Andy Birko wrote:
I was thinking of starting a new thread but I figure I'll post it here instead.
I do something similar in my banduras where the hole drilled in the bridge provides the break angle. You can see it in the picture. Now in my first build, the strings went all the way from the top of the bridge all the way to the level of the top creating a lot of down force on the top. Over my last couple of builds I've slowly been lifting the attachment point of the strings at the tail piece to eliminate the down force because my tops end up flattening (or in some cases even going concave) due to the downforce. You can see the "tailpiece lift" in the attached pic.
And you know what, my sound has been improving with each instrument and my tops have been deforming less and less. I'm not ready to put a causal relationship between the lack of downforce and the good tone but I at least believe it's not having a negative effect at this point.
In my next couple of builds I'm going to make the lift the same height as the saddle to get rid of as much downforce as possible (there will still be some due to a chromatic row of strings that will always have a significant break angle) and see what happens.
I have a thought and related question regarding how to get a good sound like this and why this wouldn't work on an archtop or violin type of instrument without modifying the design significantly.
In an archtop instrument, specifically of the violin family, you need a lot of downforce and break angle on the floating bridge to keep the strings firmly "connected" to the bridge while playing. A violin bow introduces a lot of energy into the string and without enough break angle, the string may "bounce" off of the "saddle". To deal with this effect, I glue my bridges to the top (as do the other zero torque/tension designs). This mechanical connection between the bridge and top in conjunction with the break angle provided by the holes in the bridge means that a vibrating string may displace the top "upward" without bouncing off the saddle.
The question is: how does the down force on the bridge affect the tone? It stands to reason that the downforce will raise the tap tones of the top the same way stretching a string or bending a saw raises their frequencies. One could also see that the top may vibrate differently when stressed than otherwise due to "preloading" of the top.
I've heard others say that increasing the downforce on an archtop type instrument will "increase the volume" but I just can't see how that could be so. The only thing I could see is that increasing the downforce would increase the maximum amplitude at which the strings may vibrate without bouncing or buzzing. However, for a given amount of energy put into the string, either by plucking or bowing, I just can't see how it would be any louder with more downforce on the bridge.
Andy Birko wrote:
So Al and Dennis, Any thoughts on my post as to how preloading the top ala archtops may affect tone?
Hi Andy,
It's not false modesty when I say that I'm not in the same league with Al Carruth. Al is the "real deal" - a left-brained luthier, a scientist, and a luthier with many years of experience and many instruments under his belt. If he told me the sky wasn't blue or that water wasn't wet, I'd go look it up somewhere to try to understand in what way that was true, but Al would have made the comments after his own experimentation, observations, and analysis. I try like heck to pepper my comments with "it seems" and "I believe" to show that a lot of what I write is conjecture and gut instinct. I'm not entirely right-brained, (actually I've tested right down the middle between left and right), but even the scientific side of lutherie for me always has a big dollop of instinct in the mix. Not that Al's right hemisphere is lying fallow - but he's certainly one of the "go to" guys for a scientific approach to questions in lutherie.
I think I need a "conjecture" bbcode for the forum.
Andy:
The question is: how does the down force on the bridge affect the tone? It stands to reason that the downforce will raise the tap tones of the top the same way stretching a string or bending a saw raises their frequencies. One could also see that the top may vibrate differently when stressed than otherwise due to "preloading" of the top.
I've heard others say that increasing the downforce on an archtop type instrument will "increase the volume" but I just can't see how that could be so.[conjecture]I think you already know more about this than I do. You've been experimenting with modifying the downforce.
It stands to reason that downforce will play a major role in the sound of an instrument like a bandurra or mandolin or the violin family instruments, and archtop guitars. Too little, and the parts will not physically acoustically couple very well, and too much and you could kill or severely dampen much of the potential sound of the soundboard (especially towards bass) by restricting vibrations. (less amplitude, less sustain) The sweet spots lie between those extremes, and exactly where the ideal is, is probably somewhat within the ear of the beholder. I think you're on the right track with your experiments, and you will be able to offer a bandurra buyer an instrument geared toward a specific timbre and that respond well to specific styles of playing, within a variety of possible timbres. Just like the very heavy-handed guitar flatpickers may be looking for something completely different in a guitar than a fingerstylist, you'll have the ammo (the knowledge) to be able to confidently build a bandurra that meets the desired criteria of the player.
Reducing the downforce near the extreme of too little downforce would allow you to modify the construction of the soundboard to thinner, lighter, and more flexible, which I'm confident would bring out more bass response in the instrument - but at the same time the gradient is shifting away from treble response. Since I'm much more familiar with mandolins than with bandurras, (but think of these instruments as having an upper midrange to treble overall timbre, traditionally) I'm trying to imagine someone like David Grisman playing a mandolin that had an extended bass range at the expense of the treble range - and I think it would be unacceptable. So, as a cheerleader on the sidelines as you do your experimentation, I'd encourage you to move toward less downforce, modify (thin, lighten, reduce bracing on) the tops to permit more movement and expand the mid bass and bass, and note the point where there is ANY discernable loss of the signature high-midrange and treble response - that would be my line in the sand.[/conjecture]
Not sure if that helps at all, but at least you know you have a supporter, cheering you on in your experimentation.
Dennis
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