Fir top

[renegade]

I've read this message board for several years, but never posted. I'm just a hobby builder, not a real craftsman. I've build 6 mandos (please make me stop). 2 kits, 3 from scratch, and this hybrid I'm asking advice on. I took an old Harmony f style mando. It had no sound, strings were high, generally a piece of junk. In an experiment, I carved a top out of Douglas Fir, and a back out of Poplar, and mounted to harmony body. I wedged the bolt on neck to get it to good set-up height. I used spruce for the tone bars, and left the top a little thicker than normal. I'm getting a very good sound and volumn on the tenor strongs, but the bass has no punch. It's a good sound but no ring to it. The F holes are a copy of the original Harmony and are not very large. What suggestions to increase tone on bass. I thought of increasing size of F holes. Also, taking back off and adjusting tone bar. Any help please.

[sunburst]

To adjust bass, look to the back, not the top.
Poplar, you say? There are lots of woods called poplar, and I woudn't know how to carve them for a mandolin back with good bass. It might not be a good back wood, I don't know.

From what I know of mandolins, (not much, considering all there is to know) F-hole size has only a very small effect, top thickness and tone bar size has some effect, but bass is really affected by the fundamental resonant frequency of the back , and how it couples with that of the air in the box, and that of the top.

[PaulD]

John, could it be that the top is too dense or stiff? I know all wood varies, but most of the Doug Fir I've used (in framing and building doors) has been heavy, hard, and fairly dense. It seems if the top was too stiff you would have a weak bass response... or does that make more difference in guitars where the top is so much bigger? I'm curious since that was my first impression after reading that the top was left thick.

Paul Doubek

[sunburst]

Could be, Paul.
Most of the variations I've tried in my mandolin tops have not made much difference in the bass responce. The back variations have, however. I can only speak from my own experience in saying the back has more to do with bass than the top.

[PaulD]

You're the expert... I'm just tryin' to learn sumthin!

I think I've read that increasing the stiffness of a guitar top (all other things equal) makes for a more trebbly response, but a guitar is a different beast.

pd

[Antlurz]

Being a rank beginner, comparatively, my initial thought went back to his comment about leaving the top a bit thicker.

How thick is "thicker"?

Ron

[sunburst]

I think I've read that increasing the stiffness of a guitar top (all other things equal) makes for a more trebbly response, but a guitar is a different beast.

You can loose trebble from a lack of stiffness, so, accordingly, it can be said that increasing stiffness increases trebble, at least to some extent. That doesn't necessarilly mean you loose bass when you increase trebble. More trebbly doesn't always mean less bassy.
Tops do have to be thin enough to function well, but without the back and the air working with them, thinner won't do much for bass, and can loose you some trebble in the process.

Guitars and mandolins are not entirely different beasts. In both, the function is basically the same, with the body being a modified Helmholtz resonator, and with the top and back functioning their best when strongly coupled to the Helmholtz resonance.
I can't get much more detailed than that when explaining it, but the information that has been published is available to anyone who wants to look it up.

Backs of guitars and mandolins are usually made of stiffer wood than the top, and mandolin backs usually end up carved thinner than tops to bring the fundamental resonance into proximity with the top and air resonance. That's when things start to work together and make for strong bass responce.
It gets sort of complicated, and you don't have to get much farther into it to get to the end of what I understand. ("I've already told you more than I know") If you go yet farther into it, you soon get to the end of what anybody understands...so far.

A lot of stuff has been written that is conjecture, but is stated as fact. It's tough to weed through writings and know what to believe and what not to, especially when there is so little actually known about plucked string instrument acoustics.

[ShaneJ]

John, if the stiffer wood back is carved thinner than the spruce top, is the goal to get similar "flex" or "compression" properties with the top and back?

[sunburst]

I don't know, exactly.
(Thanks for the complement, Paul, but I'm not the expert on this.)

The top and back each have to have a fundamental resonance that is close enough to the "Helmholtz" resonance to produce coupling. That is dependant on mass as well as stiffness ("flex" or "compression"), so lighter or heavier back wood would have to be more or less stiff accordingly, to resonate in the right range.
Lighter or heavier / stiffer or "floppier" top wood would be similarly different. One top might work better with one back, and another might work better with another back. Like I said, it gets complicated, and I can quickly get in over my head.

If those who are "deflexion tuning" their plates are keeping good records, and are willing to share, we might start to find the answers to that question.

[ShaneJ]

"Deflection"....that's the word I was stumbling for!

I don't even know the word, much less anything about luthiery. It seems intuitive that the stiffness and "deflection properties" of a plate would somehow translate into the resonance.

It also seems to me that all the experimenting and "trial-and-erroring" that it would take to finally come up with a way to duplicate a certain tone-producing mandolin just could never be done by one person. Maybe one day info can be compiled from lots of sources? Heck, maybe someday a group of luthiers can organize their efforts and experiment in a concerted way.

