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Finished my first acoustic instrument. yes.......




The sound is incredible for a first go, loads of bass, very loud, but sounds quite a bit like a banjo, my goal is to have it sound as much like a guitar as I can go, but in the meantime I'll just shoot for clarity and focus.

After finishing it, I decided to make some changes, those stupid pegs are gone forever, the bracing can be thinner, going to think over how to do the back, etc.

The floating bridge will also be replaced with a dreadnought guitar style bridge, I just need to figure out just how it's going to be designed. The instrument body is 9 1/2 by 10 1/2 inches and the scale length is 15 1/4 inches. the instrument is tuned DAD and the strings I have on it are about .042", .032", and .023". I tried to be as close to mandola scale length/string size as possible, but tune it DAD.

Here's the bracing pattern:


Does anyone have suggestions as to what I should keep in mind when making a bridge for this thing like thickness, length, and maybe adjust other factors of the instrument to accommodate this?

Thanks,

Ian

I dunno....I think it looks just right with the floating bridge. The bridge in the photo looks taller than the usual glued on bridges. I would worry that a dread style bridge would be too thick and heavy.
Would you keep the tail piece with the new bridge?

I'd leave it the way it is too. I'd be worried that a pin bridge would take up a lot of real estate on the relatively small top.

It's a cool instrument. The fret interval is like in an Appalachian dulcimer? I'm curious about what the original motivation was.

_________________
Jim Kirby
kirby@udel.edu

I guess the question is; what are you expecting the change to do for you that could not be done some other way? A related question is; what's it going to cost?

First: methinks the 'banjo' timbre is inherent, or, at least, would be difficult to mitigate. IMO, the shape is one of the reasons guitars sound 'guitarlike', and without that shape you're going to find that there are limits as to how close to that sound you can get. The shallow sides and relatively large hole also contribute. There's not much you can do about the side depth, but an inside 'sleeve' or a ring to make the hole smaller might help; at the cost of some power.

How high are the strings above the top at the bridge? A guitar style bridge will be carrying a static torque load that depends on the string tension and the height of the strings off the top. If the string height is much more than 14 mm I'd start to be a little concerned. Even though you're not carrying a lot of tension, you have to balance that against the restricted area you have for gluing down a bridge.

In that regard, the depth of the bridge from front to back is more important, I think, than the 'footprint'. The static torque and shear force in the glue line add up so that the peak force is seen along the back edge of the bridge, and it's that peak force that determines whether the thing peels up or not. The wider the bridge is along the line of pull, the more leverage you've got cutting down the peak force.

There is a school of thought that says the tension change signal of the vibrating string produces most of the sound. I disagree. For one thing, the transverse force is about seven times greater than the tension change on the average. So long as the string travels at least somewhat 'vertically' with respect to the top surface, the up and down force on the top will be greater than the tension change. Also, tops are built to resist deforming under bridge torque, so it's hard to get much energy out of the string and into the top that way. Finally, if the top does deform in that sense, part of it is going 'up' and part 'down', so there's some cancellation of the effort. With the transverse force the whole surface is moving in the same direction, and that's much more efficient.

The point here is that with a tailpiece you lose the tension change signal as an active driver of the top, and this is often cited as one of the big reasons why, say, archtop guitars sound different from flat tops. While the physics is certainly true, I'm not at all convinced that the acoustics, and particularly the perception we have of the sound, necessarilly follows. There are usually a lot of other differences between flat tops and archtops, and those seem to account for most of the differences in timbre to my way of thinking. If you'd like more info, with data, on all of this, there's a PDF called 'String Theory' on my web site, under 'Acoustics', that you can download.

At any rate, there are a couple of things you could try to modify the tone of your instrument that would be less work than replacing the bridge with an entirely different type. Aside from the soundhole mods I've already mentioned, you might try adding some weight to the bridge you've got. The fact that the bass is good suggests that you don't really need to drop the 'main air' resonant pitch, which is one thing the soundhole mods would do. The 'cutting' high end might be there because the light bridge is easy to move at high frequencies, and relatively stiff since it's so high. Adding some weight could drop the 'admittance' at high frequencies, and take off some of the edge, I think. Get some poster adhesive (_don't_ use modeling clay!) and stick a wad to the bridge. If it helps, add another wad. If it kills the thing, take some off. If you don't like the change in any case, no harm done.

One last thought: if you're going to switch to a pin bridge, you'll need to put in a bridge plate to keep the ball ends from chewing up the top.

First off, this is NOT the last of these instruments. I'm talking about making another one of these, just with a few modifications, so the instrument I'm talking about modifying doesn't exist yet, I'm just trying to design it to be more of what I want than this. I forgot to mention this. The top on the new instrument will be redwood, the back and sides, sapele, the rest hasn't been planned out yet, and I'm really just using the woods mentioned because I already have them. Soundhole size and shape, bracing pattern, and other aspects are wide open. I'd like to keep most of the dimensions of this instrument though because the thing feels very comfortable and I'd like to keep it small.

