Official Luthiers Forum!

Hey All,

I have been doing some bracing style testing and trying to get a more responsive and louder sound. I have been taking tops on and off of a dummy rim to test my results. I am beginning to realize that I have been just wondering around in the dark. I took a plot and posted a picture of it below. This is a guitar in the white resting on my leg, tapped 20 times on the saddle with an eraser mallet, with the sound hole uncovered. The sound is one of my better attempts. I still feel like the trebles dont sing enough and I would like more volume. This guitar is one of my louder renditions, but I still feel that it could improve. If you have any questions about the bracing, or anything at all please feel free to ask. I have been trying to take pretty good records of what I have been doing in order to try and find patterns. Thank you for reading and hopefully you can help steer me a little.


Thanks,
Rob

What body size?

_________________
Formerly known as Adaboy.......

I knew I was going to forget something....It is pretty much an OM, slightly bigger, but not by much. I believe the body is about 4 1/8" deep at the tail and 3 7/8" deep at the neck.

Thanks

It's really hard to know by just looking at the one plot and not knowing anything else about the guitar. I don't even know if this is a steel string or nylon string, I suspect steel string though.
I'm guessing this is a large-ish bodied guitar with the low main air of under 90 hz.
It looks like the main top is about 162 or so, pretty low and I suspect this is either a pretty floppy top or pretty heavy top, a heavy bridge or some combination of these.
It's been my experience that guitars with a higher main top mode have better trebles and a brighter clearer sound, though they can lack other things.
Now all this is conjecture on my part since I know nothing about the guitar.
Trevor Gore is the resident expert on this, hopefully he'll come along and give a more pertinent reading of the tea leaves.

Edit: I see the body size was posted while I was typing. Everything seems low to me for an om.

_________________
Jim Watts
http://jameswattsguitars.com

Agreed with Jim -- main air and main top are both low which explains the lack of treble. Need a thicker/stiffer/better top, stiffer bracing, heavier sides, and/or lighter bridge/plate. The heavier sides and lighter bridge/plate would help volume; stiffer bracing and a thicker top would probably make it a bit quieter albeit better balanced. Can you post details on the top, thickness criteria, and bracing?

Also, how did you get Audacity to zoom in on the frequency scale (0-800 Hz)?

_________________
David Malicky

i'm all for technology advancing design, but....tens of thousands, maybe hundreds of thousands, of luthiers have experimented with different acoustic guitar designs over a span of least 250 years, and most of those designs have been auditioned by millions of musicians- the best sounding/performing designs(using wood) are the standard designs that remain today....somehow i don't think an iphone app is going to exactly revolutionize guitar top bracing design at this point.....no need to "wonder around in the dark"

You feel it's lacking in the higher frequencies but how about the rest? Is the bass strong but undefined? How about the mids?


Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

So this top`s bracing layout is a lattice below the X, with a thinner but taller X brace. The top was deflection tested with 5 lbs. at an 18" span to 0.33, which brought the thickness in around .090. I know this is crazy thin, but I have read of other guys going even thinner than this. The bridge plate itself weighed 14.9 grams. The bridge minus the saddle and the pins weighed in around 25 grams maybe a little lighter. I know that this set up is light and maybe has longevity issues, but the top doesnt have a belly and really didnt move all that much when I strung it up. I have had this one together for a few weeks now and still no weird bulging.



I just kept on enlarging the plot until it got to a readable size. If you were to see the whole plot it is around 4ft long.



As far as tone goes, I don't think it sounds horrible. The bass is nice and lush in my opinion and the mids feel nice too. It does very well in the first position. I would like to have thicker trebles and more volume. I can record a quick clip if you would like.

This guitar is never going to be finished it is merely a bracing/learning exercise to try and get more out of my guitars. This is the 3rd tops that I have pulled off of this rim now. I recently read Somogyi`s book which really helped me, but I am still looking to get more monopole mobility out of my tops. I have a borrowed version of Trevor Gore`s book and he mentions something about the volume he is getting from his instruments. I recently heard another builder`s guitar that had the volume that Trevor was talking about. With out hearing one for myself, I am not sure I would have believed it. After hearing this guy`s guitar I couldn't believe how loud it was. After hearing this guitar I know that there is room to improve.


Thank you guys for all of the help.

.33 is far far far beyond where I go. Very far. Different country far. Wrong exit far. There's your trebles up the dust chute far... Stick with the Gore/Gilet books.

What do you shoot for?

I dont normally go this far but I just wanted to kind of see what would happen. I have gone even farther than this on other tests.

I've had problems getting good treble response from a really thin top. I feel like you need some good consistent stiffness around the bridge for at least a few inches which is tough to do with a floppy plate. To keep the bass strong with a thicker top I would loosen up the edges to the point where there is pretty much no bracing around the rim and might even thin the plate out at the edges as well. In my experience you can really pump up the volume by using a super light bridge (under 20g) and bridge plate (10g or so) and the difference can be quite dramatic. All of this is just my opinion of course; there are hundreds of ways to skin this cat.

