapace]

Thank you for the helpful replies. What I am trying to achieve is a bass that's been properly re-graduated, given the general standards of acceptance. I had to ship the bass to have the top work done, so I'd like to make sure the bass is correct before it gets shipped back. I was not sure how important the edges of the back plate were, if I understand the above reply, they are not as important as the overall graduation of the back plate. The luthier feels the benefit of taking off the back and redoing the edges might be marginal. I mostly wanted to make sure that the work done is correct, for both achieving it's full potential tonally, and also for when the time comes that I might have to sell it. Thanks for the info on the tapping experiment too, have to try it. I'll work on getting the numbers on thicknesses.

[Arnold Schnitzer]

Both Kens gave excellent information above. To stir the pot a little: 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.

[Ken Smith]

Originally Posted by Arnold Schnitzer

Both Kens gave excellent information above. To stir the pot a little: 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.

14mm? Did we both see that Bass?

Anyway.. Of the Basses I have owned, those with harder maple backs seem to project more volume providing they were properly thicknessed within. Those with softer backs seem to put out a softer tone, less penetrating in the mids. The only softer maple Backs I have seen recently are those of Italian Oppio, a local wood of north Italy used often on Italian basses and Cellos. I currently have two Oppio Backed basses and although both loud and deep, they do not 'throw' out the sound as well as some other basses with harder maple regarsdless of them being round or flatback.

[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.