Good vibrations???

[arnoldschnitzer]

Originally Posted by Ken McKay

If Arnold is still out there, my question is: How thin/flexible would you recommend the lower bout to be? Do you use your fingers to tap tones to make that area flexible. I really don't think that area needs to be as stiff as the areas closer to the bridge and I think there is a lot to gain by making it flexible there. Am I wrong?

I do believe in graduating thinner as you move away from the center. But if you make the area around the ends of the bass bar thin you will almost certainly end up with cracking or sinkage in that region. How thick should it be? Depends on the wood and the size/shape of the instrument.

[Matthew Tucker]

Originally Posted by Ken McKay

the frown in the lower bout at the bottom is proving that it is stiff there. It seems to me that you would want to utilize that vibrating area by making it larger inside of the mode 5 lines. ie push down the frownie. Theoretically to do this you would remove wood between the line and the tail block flat.

Well ... if the plate were uniformly arched I'd say yeah, but this plate is quite thin now between the frown and the block. If the bottom wasn't going to be glued to the block, then that area could be pumping too, but on this bass, at least, the block is unavoidable. It's going to stiffen that part.

I did take about 1cm wood off the edge of the block pad just to see, but it made no difference to the pattern.
I'm tempted to see what would happen if I cut away the WHOLE of the block pad, and glued the top to the ribs at the edges only. But I'm uncertain what this would do structurally. The block is nicely glued to the back and ribs, so in terms of holding the bass's weight via the endpin it could be fine. But ... I don't think I dare try it just yet. Any thoughts??

*i* think the Mode 5 pattern is like that because of the change in long arch curvature at the middle of the lower bout to a curtate cycloid; it gives a stiffness boost at that point along the long wood fibres. I think it would take a lot more thinning to "improve" the mode 5 pattern ... and I'm not convinced that an evenly "smiley" pattern is necessarily an improvement. But who knows?

Is Don Harris watching? What was your mode 5 like? Care to post a pic?

[Ken McKay]

thanks Arnold, I will look at where my bar ends. Because that is what I am doing next. Only problem is I have to order some wood. I thought I had a piece for it but it is the wrong grain orientation.

Is International violin a good place to get a bass bar?

[Matthew Tucker]

Well I don't know if anyone's still watching, but the bass bar is fitted and more Chladni patterns on offer.

Look here: http://home.exetel.com.au/studio205/...?18.10#post_53

[Jake deVilliers]

Of course we're following!

Hey Matthew, what note do you get when you hold the top and smack the bass bar?

I'm no expert, but your bass bar looks very substantial to me.

[Matthew Tucker]

Well you get a whole range of tones, that's what the modes tell you. It depends where and how hard you smack. I only measured the first four or so. I guess the most audible of these is 85Hz which is just below F. If I hold the bar and tap gently, I can hear a whole lot of almost subsonic LF stuff going on. Sounds nice.

[Jake deVilliers]

Very nice Matt.

There will definitely be a lot going on but there's usually a fundamental note. If you hold it up off your shoulder between your fingertips and your clavicle, hanging freely, and tap the bar you'll get the dominant note.

Which may very well be that F.

[Matthew Tucker]

If I tap the bar, I get an A.

I'm wondering; how much is what "looks" right a good determinant of what a good bassbar shape should be? In this case i have little to compare with; I'm using a cedar top and bar, not spruce, its a cornerless shape, using cycloids for the transverse arches and a gut feeling about thicknesses and long arch shape. I have a feeling a bit of extra weight in under the bass foot really helps the sound along. But cedar is lighter than spruce, so I have left it a bit more substantial. it still weighs only 133 grams, is 43 mm tall.

I don't think it's completely finished yet though.

[Felessan]

I'm going to sound like a noob here, and that's because, in terms of luthiery, I am.

From what I've learned, the bassbar should be tuned as well. Each end should have a tap tone (tapped with something hard), and those tones should match... you just shape the bar and cut the ends in a short ramp to be flush with the top.

I've heard of a couple of different ways of shaping the top of the bar (the view as you look down the length of it, light sighting through a gun). Some people make this shape an arch, some people leave it flat but round off the edges. Leaving it flatter would make it more flexible--a good thing, I'm guessing?

If anyone can tell me more about the "why" of all of that, I'd really like to know. As far as "ramping" the two ends, I don't know how long the ramp should be, nor why you should even do that... it makes it a bit more artistic, that's for sure.

-Trevor

[Matthew Tucker]

The "Why?" is hard to come by.

I've done a few bass bars now, both new and replacements. The way I see it; the centre of the bar is attached to a thickish part of the plate which is the least flexible anyway, so the bar needs not to be flexible right there. So it can remain as thick as I want for weight reasons. However, towards the ends of the bar, the bar needs to flex so as not to restrict the pumping movement of the top, but not so much as to dampen the transmission of string vibrations into the upper and lower bouts. If it was made of rubber, it would do nothing. So its a balance. Right at the ends is where any lifting of the bar may occur, and I'm assuming this is often triggered by a thump on the lower bout, in which the relatively inflexible bar gets cracked away from the moving plate. So the end of the bar is further feathered away to give even more flexibility at that thinnish point of the plate.

I think the shape of the bar must work with the arching shape and the graduations of the top, so there isn't really a formula. Some people "tune" the bar - and certainly Traeger has his methods - but personally i haven't had much success with the process he describes. So the bar taps a tone higher than the plate? What does that mean? So the bar taps the same tone end to end? Is that good? Why?

So I'm going more by the flexibility of the bar itself, and the weight of the bar, and how the outside arching is designed. A long flat arch for example might need extra stiffness in the bar in the middle. I also like the idea of a bit of extra weight under the E foot.

And then there's "how does it look?" because the fairies will see it, and tell on you. Now will i sign the bar, or not??

[ctregan]

Hey Matthew, Have you tried this with your top plate?

http://www.sharmusic.com/video/firstunofficialsound.wmv

[Matthew Tucker]

Nope. Too busy walking around and around the bass bar ...