The bridge height is actually 13mm, and what I was hoping to do is just add some aesthetic appeal and see how the sound would either contribute or take away from the thing. Also thinking about scale length, if I used a longer scale length that would allow me to use thinner strings. Would that help me get the sound I want? It's hard to understand just what you're saying because I learn by seeing, feeling, and hearing, but I think I get the gist of it. Are you suggesting dampening in certain areas to make the sound, say, "thinner"? In which case I think putting a large mass on the top would help. I feel very uneducated right now and I have no one to teach me these things first hand. I greatly appreciate your efforts though and encourage you to keep trying.

Thats awesome! I dont have any advice for you because im pretty new to building myself.
But I just wanted to say so many people wont do anything if someone doesn't show them how.
The fact that you dig in and make it happen on your own without someone holding your hand on the way
shows intelligence. Keep it up I look forward to seeing more pics!

Is the fret interval like a dulcimer?

Yessir, and thanks. Tried to do something no one's done before, at least not apparently. Being an Indigo I always wanted to do stuff differently and better but never got the guidance I needed until recently, and even now it's limited because my favorite mentor lives in Canada of all places lol. I didn't even hear the word "luthier" until about four years ago, but I think it's been in me ever since, just now getting some results of my labors planning the dadgum thing out and teaching myself how to put it together. I love forums!

Ian wrote:
"Are you suggesting dampening in certain areas to make the sound, say, "thinner"? In which case I think putting a large mass on the top would help. "

First: I tend to learn best by 'getting the feel of it' too. People who start out by writing equations bug me. What I was talking about there was the notion of 'impedance', which is a little tricky. Basically, it has to do with how easy it is to move something at a given frequency. It's not that hard to get something of a feel for it, though.

Imagine you've got something like a ruler that you've clamped to the edge of the bench, with most of it standing out sideways and free to vibrate. Stick a lump of something fairly heavy on the end. Now you've got a pretty good 'spring and mass system': most of the mass is in the lump on the end, and most of the 'springyness' is in the ruler. If you pull it aside and let go it will 'twang', and we all know that the stiffer the ruler is the higher the pitch will be, while more mass will make the pitch lower.

Now, push the mass aside and hold it. You've got to do some work (apply a force through a distance) to get it moved aside, and you've got to keep exerting force to hold it there. You're working against the stiffness of the ruler here. If you push the mass slowly back and forth, the stiffness is what's 'impeding' the motion: so that's the 'stiffness impedance'.

If you try to move the mass at a higher frequency you've still got to work against the stiffness to move the mass off center, but as it's swinging back you can allow the spring to do some of that, so as the frequency goes up you have to fight the stiffness impedance less and less. On the other hand if you are always trying to keep the same amplitude of motion the mass has to move faster at higher frequencies, so you've got to do work more to overcome the inertia of the mass; that is, more mass will 'impede' the motion at higher frequencies, but not so much at low ones.

There will be some frequency at which the energy stored in the displacement of the spring when it's all the way over will be the same as the energy stored in the inertia of the mass when it's in the center and moving it's fastest. The spring and the mass will trade that energy back and forth, and you'll be able to get a lot of amplitude by just feeding in enough power to overcome the friction in the system. This is the 'resonant frequency'. The physics types would say that the 'capacitative (stiffness) reactance cancels the inductive (mass) reactance, leaving only the resistive (loss) reactance'. But I wouldn't.

The point here is that the impedance of stiffness is high at low frequcies, and falls off at higher ones, while the impedance of mass is low at low frequencies and rises as you go up.

The bridge and top of your instrument have stiffness and mass, so there's an impedance there. The same holds for the strings, although with them it's 'tension' rather than 'stiffness'. It turns out that when the impdeance of two things, like a string and a bridge, that are hooked together is the same, the energy can pass from one to the other easily. You want _some_ of that on your instrument , but not _too_ much, or you'd have no sustain.

But remember, impedance varies with frequency. A light stiff bridge is likely to have an impedance that more or less closely matches that of the strings at high frequencies, so that energy gets out and into the top easily. Making the bridge heavier will raise the impedance, but most especially at high frequencies, so you won't get quite as much treble in the sound.

Luckily, it's easy to check this out and get a feel for it. Get some poster adhesive ('Blu-Tac' and 'Fun-Tac' are a couple of brands) and stick a wad of it onto the bridge. Don't use modeling clay; it has oil in it that can get into the wood. With the poster adhesive it's pretty easy to home in on the amount of mass that will just be enough to take some of the edge off.

One of my students got a little carried away a couple of years ago when carving her bridge, and ended up with a 'sharper' sound than she wanted. We ended up adding about 3-5 grams of weight to the bridge to get it to sound about right. Later, when we had time to get some finish on it, we were able to remove the weight, as the added mass and damping of the finish did the trick.

Thanks again for the effort. I really appreciate it. I'll have to read this over a few times before it clicks I can tell, but I'll tell you what I come up with.