MikeC Posted November 29, 2020 Report Share Posted November 29, 2020 After finishing the back plate I wanted to try and analyze it with Audacity. I don't know if I'm doing it right. Does this look like a normal plot from tapping a plate? Quote Link to comment Share on other sites More sharing options...
mbrancalion Posted November 29, 2020 Report Share Posted November 29, 2020 You could try "axis: logarithmic". Quote Link to comment Share on other sites More sharing options...
J.DiLisio Posted November 29, 2020 Report Share Posted November 29, 2020 You want to be focused in the 200-600hz range to see the typical resonances. Logarithmic should help. You may find this chart useful if you haven't come across it already.. Quote Link to comment Share on other sites More sharing options...
MikeC Posted November 29, 2020 Author Report Share Posted November 29, 2020 Thanks, it looks more readable now. Quote Link to comment Share on other sites More sharing options...
J.DiLisio Posted November 29, 2020 Report Share Posted November 29, 2020 My mistake, I see that you're tapping plates. Quote Link to comment Share on other sites More sharing options...
Don Noon Posted November 29, 2020 Report Share Posted November 29, 2020 The first plot was at far too low resolution (size 512). The second one is at a more reasonable 8192 setting. You don't mention how the plate is being held, tapped, or where the microphone is positioned. All 3 of those things are critical to highlighting specific modes. Quote Link to comment Share on other sites More sharing options...
MikeC Posted November 29, 2020 Author Report Share Posted November 29, 2020 Holding on the nodes about where I think they are and tapping with my finger. Mic directly in front of the plate. I get various results depending on where I hold it. Mode5 seems to be about 385hz. I'm new to this so just trying to figure it out and hopefully get my plates in the ballpark of what's reasonable. Quote Link to comment Share on other sites More sharing options...
jowl Posted November 30, 2020 Report Share Posted November 30, 2020 13 hours ago, MikeC said: Holding on the nodes about where I think they are and tapping with my finger. Mic directly in front of the plate. I get various results depending on where I hold it. Mode5 seems to be about 385hz. I'm new to this so just trying to figure it out and hopefully get my plates in the ballpark of what's reasonable. have a look at Sora's video: https://www.youtube.com/playlist?list=PLaxadm6POX7Hy2CxMIMU5wcOOBxxNsq5r Quote Link to comment Share on other sites More sharing options...
Don Noon Posted November 30, 2020 Report Share Posted November 30, 2020 Fingers aren't the best thing for plate tapping, as there's a heavy hand and arm attached to the finger, and the plate moves around a lot. It's plenty good enough for just getting plate mode frequencies, though. I use a 5g "hammer" with a vinyl cabinet bumper on the end for tapping. It's overkill for plate frequencies, but I have it for damping measurements, where the test piece has to be held at a consistent distance from the microphone. The amplitude variations (see how much Davide's plates move relative to the microphone) don't matter for frequency, but totally mess up damping calculations. Since I have the hammer, I use it for plate frequencies as well. Quote Link to comment Share on other sites More sharing options...
MikeC Posted November 30, 2020 Author Report Share Posted November 30, 2020 2 hours ago, jowl said: have a look at Sora's video: https://www.youtube.com/playlist?list=PLaxadm6POX7Hy2CxMIMU5wcOOBxxNsq5r Thanks for the link. That's one of his videos that I have not watched until now since I've just now got that stage of the build. There's a lot of good info on modes in that video. Quote Link to comment Share on other sites More sharing options...
MikeC Posted November 30, 2020 Author Report Share Posted November 30, 2020 39 minutes ago, Don Noon said: Fingers aren't the best thing for plate tapping, as there's a heavy hand and arm attached to the finger, and the plate moves around a lot. It's plenty good enough for just getting plate mode frequencies, though. I use a 5g "hammer" with a vinyl cabinet bumper on the end for tapping. It's overkill for plate frequencies, but I have it for damping measurements, where the test piece has to be held at a consistent distance from the microphone. The amplitude variations (see how much Davide's plates move relative to the microphone) don't matter for frequency, but totally mess up damping calculations. Since I have the hammer, I use it for plate frequencies as well. For now I'm low tech so fingers will have to do. I don't think I'll get more involved than simple mode testing. For M5 385hz is a bit on the high side isn't it? I wonder should I try to lower it by thinning the plate some more. Quote Link to comment Share on other sites More sharing options...
Don Noon Posted November 30, 2020 Report Share Posted November 30, 2020 1 hour ago, MikeC said: For M5 385hz is a bit on the high side isn't it? I wonder should I try to lower it by thinning the plate some more. Three huge factors enter into deciding if 385Hz M5 for a back plate is high or not: What size model is it? What is the weight? What is your aim... compliant or resistant character? 385Hz is high for what I normally make, but my last violin was a small (352mm) model and the back plate was 389Hz at 95grams. I generally pay more attention to the weight, and less to the Hz. Quote Link to comment Share on other sites More sharing options...
MikeC Posted November 30, 2020 Author Report Share Posted November 30, 2020 I have to pick up a few things at the store tonight. A gram scale is on the list. I'm building from a copy of Strad's PG mould but don't remember the exact measurement off hand. Ribs may not be tight to the mould but they are close. What do you mean by compliant or resistant? Quote Link to comment Share on other sites More sharing options...