.200 for dreads, .235 for OM and smaller...

I am little confused. So in the gore/gilet book, figure 2.3-7 pg. 2-19, the first peak is at 90.2 Hz, the first dip is at 109 Hz, and the second peak is at 172.3 Hz. I am pretty close to that on this top first peak is around 88Hz, my first dip is at 110Hz, and my second peak is 163Hz. It seems that I could use some stiffness, but not that much. He mentions the uncoupled back to be 210Hz and my back is around 183 Hz. Maybe my back could use a little more stiffness and this would possibly give me a little more volume which may bring out the trebles. Now this frequency response wasn't stated to be ideal, I am just using it as a reference. Maybe I could open the sound hole up some, to bring things up a little. The soundhole diameter is around 3 7/8". My understanding of all of this is very weak so please bear with me.

Thanks for the tips. I will look into ways to reduce the mass of my bridge and bridge plate. I debating between decreasing size or using the spruce ebony lamination for the bridge plate.

nyazzip, Certainly, the standard designs are excellent. But it's not fair to say every one of the newer designs offer no improvements. E.g., Smallman, Somogyi, Gore, Bashkin. The point of doing frequency analysis is to understand why a guitar sounds as it does, and give reliable and repeatable targets for obtaining a certain sound. Some luthiers do just fine without it, of course, but some others do better with it.

Robbie, for Somogyi's thin top approach, I think it's helpful to have more arch, more interconnection between braces, more braces (like your lattice), and thicker bridge wings--those bring back the stiffness lost by the thin top. Also changing kerfed linings to solid/laminated helps treble in my builds. My guess is it will take a lot of trial and error to fine tune the Somogyi approach, given how different it is and the few design details in the book. And also a great top with very high cross-grain stiffness. I've read that many people attending Somogyi's workshops tried thin tops but eventually returned to more conventional approaches, so there is likely a set of variables that all have to be right for it to work well. If going down a new road, I'd go with Gore's Falcate as he lays out everything in the book and has data to back it up. 15g is a heavy bridgeplate; 25g is about average for the bridge. What OS are you using with Audacity (I haven't been able to enlarge my plot that way)?
For reference, here are my tap testing results for a number of guitars, mostly larger bodies (3rd post): viewtopic.php?f=10101&t=31330
The best sounding OM of that bunch was the H&D OM-R (111 Air, 202 Top, 239 Back). Some pictures of your bracing would probably help suggest changes.

_________________
David Malicky

I will look into those things you have suggested in my next attempt. Do you think that I should only look at the top stiffness or should I also look into beefing up the back a little?

I will definitely look into doing a falcate top. I just have to finish up reading the Gore/Gilet book. I am definitely going to eventually order my own book. This book makes me want to break out the highlighter and take notes like an academic book. I have been trying to take in everything I can and produce better guitars. I am still struggling with ways to get a louder guitar. I intuitively kind of know how to get treble or bass, but volume is still kind of a mystery. I suppose as I digest the book I will hopefully I will gain a better understanding of this. I have been hinting to my girlfriend that the gore/gilet books would be a great christmas gift.

My operating system is OSX. I just pulled the little hashed corner on the lower right to expand it.

Its funny that you reference that thread because I already have that document saved on my computer for reference.

It's great to see that you have the confidence to experiment!

Given what you've told us about the guitar, I think many of the others have got it right; not stiff enough or too heavy or both. The resonances you have on this guitar are pretty close to what I've seen on some of Somogyi's work, and I know for a fact that he has occasional issues with the trebles, too, so don't feel too bad about it!

For a high monopole mobility medium sized SS guitar for finger picking the lowest I'd want to go is 90, 170, 214Hz for the T(1,1)x, but that can get a bit "flamenco" (brash sounding). Total mass for bridge (no slot) and bridge plate together ranges from ~25gm to 30gm when I'm after high monopole mobility. So looking at your numbers, you're significantly up on that at ~40 grms (combined) which is enough to make quite a difference. So the 10gms to 15 gms and the fact that I also have two layers of CF in there, too, would easily get me up to 170Hz for the T(1,1)2 leaving the rest of the structure the same. 10Hz or so at these frequencies makes for quite a different sounding guitar.

Monopole mobility correlates strongly with perceived loudness. So if you look up monopole mobility (p1-89), you can measure it and see where you stand. Monopole mobility rises (generally a good thing) when both stiffness and mass are reduced. With the stiffness, you still have to satisfy static criteria (withstanding string pull over the long haul) so there's only so far you can go, but by using CF you can eliminate much of the material you have to put in just to delay cold creep, and that's where you really win out with composite structures.

Finally, if you make the back too floppy, that can be a serious tone and volume killer. With an uncoupled back response at 183Hz, you're pushing the limits if you've not already exceeded them.

Have fun and good luck!

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au