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"Testing" a box for bridge placement

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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Replies to This Discussion

Permalink Reply by Ron "Oily" Sprague on July 21, 2011 at 11:36am

John,
Was it something we said?

Seriously, if you studied violin and viola, and spent any time at RAM, then I shouldn't even be swimming in the same pool with you! I'm merely self-taught, with enough of an engineer / architect's brain to get me in trouble with real practitioners of those disciplines. One wonders if your years of musical training didn't rub off on your builder's brain. While I was poking around on the Web for guitar engineers, I ran across this guy, whose research into the acoustics of the violin you might find interesting: http://www.phys.unsw.edu.au/music/people/mclennan.html

Needless to say, I relearned a lot of acoustic physics this morning, and better yet, learned some new stuff. All because
you asked a great question! Re your observation that body size reflects the register in which the instrument play: that's pretty much what all the sites I visited this morning said. Also, from reading the FEM thesis I referenced above, plus links to the UNSW Acoustic Research Group, I found that my empirical observations of my acoustic guitars resonating best to an A were spot on, due primarily to top bracing patterns, and thickness of the soundboard. That's why I like engineers as much as I do: they have the patience and the maths to figure out why what I tend to do ( work empirically and logically) works as well as it does.

Keep asking keel questions, John; we'll dig up the maths. BTW, just from my reading this morning, there are several general things for builders I think we can reliably say about the acoustics of any guitar, including CBGs:

1) Thin braced tops and bottoms are good.
2) Tone and bass bars and X-bracing should be able to improve bass responses, and maximize frequency response.
3) Soundhole size, number and placement can help to improve frequency response, but only after 1 and 2 are implemented.
4) Bridge coupling to the soundboard is important.
5) Maximizing volume and frequency response involve a number of design compromises and trade- offs.
6) You can build CBGs reliably without actually "knowing" any of this.

This is fascinating stuff...

Permalink Reply by Roadkill a.k.a. John Maw on July 21, 2011 at 1:10pm

I particularly like point number 6. I appreciate all the extra information. By the way, having been to the RAM doesn't make someone a good player. There were lots of mediocre players there, as well as a few truly gifted ones. Remember, I gave it up for a reason.

Permalink Reply by Ron "Oily" Sprague on July 21, 2011 at 5:30pm

You gave it up because Life had something else in store for you?;-)

Maybe attending the RAM didn't make you a good player, but it also means you're not unfamiliar with a lot more music than many people here. You certainly can't claim ( not that you did ) to be a musical novice, even where CBGs are concerned. After all, violins are just 4-string fiddles with a fancy pedigree.

Permalink Reply by Mark Bliss on July 28, 2011 at 6:12am

As usual we find Johns humility to be chronic. (Thats a good thing John.)

And no surprise, he politely points out that I was making some gross generalizations.

For instance I didnt mean I feel that the sound hole isnt important, just that some people seem to think its size, and location is far more critical than I do. Yeah it frees up the top to move and allows some air movement, but in my opinion it is far more important to consider the top as a whole as a soundboard, and to keep in mind it must transmit a broad range of frequencys, often many at the same time. And I also think that subtle nuance and response is just as big a consideration as acoustic output as far as volume and so on.

The comparison to a speakers bass port air movement is interesting, but to me there are two considerations, that the air movement is the RESULT of allowing the speaker to move freely, and is initiated as a mirror image to the air displaced by speaker cone movement. And that the air movement is a factor of certain frequencys and tones, certainly not the full spectrum of reproduced sound. I dont know what is common these days, (I dont see much interest in accurate sound reproduction to be honest, since the main objective is to create a thudding DUNG and I am not aware of any instrument that sounds like that! But I digress.....) When I messed with "Hi-Fi" speakers, tweaters and usually midrange speakers were housed in a sealed cabinet, while bass and lower end speakers were housed in some variation of a ported cabinet, tuned in such a way that the air movement would aid, not cancel the speakers production of air movement.

Just as Oilyfool, I make no claim to higher knowledge or training. Much of this is just opinion and "reckoning" on my part. A lot of what I read and study goes right over my head, but I would like to think I pick up something along the way. Also similar to what Oilyfool said, I have always seemed to have this inate ability to look at something and not only seem to have an understanding of how it works, but to be able to analyze it for "weaknesses" or see "room for improvement". It just seems to be part of my nature.