On second thought, maybe they won't. That might take all the mystery and fun out of it. I AM constantly amazed and appreciative for all the info from the pros that is shared here and accessible to boneheads like me too. Thanks!

[arbarnhart]

Say, who is this Hemholtz guy? Is he a member? Maybe he'll post.

This is interesting. I am learning a lot about backs in this thread. My impression had been that the back could be extremely stiff (and therefore thick) as long as it had good acoustic properties (between the extremes of absorbibg sound and echoing too freely) and was attached in a manner that had no dampening effect on the top. I got this from comments made by builders saying it was mostly the top and the back's job was to not screw up the job the top does (I am paraphrasing).

[pelone]

The light bulb just went on!!! I have wanted a bit more woody-fruity-bass and have been struggling with the top. Plenty of treble. Somehow, I never considered the back other than an opportunity to show off some great flame. But, I find it interesting that when I play for a bit the good vibrations seem to pass through the back of the mando into my body and I feel good all over, kind of like mando therapy. My backs have been kept significantly thicker than the tops. Thanks for the heads up!!!

[troika]

Is the recurve thinner on the back than the top or the same. Maple must be carved a little thinner yes?

[van]

Douglas Fir as my dad once said, "That's as hard as a preachers D!*#!"
I suggest that you thin the top and the back and open the F-Hole slightly.
You can go thinner than you think with ol' Doug, he is a tuff character.

[LeoR]

Darby Boofer's Rose mandolins are generally fir tops. He left the tone bars really thin on the one he made for me. I'm not sure what he does with the tops as far as tuning or thinning.

They have a unique sound - sort of a wallop (ka-boom not chop...)

You can definitely hear it!

[Jim Rowland]

Getting back to John's point for a moment. I bought Bob Benedetto's book straight from him before it had been shipped for general distribution. Of course,I devoured it immediately....but I didn't digest some important details. He is obsessively concerned with the back of the instrument,something I didn't get for a couple of years. I reasoned (rationalized) that the top was all,and that any vibrations in the back were doomed to the black hole of the player's belly. Wrong,as I often am.
Getting the plates in sync can do magic for carved instruments. Others too,but you don't have as much delicate control on flat plates. Scraping and sanding and pecking and listening are great fun and DO make a difference. Work slowly and listen carefully and you probably won't go too far. So what? If you do carve a little beyond the optimum,it's no worse than quitting a little before. At least, that's what I tell myself. Seriously,back tuning is one of the most enjoyable parts of the process.
Jim

[sunburst]

Is the recurve thinner on the back than the top or the same. Maple must be carved a little thinner yes?

Most builders have a thinner recurve area in their backs than in their tops. Maple can be hard and dense, and, to some extent, you can expect to carve thinner as hardness and density increase.

There's only so much to be done in the recurve area, however. If it's too thin, there can be structural issues. Furthermore, removing wood from the recurve area doesn't affect the back (or top) resonance nearly as much as removing wood from the center of the plate.

[PaulD]

For what it's worth... this link to a F-style mando building photo-essay was in another thread and had the following quote:

I used Douglas Fir which is a slightly heavier or denser wood so I carved the thickness to about 10 to 15% thinner then these dimensions.

I don't know anything about this guy and I didn't read to the end to see how he thought the mando turned out, but I thought it kinda related to this thread.

BTW: John... I consider you one of the experts on this forum based on your experience; hence the reference.

Paul Doubek

[Antlurz]

I used spruce for the tone bars, and left the top a little thicker than normal.

I'd still like to know what his definition of "a little thicker" means.

3/8th's of an inch is a "little" bit of snow, and I think it's fairly well agreed this kind of wood can be carved a "little thinner", rather than a "little thicker"...

Ron

[Dale Ludewig]

I think John is right on the money with this issue. I do think the recurve on the top is very important for response issues, but you really have structural issues in play at the same time. There's much more freedom, from my experience, with the back. And it's a combination of species, piece of wood, and how it's carved. If the back wasn't important, why the "tone-guard"? And I also agree that removing wood from the center of either the top of back is a completely different aspect than the recurve. I also suggest on the top- watch it between the bridge and the tailpiece. Don't go too thin there or you could have structural problems. Use a caliper. And use a light bulb.

As far as thicker or thinner goes, we each have to decide that personally on each plate, IMHO.

[whistler]

Poplar (Populus spp.) is sometimes used for viola backs and sides, giving a mellower, less brilliant tone than maple. #There are many species - I don't know which, if any, are particularly favoured.
The true poplars are not to be confused with yellow poplar or tulip poplar (Liriodendron tulipifera), which I have never heard of being used in instrument making.

[sunburst]

Yellow poplar has been used in instruments. I've seen it mostly in old, inexpensive, manufactured guitars as backs, sides and necks. It also shows up in old american banjos.
It's been used for linings, head and tail blocks too.
I don't know of it ever being used for a carved back, but it's just too commonly available not to have been tried.