I've been searching through the articles on this site for information on selecting the position where the bridge should be. Obviously on an unfretted instrument you can move the bridge and experiment, but on a fretted guitar, once the frets are on and the neck is fixed that's it. You have committed yourself.

 

So is there any way to know on a box for box basis (not using formulas because I'm pretty sure that they are worthless unless you always use identical boxes) where the bridge should be to get the best results?

 

Now I've already done some test, but I don't know if they are the work of a visionary (don't laugh) or a pointless waste of time (odds on the latter). I was thinking that as far as the box is concerned it gets most of the string vibration through the bridge. If you want to simulate vibrations coming from the bridge can you use something else that will transmit vibrations and see (or rather hear) how they sound and make a choice based on that. I found a tuning fork and tried it at various places on three boxes that I intend to use soon.

The results from the three were quite different. Not surprisingly all gave the warmest and clearest sound in the centre of the box. The top one gave quite progressive results getting better quite gradually towards the centre. The left hand one was very even across it's width until right near the edge. The right hand one was a surprise as it sounded best right in the middle, tone falling off and getting soft of nasal even an inch away from the motif. Shame as I don't really want to put a bridge right on the motif but my tuning fork test suggests that it may be the best place.

 

So, finally, to the question. Has anyone done any tests - similar or otherwise - that give a good indication of where to put a bridge and that do not rely on ratios or formulas, but take into account the different characteristics of each box?

 

 

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John Maw said:
This was meant to be simple. I think it still is. The only unanswered question is weather it can tell us anything useful at all. I don't know and it remains to be seen (or rather heard).


Part of my point was the possibility that it might not tell us anything useful or - worse - it might tell us things that are misleading. That's not meant to be a put down or to suggest your original post was pointless - far from it. You have definitely started an intersting strand here. There is certainly some work to be done on figuring out how and why the response of boxes varies depending where you drive them with a tuning fork - and if I had a set of tuning forks I'd do it.

I also think you might have misunderstood what I said about how I think it's best to proceed in designing CBGs. I don't think I'm advocating "pinning the tail on the donkey". I do think it's worth looking at what design features have worked well repeatedly and seeing if it's possible to deduce things from that - although I conceded this isn't necessarily straightforward as people have mentioned that sometimes they seem to get different results from apparently similar boxes or designs.

I also think it's absolutely worth fiddling around with whatever experiments one can do with whatever one has to hand. But it's important to be clear about what these experiments do and don't tell us.

My feeling at the moment is that testing with a tuning fork can tell you something about the overall properties of a box (or a finished instrument) - for example, tests with multiple forks might indicate whether it's better for a higher or lower tuning. But I'm skeptical as to whether the tuning fork tells you very much directly about exactly where to put the bridge. I guess it might help you locate nodes and anti-nodes for the various principal modes of oscillation. And there might turn out to be principles for using that information to calculate optimum positions for the bridge. But that needs more work.

(How surprised should we be, I wonder, if it turns out that by coincidence the principles, when they emerge, produce similar results to some of that golden mean construction...)

Finally, for people interested in these sorts of questions, I thoroughly recommend the following book:

"Musical Instrument Design: Practical Information for Instrument Making" by Bart Hopkin (pub. See Sharp Press, 1996)

I got it as a christmas present and it's already proving fascinating and useful.

 

 

Of course it's possible that lots of people will now build CBGs with the bridge smack bang in the middle of the box and they'll sound great and I'll look like a fool. C'est la vie...

Well first I would like to welcome Mark AKA into the discussion, and thank him for his contribution. He has made his point eloquently and pretty much said exactly what I have been thinking.

And another nod of appreciation to all for keeping it interesting, stimulating, and yet somehow still fun. Best thread in a long time in my opinion. No John there may be no simple answer. But there sure is a lot of very interesting opinions.

I personally am not sure about a tap test. or a tuning fork test in the end, on a loose topped, paper covered box of vastly varying assembly quality. (As an example) But if useful in the end, it might be more suited for sorting out the sure duds (acoustic wise) than selecting any sure winners.

Anyway, just for the record, I cant find my tuning fork, which is bad since it was made by hand, by my grandfather for tuning pianos. (Another hobby, quite a character.) But the discussion has me thinking about how four or more decades ago he would use a DB meter to analyse a room, "tune" his custom built loudspeakers, set his crossovers..... A whole story in itself. Maybe another time!

Hi to both Mark and Mark.

 

Just a quick point or two. I hope it didn't sound as if I thought you were advocating the "tail on donkey" approach. The opposite is true in fact. My point is that as far as acoustic information about the box goes, most people have nothing to go on. That only leaves factors such as playability and looks (and I'm not saying that there are not also important). I have no doubt from what I have read that neither of you fall into the category of haphazard building, and it is obvious that both of you are much more knowledgeable than I am about instrument construction. Where I see this test possibly being useful is for those who haven't done much reading on the subject and just want some small indication of how the box might respond to different bridge placement.

