Audacity plot question

[MikeC]

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?  

 

[mbrancalion]

You could try "axis: logarithmic".

[J.DiLisio]

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..

[MikeC]

Thanks, it looks more readable now. 

 

[J.DiLisio]

My mistake, I see that you're tapping plates.

[Don Noon]

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.

[MikeC]

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.  

[jowl]

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

 

[Don Noon]

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.

[MikeC]

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. 

[MikeC]

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.   

[Don Noon]

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:

1. What size model is it?
2. What is the weight?
3. 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.

[MikeC]

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?  

[Don Noon]

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.

[David Burgess]

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.

[MikeC]

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.   

[MikeC]

So the plate weight is 135.9 grams.  That's a bit on the heavy side isn't it? 

 

[Don Noon]

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.

[MikeC]

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? 

[Don Noon]

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.

[J.DiLisio]

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

 

[MikeC]

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.   

[MikeC]

well it's now at 125 grams

 

[J.DiLisio]

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 .. 

 

[Bodacious Cowboy]

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.