Don Noon Posted November 30, 2020 Report Share Posted November 30, 2020 58 minutes ago, MikeC said: What do you mean by compliant or resistant? Thinly built Strads I would call compliant, where higher bow speed and lighter bow pressure works best. Thickly built Guarneris I would call resistant, where higher bow pressure can be used. It's technically more of an impedance thing, but I'm using more common player terms. Quote Link to comment Share on other sites More sharing options...
David Burgess Posted November 30, 2020 Report Share Posted November 30, 2020 2 minutes ago, Don Noon said: Thinly built Strads I would call compliant, where higher bow speed and lighter bow pressure works best. Thickly built Guarneris I would call resistant, where higher bow pressure can be used Agreed. Quote Link to comment Share on other sites More sharing options...
MikeC Posted November 30, 2020 Author Report Share Posted November 30, 2020 Well since I'm using a Strad mould and have templates from the Titian poster I was thinking of going kind of thin and Strad like. Although I like some of the roughness of DelGesu so I may leave some visible tool marks here and there. Quote Link to comment Share on other sites More sharing options...
MikeC Posted December 1, 2020 Author Report Share Posted December 1, 2020 So the plate weight is 135.9 grams. That's a bit on the heavy side isn't it? Quote Link to comment Share on other sites More sharing options...
Don Noon Posted December 1, 2020 Report Share Posted December 1, 2020 If you're making a Titian model (approx. 353 x 206mm), then 135.9g might be good... if you're making a solid-body electric version. If it was me, I would not go over 100g for that size. Maybe a little higher for extremely dense wood. Quote Link to comment Share on other sites More sharing options...
MikeC Posted December 1, 2020 Author Report Share Posted December 1, 2020 6 hours ago, Don Noon said: If you're making a Titian model (approx. 353 x 206mm), then 135.9g might be good... if you're making a solid-body electric version. If it was me, I would not go over 100g for that size. Maybe a little higher for extremely dense wood. that made me laugh. Maybe a solid body next... It's strange, I think the grads are about right, a little over 4mm in the center tapering to about 2.5mm in the upper and lower bouts. I could go thinner but I thought it was about right so I'm not sure where all the extra weight is. Could it be that the maple is unusually dense? Or am I just unsually dense? Quote Link to comment Share on other sites More sharing options...
Don Noon Posted December 1, 2020 Report Share Posted December 1, 2020 2 hours ago, MikeC said: 1) Could it be that the maple is unusually dense? 2) Or am I just unsually dense? 1) Probably. 2) I can't tell from here. If you can take a full breath of air and sink in fresh water, you're unusually dense. Quote Link to comment Share on other sites More sharing options...
J.DiLisio Posted December 1, 2020 Report Share Posted December 1, 2020 2 hours ago, MikeC said: that made me laugh. Maybe a solid body next... It's strange, I think the grads are about right, a little over 4mm in the center tapering to about 2.5mm in the upper and lower bouts. I could go thinner but I thought it was about right so I'm not sure where all the extra weight is. Could it be that the maple is unusually dense? Or am I just unsually dense? Maybe check your thickness in the channels around the edges and minimize the block platforms. Or you could try cutting ff's in the back Quote Link to comment Share on other sites More sharing options...
MikeC Posted December 1, 2020 Author Report Share Posted December 1, 2020 I guess if I cut Fs in the back the violin would play in stereo? Actually thinking about it this morning it may be a bit thick still near the edges. When I get time tonight I'll examine the CT scans and compare to my arching and thickness around the edges. I think my arches may still be a bit full in the recurve area so may still need some thinning there. Quote Link to comment Share on other sites More sharing options...
MikeC Posted December 2, 2020 Author Report Share Posted December 2, 2020 well it's now at 125 grams Quote Link to comment Share on other sites More sharing options...
J.DiLisio Posted December 2, 2020 Report Share Posted December 2, 2020 19 hours ago, MikeC said: I guess if I cut Fs in the back the violin would play in stereo? Actually thinking about it this morning it may be a bit thick still near the edges. When I get time tonight I'll examine the CT scans and compare to my arching and thickness around the edges. I think my arches may still be a bit full in the recurve area so may still need some thinning there. The reason I mentioned it is because my first couple instruments all had heavy plates and this was due to my misunderstanding the importance of thicknessing that fluting area correctly. I was going off of the Johnson/Courtnail book which glosses over the subject. That and I was letting the neck/end block platforms run all the way across the width which is something you see in the Sacconi illustrations but not so much in the real world. Thankfully, this forum exists to help amateurs like myself correct these pitfalls. Here is a Davide Sora video I found very helpful .. Quote Link to comment Share on other sites More sharing options...
Bodacious Cowboy Posted December 2, 2020 Report Share Posted December 2, 2020 1 hour ago, J.DiLisio said: . That and I was letting the neck/end block platforms run all the way across the width which is something you see in the Sacconi illustrations but not so much in the real world. Thankfully, this forum exists to help amateurs like myself correct these pitfalls. Well, I know plenty of really good pro makers who do this. It isn't some kind of amateur "thing". Makes little difference from a weight point of view. Unless you're digging things out to the point of introducing structural weakness. Quote Link to comment Share on other sites More sharing options...
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