 

As ever, the one question that isn't being answered is weather the test is of any use. My gut (cat or otherwise) feeling is that it can be helpful but I have no more to go on than that. If anyone wants to test the test, please let the rest of us know the results.

 

P.S. Mark B. I hope you find that tuning fork.

 

 

 

 

I am amused, thank you.

 

Hi, This is a bit lengthy but stumbled accross it when looking for F hole templates. Its a paper on the stradivarious violin & the proprotion and placements of f-holes. He used the golden mean to do his calculations. I know its a fair stretch from cigar box to stradivarius but I thought it might add something to the Golden Mean part of the discussion that Hoov bought up.

placement of F holes & the Golden Mean

Hi Penny.

I was more interested in a test than a rule (as they say, no rules). A test can take account of variations in materials and construction. A rule based only on dimensions can't.

I think Stradivari would have made cigar box guitars if he had known about them. Shame he didn't have the internet to inform him.

Late to the party as usual, but fascinating to see the thought processes. Quicky Googlization revealed these:

http://www.indigo.com/tuning/music-tuning-fork-sets.html

And these (much more affordable):

http://www.amazon.com/Musical-Tuning-Forks-13-Set/dp/B0017Y1EXA

And now, my $0.02 as an amateur musician, and professional scientist (geologist / geophysicist):

Not being able to find my tuning fork in the melange of my workshop, I instead opted for the tap test (which many luthiers do to judge the vibrational qualities of tone woods; I also seem to remember reading, maybe in GuitarPlayer some years ago, about some outguess using tuning forks to test tone woods.). You have to have good ears for this, and almost by definition, nearly everyone does. What!?, you say. but I'm nearly tone deaf. Not if you've learned how to speak! You did, didn't you? Human speech falls in the frequency range of 85 - 185 Hz for the average male, and 165 - 255 Hz for the average female. As one of the Marks earlier said, the average 6- string resonates between 80 - 600 or so Hz. So a standard guitar spans the frequency content of all human speech, which is why we probably like it so much.

Tapping the box will reveal a variety of places where the wood resonates. Using a tuning fork will only tell you at which points the box resonates at the calibrated frequency of the tuning fork. Most people who are not pros use an A tuning fork. Interestingly, I've noted empirically that my acoustic guitars seem to resonate best on the A string, where A = 436 - 438 Hz (Baroque A, or close enough); I've checked this with one of my fancy $20.00 chromatic tuners.

I also note that many old CBGs had a central sound hole, as do most acoustic guitars, e.g., right where the soundboard resonates the most! I suspect this is the spot John has actually found with his tuning fork test. Kaman Musical Instruments, who make the Ovation, have done extensive sound tests since the 1960's on bracing patterns, sound hole placement, soundboard thickness and resonance. They used to have the charts and graphs, with glossy photographs with circles and arrows and a paragraph on the back...sorry, Arlo just popped into my head. Anyhow, for a long time they used to have this info on their website; remember, Charlie Kaman was a helicopter design engineer and musician, and combined what he knew about the resonance of helicopter blades with what had never been previously scientifically analyzed about musical instrument construction.

So, you may ask, why is the sound hole so big, and in the center of a guitar soundboard? Vibration of the board causes the air molecules in the box to move, further interacting with the top, base and sides of the box, creating sound.The hole lets the sound out, and depending on its size, can be tuned for the dominant frequencies it lets pass. That's why the Ovations often have different sized shoulder sound hole clusters: the shoulders of a standard guitar are secondary resonating sweet spots, and the various hole sizes are frequency-tuned ports.

So, I suspect that John has discovered, not where the bridge should go, but instead, where his optimum sound hole should be. Here's where a design compromise comes in - that tends to be where cigar box makers put their logos, some of which are right nice. So taking a leaf, so to speak, from Kaman's Tree of Knowledge, the next best spot for sound holes would be one or both of the front (nearest the neck join) shoulders. Why not the back (nearest the tail) corners? Because the bridge is designed to transmit, or drive, the string vibration to the soundboard. And it gets placed where? Uh-huh, another design compromise! For further confusion, take a look at a Selmer Maccaferri from the 1930's: thin bridge very near the sound hole, and relatively small sound hole for such a big box. Why? To drive the air inside the box to produce a deeper driving rhythm tone over an unamplified orchestra in a club or dance hall - go Django!

Lest you think I've gone off the deep end, most luthiers will tell you that guitar or wooden instrument building is a series of compromises. Further proof of this is straight frets for well tempered tuning. Try "just" tuning a guitar, and see how bad that sounds, even if it is mathematically correct. Only in the last few years have weird fretboards attempted to deal with the design compromise that is tempered tuning - some have fanned frets, while others have mangled-looking offset fret designs, to arrive at something playable that is also justly intonated.

I've done the tap / fork test on my first two unfinished boxes; one of them resonates beautifully in the exact center of the bottom, while the other has a dead spot there, but resonates increasingly better toward, but not at, the glued and screwed edges. I expect them to sound different. Both beautiful, but with their own sonic personalities.

Build on!

Thanks Mr. Oily.

