bridge rotation, brace stiffness and modal tuning

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soundshooter
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bridge rotation, brace stiffness and modal tuning

Post by soundshooter » Mon May 06, 2013 12:32 pm

I recently completed my second guitar and posted some picks and details in the Gallery. Redwood Black Limba Falcate. viewtopic.php?f=25&t=5390&p=62091#p62091

The T(1,1)1 was higher than i was shooting for. (95 vs. 90) I suspect that my bracing was on the stiff side seeing how I also fell short on bridge rotation as well. It seems to me if the bracing was less stiff then all the modes would go down.

My question is how does one go about getting a 2 degree bridge rotation while still being able to tune to higher frequencies like 190 or 202 . Do I need to change the "f" in emu.4.5-7? Is it a function of edge thinning?

Frank E T

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Trevor Gore
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Re: bridge rotation, brace stiffness and modal tuning

Post by Trevor Gore » Mon May 06, 2013 8:37 pm

soundshooter wrote:The T(1,1)1 was higher than i was shooting for. (95 vs. 90) I suspect that my bracing was on the stiff side seeing how I also fell short on bridge rotation as well. It seems to me if the bracing was less stiff then all the modes would go down.
If you are short of 2 degrees on bridge rotation, lower bracing stiffness will help you get closer to 2 degrees and will drop the T(1,1)2 as well. You can get the lower bracing stiffness by reducing the height a little or by using a less stiff brace material (e.g. like AK yellow cedar, seeing as there is no way you are likely to get hold of any King Billy).

The T(1,1)1 is strongly effected by the "swept volume" of the box, that is the volume change as it vibrates. Thinning the lower bout edges tends to increase the swept volume and so is a means of dropping the T(1,1)1 (as well as the T(1,1)2; so you'll not need as much side mass). You can drop the T(1,1)1 some more by using a slightly smaller sound hole.
soundshooter wrote:My question is how does one go about getting a 2 degree bridge rotation while still being able to tune to higher frequencies like 190 or 202 . Do I need to change the "f" in emu.4.5-7? Is it a function of edge thinning?
Go lighter. Low density top, low mass bridge, low mass bridge pins, low mass saddle. Stiffen the back, because you still need the T(1,1)3 four semitones above the T(1,1)2. Remember that coupled resonances repel, so just stiffening the back will raise the T(1,1)2.

Having said that, don't get overly hung up on the two degrees. That just gets you in the right ball park to get the modal frequencies right. You hear the modal frequencies, whilst you only see the 2 degrees if you're looking very closely.

No matter how close I look, I still can't find the "f"in' emu. :mrgreen:

BTW, nice build, Frank.

soundshooter
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Re: bridge rotation, brace stiffness and modal tuning

Post by soundshooter » Tue May 07, 2013 11:17 am

Thanks Trevor

Good to know about the Swept Volume. When tinning the edges I was looking more to the main top mode. I didn't want to get to close to target not knowing what the bridge would do. Just taped on, it is dead mass and brought the t(1,1)2 way down. Once glued it hardly brought the frequency down at all. I'll be more daring next time.

I've been wondering about the saddle being 5mm thick. Getting the bridge as light as posable and then putting a honking piece of bone almost 8 gm on there. Can I drill holes through it to reduce weight? It is great to have the width to intonate, string contact and a good solid purchase in the bridge. I've seen some folks reduce material between the strings. Just glueing on the K&K pick up made a big difference and that couldn't be more then a couple of grams.

The sound hole in your SS Falcate design need not go smaller to hit a target of 90 T(1,1)1? I should have been able to hit it with appropriate bracing and or top thickness, right?

I have a guitar with a solid 2degree bridge rotation and it is very responsive and powerful. You might not see it but Im guessing you feel it. Or is there more at play. You are right though, the guitar sounds great.

Equation 4.5-7 Sorry I need to proofread better.

Thanks again.
Frank E

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Trevor Gore
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Re: bridge rotation, brace stiffness and modal tuning

Post by Trevor Gore » Tue May 07, 2013 7:58 pm

soundshooter wrote:I've been wondering about the saddle being 5mm thick. Getting the bridge as light as posable and then putting a honking piece of bone almost 8 gm on there. Can I drill holes through it to reduce weight?
Using Tusq instead of bone will save you a gram or two, and I prefer the sound, though Tusq is not quite as hard as hard bone. You could try a two piece angled saddle and make it thinner, but you won't have much room for error. Never tried drilling out a saddle. Give it a go and tell us what happens.
soundshooter wrote:The sound hole in your SS Falcate design need not go smaller to hit a target of 90 T(1,1)1? I should have been able to hit it with appropriate bracing and or top thickness, right?
I've made a few at 90,170, 214, but I prefer the balance of 95, 180, 226. I go with a really light bridge (~14 grms) and a "soft" top with KB braces, so very high mobility and swept area and get 90, 170, 214 with a 95mm diameter soundhole. Generally, there are no side masses in these guitars, so you have to be confident of hitting your frequency targets, and sound-wise they tend to be of a flamenco nature. There doesn't seem to be much downside of going smaller (at least to 90mm) but choose the right rosette design so you don't have to worry about screwing up the look.
soundshooter wrote: ...the guitar sounds great.
That's what we're after!

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