Robbie O'Brien wrote:
you can certainly analyze this stuff to death.
Yea, that darned engineering degree can drive one in that direction
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I'm trying to learn the "Whys" of stuff primarily because I don't build guitars (although I'm building one now) and there are no good plans or direction for the instrument I do build so I figure if I can understand a little about how a guitar works perhaps I can transfer that knowledge to bandura, or at least make better guesses. My current bandura design works pretty well but can certainly use some improvement/refinement.
I just went down stairs, held the top in the upper bout sweet spot and tapped the top for the guitar I'm building right now and could get a large variety of tones depending on where I tapped. I also held the X-brace about where you peak them and I could definitely spot the node you were speaking of right about where you said. The tap tone there was definitely louder and less damped when I held it in the right spot but was lacking the bottom end that it had when holding at the sound hole or the sweet spot in the upper bout.
To further explain my question a bit, the way I see it is this - from a structural standpoint (i.e. resisting bridge torque) I don't think that the peaks of the scallops are helping you very much. i.e. you could probably cut the peaks down to be the same height as the thinnest part of the brace and you'll still get mostly the same "strength" in the top (That's probably not entirely true but for the most part would be correct - weakest link sort of thing). I think we all agree thought that less isn't always more in the case of tone and adding mass can sometimes help rather than hinder.
If we look at these pics:
http://www.phys.unsw.edu.au/~jw/guitar/ ... _engl.html one can easily see that sweet spot up in the upper left/right you were speaking of - a lot of modes have that spot as a node. Holding there and tapping must let a lot of the modes ring leading to a full sounding tap.
On the other hand, the area 3" in from the rim along the X-brace and tone bars don't seem to show up in very many of the modes - At least not to my eye. It does however show up in the mode at 221Hz (I think it's called ring and a half). I also recall reading that guys Like Al Carruth try to "close" that ring when it's open like the one in the link. That's implying to me that this is an "important" mode for the tone he's after.
I'm thinking leaving the mass in the peak at that node line should have less effect on that mode and any other mode which has nodes there but would lower the amplitude of modes that have antinodes in that spot. I can't really guess what the added stiffness in that local area would do but you'd think it would have to do something for tone as well (hopefully Al will chime in as I seem to recall reading a post of his a while back that directed how to shave to get certain effects in the chladni pattern). The more I think about it, it could be that the stiffness is way more important than the mass since shaving only changes mass linearly while it changes stiffness exponentially.
Here's a swag hypothesis I just came up with:
Thinning the braces loosens the top for added bass, but perhaps doing it by scalloping rather than just uniform thinning suppresses some, but not all of the lower frequency modes while favoring the "ring and a half" (if that's what it's called) to give the instrument a more balanced tone. Or to put it another way, a tone we like.
Thoughts?
p.s. sorry to ramble but it's difficult for me to get a grip on this stuff.
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![[clap]](./images/smilies/smiley32.gif "adulations")
Sounds like you are losing sleep over this. Remember that if there were a recipe for this everyone would just follow the same recipe and bake the same cake. Fortunately in our case what works for some may not work for others. Some guys get satisfactory results one way and others another way. This is what keeps this game exciting. What I am trying to convey in the video is to not try and re-invent the wheel. I get this all the time in my classes where guys come in re-designing everything about the instrument before they have even built one. I recommend that folks draw on the knowledge of past generations. Use what has been given to us. What makes us think we can build a better mouse trap when past generations have already stayed up at night thinking about it? Take what has been given us and then make it your own. Add you own personal experience and touch. ![[uncle]](./images/smilies/surrenderflagqv6.gif "Uncle")