That really is a very interesting contribution. Detailed but easily understood and well described.

The one bit that I don't understand is why you think that the test is likely more useful for sound hole placement rather than bridge placement. Sound holes don't transmit vibrations to the box surface. The bridge does and the tuning fork does too. Not saying you are wrong. Just that I don't understand.

I was intending to add a bit to this post in the next few days. One of the reasons that I became interested in the tuning fork test was that when I tried it on one particular box I got very different results from others that I had tried. Most seemed to be clearest near the centre but loose that clarity only gradually as the fork was placed further and further out, suggesting that a compromised position (because of the cigar makers logo) would not be too bad.

One box (the Padron, on the right in the photo at the top) had an extreme sweet spot right where the logo was and some really duff or buzzy sounds moving even a small way from the logo. I didn't want to cover the logo but was concerned that I was compromising the tone of that box.

What I can say now that the box is nearing completion is that the characteristics of the box changed dramatically during the making. I thought that cutting the box for the neck or for sound holes or the mag pickup might change things, but they didn't. What did, was adding a couple of internal bracing strips similar to a violin bass bar. The sweet spot spread out wide and with progressive and only slight falloff.

I now have a temporary bridge on and am very pleased with the sound.

I agree John but always useful to take a look at how others have approached it in the past. There was an interesting throw away line in there somewhere that whilst the distance between the circles in the f hole was consistent in some instruments the placement was not symetrical which may indicate that they were placing for maximum resonance.

Found another link with a very funky hand carved violin and he had modified the bridge to maximise resonance. 

I find all the little bits of stuff like this all food for further thought.

Roadkill a.k.a. John Maw said:

Hi Penny.

I was more interested in a test than a rule (as they say, no rules). A test can take account of variations in materials and construction. A rule based only on dimensions can't.

I think Stradivari would have made cigar box guitars if he had known about them. Shame he didn't have the internet to inform him.

John,

Yes, the tuning fork transmits vibrations of a certain frequency to the box soundboard. And yes, the soundboard will vibrate best at a certain point or points at that particular frequency. That may indeed be the middle of the box, for a particular size box, as the ends of the soundboard will be fixed nodes (what one of the Marks was alluding to in an earlier post), exactly like a string tied at a tuning peg and a bridge. Halfway from the nodes will be the anti-node, where the point of greatest vibration should be. Without bracing, this will also be the point at which the soundboard flexes the most. Putting a bridge there should indeed result in max flex - over time, however, this could also result in bowing and weakening of the soundboard. So one design compromise is to move the bridge further back.

That's why I mentioned that the max flex spot could still represent an optimum sound hole location. If you put a sound hole at this point, you will set up two smaller nodes( the edges of the sound hole), but overall the soundboard will still flex / vibrate the most there. That being where it vibes the most, is also the point at which max air is being pushed around inside the box. Vibrating air = sound. Max vibe = max sound. Best place to let the max sound out would then be...

That doesn't make me correct. That's just what logic tells me. Which is why I like your idea of a simple test. In my day job, we routinely spend $20 mil drilling wells on not much more logic. And quite often, we're wrong, because of some unforeseen niggly yittle detail. Problem is, that $20 mil hole IS our simple test. Life 's a lot easier with a tuning fork ;-)

And you're correct, the sound holes do not drive the box by transmitting vibration


oilyfool said:
John,

Yes, the tuning fork transmits vibrations of a certain frequency to the box soundboard. And yes, the soundboard will vibrate best at a certain point or points at that particular frequency. That may indeed be the middle of the box, for a particular size box, as the ends of the soundboard will be fixed nodes (what one of the Marks was alluding to in an earlier post), exactly like a string tied at a tuning peg and a bridge. Halfway from the nodes will be the anti-node, where the point of greatest vibration should be. Without bracing, this will also be the point at which the soundboard flexes the most. Putting a bridge there should indeed result in max flex - over time, however, this could also result in bowing and weakening of the soundboard. So one design compromise is to move the bridge further back.

That's why I mentioned that the max flex spot could still represent an optimum sound hole location. If you put a sound hole at this point, you will set up two smaller nodes( the edges of the sound hole), but overall the soundboard will still flex / vibrate the most there. That being where it vibes the most, is also the point at which max air is being pushed around inside the box. Vibrating air = sound. Max vibe = max sound. Best place to let the max sound out would then be...

That doesn't make me correct. That's just what logic tells me. Which is why I like your idea of a simple test. In my day job, we routinely spend $20 mil drilling wells on not much more logic. And quite often, we're wrong, because of some unforeseen niggly yittle detail. Problem is, that $20 mil hole IS our simple test. Life 's a lot easier with a tuning fork ;-)

And you're correct, the sound holes do not drive the box by transmitting vibration; however, they do allow those good vibes to escape! The addition of your tone bar created further end nodes, thus moving the endpoints ( moving the goalposts in American football / management- speak), and causing the box top to vibrate better between those new end nodes. Just curious: did you glue the tone bar transversely, or lengthwise? Even that makes difference.

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