The thoughts on utilizing the strengths of the violin design are very general "thinking out loud" theorys. I just think there are some things to consider here.

John makes an interesting observation on the comparison of output, by pointing out the observed weaknesses of the "chin cello" (Viola). My response would include an observation that goes "yes, but you cant compare the output of notes out of the instruments range!" What I mean is that each instrument, if optimized, is only optimum for a certain limited range.

Every instrument is going to have its own dynamic charactaristics, and just as each sound board or surface has its "nodes" each instruments charactaristics are only going to be ideal for a certain tonal range. Maybe each box size being similar to a "node" itself?

I am finding the words to describe my thoughts difficult to come by at the moment, but I want to thank you folks for the conversation and for making me think deeper about this........

Have fun!

Mark

Permalink Reply by Mark Bliss on July 29, 2011 at 6:30am

Heres a couple of videos that touch on some of what we have discussed here, and perhaps of interest to some.

The first is highlights from a lecture available on DVD: "Ervin Somogyi: Voicing the Guitar DVD • The 2009 Healdsburg Guitar Festival Lecture"

The second is a more similar highlights from Ervin.

Permalink Reply by Brian Reagor on August 11, 2011 at 10:55pm

Hello John, alot of good input but I think, for what its worth, you may want to research violin making, especially historical information of violin makers of old, the way they developed the violin body specifically, a cigar box made of Cedar is actually similar in a general sense and researching the violin making might just be of some help, however I must say that I think you already found your answer by testing your boxes the way you did, you could change the tone of the box after you determine at which point it sounds the best by modifying, ie: adding to or taking away from the structure of the box to adjust the tone that you have found.

Interesting notion, Good luck and let us know what you come up with. Brian

Permalink Reply by MichaelS Country Boy Guitars on August 12, 2011 at 2:20am

Mark and John, I am back, wow you guys have been doing a lot of discussing. John, the Padron does make me smile, fantastic job, is the steel bar there so you can play heavy metal with it, lol. just kidding. The only thing I can add to the discussion is that I have put heart shaped sound holes in my latest cbg and I think it makes the sound warmer.

Permalink Reply by Mark Bliss on August 12, 2011 at 2:32am

Good one Michael, warms my heart too!

Permalink Reply by Roadkill a.k.a. John Maw on August 13, 2011 at 5:35am

Who the hell started this damned topic anyhow?

P.S. Michael. The steel bar is a lightning conductor in case someone up there doesn't like my playing (which seems likely).

Permalink Reply by Brian Reagor on August 13, 2011 at 6:04am

Hi John, I thought of a simple way I think that you can test different boxes for sound.

If you make a small wooden frame with 4 guitar strings attatched to a slightly smaller frame placed within the outer frame, you could set this unit on top of each box that you intend to build initially, (maybe put small rubber or foam pieces on bottom of unit), and have the 4 strings tuned to what the cbg will be tuned to, and if you make the smaller framed section so that the strings are across the box instead of with the length of it and be able to move the smaller frame to the left and right across the box you might be able to hear differences in tone, (pitch) and be able to adjust your build, ie: bridge placement, nuts, scale length to accomadate the tone of the box, meaning have the tone you want determined before you actually build the guitar. I don't know if this will work or how accurate it would be if it did work but might be worth considering. Good luck...Brian

Permalink Reply by Mark Bliss on August 13, 2011 at 9:49am

Or just build ten different combinations and see which one you like......

LOL! Hey were just supposin' here anyway.

Permalink Reply by Ron "Oily" Sprague on August 13, 2011 at 11:48pm

John,

What Brian is proposing is essentially using a sweep frequency generator to input sound into your box, at different frequencies, to see which freqs make the box resonate the best (to your ears). In the lab, you'd do this with a sweep freq gen and an oscilloscope; I found essentially this concept at ScienceBuddies (especially look down the page at Advanced; Soundboard Frequency Analysis), along with the following projects, that are excellent acoustic physics backgrounders for musicians, builders and everyday people:

http://www.sciencebuddies.org/science-fair-projects/project_ideas/M...

BTW, just ran across this set of fun facts / food for thought at Taylor Guitars:

http://www.taylorguitars.com/guitars/features/woods/Tone/

Note that they claim not to do any scientific frequency testing of tone woods (except tap testing).

Oily