Collecting 4DOF models

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seeaxe
Blackwood
Posts: 753
Joined: Thu Dec 18, 2008 7:20 pm
Location: Auckland NZ

Re: Collecting 4DOF models

Post by seeaxe » Thu Aug 19, 2021 5:19 am

So if I mold the cylinder shape with axis parallel to the grain (i.e. sides of the top fall away from the center under the strings), then that increases cross-grain stiffness, I think. So when I apply the downward-only force of the strings as in an archtop, it acts like a compression arch bridge. And Elong/Ecross for Engelmann Spruce goes from 14.5 to 1.7. Did I get that wrong? Am I confusing strength with stiffness?



Well you might be right but...arch action comes from inducing compression in the arch and thus relies on support at the spring of the arch which is questionable on something as light as a guitar. Its not flexural so vibration modss will be very different. Think Hoover Dam versus Tacoma Narrows Bridge (not quite the same but you get the mental picture)

The problem you will have esp with an archtop is that even with building lots prototype tops you wont know how they behave until you actually build them into a guitar.
Richard

GregHolmberg
Beefwood
Posts: 27
Joined: Mon May 17, 2021 5:05 am
Location: California, USA

Re: Collecting 4DOF models

Post by GregHolmberg » Thu Aug 19, 2021 7:53 am

seeaxe wrote:
Thu Aug 19, 2021 5:19 am
Well you might be right but...arch action comes from inducing compression in the arch and thus relies on support at the spring of the arch which is questionable on something as light as a guitar. Its not flexural so vibration modss will be very different. Think Hoover Dam versus Tacoma Narrows Bridge (not quite the same but you get the mental picture)

The problem you will have esp with an archtop is that even with building lots prototype tops you wont know how they behave until you actually build them into a guitar.
What's the "spring" of an arch? The base, maybe? I think you're saying that the sides of the guitar must be very rigid. Yes, I think, for this reason and also for increased mass (increasing the area of the main top mode, as in Fig. 2.3-13), I will create a "tone-well" chassis as shown in the Build book, p 11-40. This structure below the top will resist the arch wanting to push outward.

Yes, I think there will be many prototype tops. I'll have to make a body that is easy to pop in a new top for fully-coupled testing of frequency response. It would be nice to find a good starting point for these prototypes though--thickness, brace dimensions, radius of arch. All working toward some target stiffnesses. And that's what all this modeling is about for me. In the 4DOF modeling I've done, I've found that it's very easy to the get sound hole size and top and back stiffness wrong, and end up with frequencies wildly off. At least I now know that for the dimensions of the cavity I've designed (17" x 3.3") and target frequencies, what stiffnesses I should be working toward in my prototypes. And hopefully, if the 4DOF model is correct, when assembled, the coupled frequencies will come out on target. We'll see...

Greg

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