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PostPosted: Sat Aug 30, 2014 9:20 am 
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Mahogany
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I'm building with Adi for the first time, and the top is braced and ready to attach to the rim. I'm a little nervous.

As sort of a check, I tap the soundboard at the bridge on most of my builds to check frequencies, and usually get a peak around 200 hz plus or minus, with a few other "peaks" I guess you could say (it doesn't show 200 every time I tap, it bounces between numbers but always the same range, say 50, 115, 195..). That seems to be in the ballpark of what I should expect. This one shows three frequencies, around 60, 120, and 250 hz. 250?? That seems pretty high, top doesn't sound overly tight, doesn't seem overbraced. At this point I think I'm going to glue it to the rim and go from there.

How much do free plate frequencies even matter?

Top is 117 thick, braces .280 thick, .6 at the X, .45 at peaks, .3 at scallops....


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PostPosted: Sat Aug 30, 2014 11:08 am 
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Koa
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TJP the only thing I know about tap tone is what I've learned watching a video. I know builders tap their wood and know exactly how it's going to sound. Brings me to the question what did the papier-mâché guitar tap tone sound like. When I tap a top it sounds like someone tapping a piece of wood to me. Somebody else probably could answer this question with better authority.


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PostPosted: Sat Aug 30, 2014 11:44 am 
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Brazilian Rosewood
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Alan Carruth seems to be able to use free plate tuning with some good results.

As for myself, it's not in line with my methods. Whatever you have as a free plate tone will immediately go out the window as soon as you attach it to the rims, in an unpredictable way.

Since it's the final coupled modes that matter, I do all tap tone testing after the top is glued to the rims, and again after the back is on.

It's certainly possible that your adi is stiffer than what you're used to using. Every piece of wood is different of course, but I haven't had a top need to be .117 since I've started tracking that data. Mine are usually between .095 and .110 max...

But my numbers will likely have little relevance to your building...


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PostPosted: Sat Aug 30, 2014 12:02 pm 
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What size guitar?
0.117 does sound a bit thick for Adi, even if it's a big guitar. Do you have a density on it?
I believe Alan Carruth goes by Chandli patterns for free/braced plates, but I'll leave him to comment if he wants.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.


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PostPosted: Sat Aug 30, 2014 12:31 pm 
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Brazilian Rosewood
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You'd be we'll served to invest in the first Gore/Gilet book...


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PostPosted: Sat Aug 30, 2014 2:35 pm 
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I don't think Gore Gilet talks about free plates.

If you've liked your previous builds that had lower frequencies for the same size top, I'd say this one is over braced or too thick. Frequency is related to stiffness divided by mass (or is it density). Higher freq for equivalent masses means higher stiffness.

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PostPosted: Sat Aug 30, 2014 4:30 pm 
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Brazilian Rosewood
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Nope, Gore does not talk about free plates. But if you want to start in on frequency analysis, it's a wealth of information.


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PostPosted: Sat Aug 30, 2014 5:27 pm 
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Mahogany
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Thanks all, based on your suggestions, I think I will glue it to the sides, then sand a few thou off of it, while keeping track to see if frequency goes in a direction that makes me more comfortable. Turns out after checking my notes more carefully (my memory betrayed me) I may be in the ballpark...my last few were in the 250 range, I was remembering the freq after they were attached to the sides, which was about 200-210. Those three were "successful" enough that I've had to tell several folks to hold off while I build one for myself. I'm busy for about the next year and a half... If I could just nail the finish down...

I'm an engineer by day, so the last thing I want to do for fun is crunch numbers, but I DO like having "checks" to see if I'm headed in the right direction. The Gore book, while useful and fascinating, is exactly how I don't want to approach the craft. This is more of an artistic outlet than a technical one. So here are my predictions - After I glue this thing up, I'm guessing I'll get around 210-215. Box will be around 205-210. We should start a pool...

Tap does sound good, resonant, and not the ping I would associate with a too-tight top. This is all such a beautiful mystery...


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PostPosted: Sat Aug 30, 2014 6:54 pm 
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Brazilian Rosewood
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Free plate 'tuning' is more like tuning up a car than a piano: it's not the specific pitches of the modes that matter, but rather how well they work. The pitches do tell you something about the relationship of mass to stiffness, and that's helpful in keeping things more or less in line. The shapes of the modes and how well they 'ring' tell you about the distribution of mass and stiffness within the plate; whether you have 'hard' or 'soft' spots, and how the braces and the top work together. As far as I can tell that does have some bearing on the final tone.

As has been said ,when you glue the top to the rim everything changes. Once the edges are fixed it's harder to recover information about the mass and stiffness distribution in the top. David Hurd came up with a way, using deflection mapping over a grid, that seems to work for him. When I sent him one of my 'test mules' to check out he got exactly the sort of deflection map he'd like to see, so it's possible there's more than one way to skin that cat. Of the two methods, I think 'free' plate tuning is easier, but that's only my opinion.

If I understand the theory correctly, the pitch of a given mode should be proportional to the square root of (stiffness/mass).


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PostPosted: Sun Aug 31, 2014 7:39 am 
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tjp wrote:
I'm an engineer by day, so the last thing I want to do for fun is crunch numbers...

:D I can roll with that!
tjp wrote:
...but I DO like having "checks" to see if I'm headed in the right direction.

In which case, be sure you're checking the right things. Most of us have found that free plate frequencies are not a good predictor of closed box frequencies, if those are what you're interested in. Go around that loop a few times and you end up where I did. Or you choose not to think about those sorts of things at all.

The important thing is to just make sure you're having fun!

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PostPosted: Sun Aug 31, 2014 8:55 am 
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Koa
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Lonnie J Barber wrote:
... I know builders tap their wood and know exactly how it's going to sound...


You mean they know from tapping exactly how the finished guitar is going to sound?


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PostPosted: Sun Aug 31, 2014 8:57 am 
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Koa
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You would think so wouldn't you. Like I said sounds like someone tapping on a piece of wood to me. I learned through experience a guitar sounds better when a better player is playing it. Funny how that works out. Lol


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PostPosted: Sun Aug 31, 2014 9:13 pm 
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Mahogany
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Thanks for that advice, Trevor, and I appreciate your humor - I would have hated to offend someone who's work I admire. I'm hoping to figure out, over time, a way to be confident I'm headed the right direction. Right now I carve, worry, fuss, sand, wait, and after about a week of futzing there always comes a time when I know in my gut the top is ready. How's that for a scientific method?


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PostPosted: Mon Sep 01, 2014 7:00 am 
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Brazilian Rosewood
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On the older Martins I have seen the tops did vary a bit as there was a lot of hand sanding. Most fell into .100 +/- .010 . I have seen one as low as .090. .117 was the thickness it came into the factory.
I like to rough my tops to .110 but till I am done they are around .100 to .105.

I use patterns mostly now. I used to do deflection testing on my tops , also a mech engineer so I liked to see numbers. I used a pre brace and post brace scale and looked to get a certain ratio.I also wanted to see a certain movement in different areas. I notice that after about 10 tops the braces took a very similar shape and now I just use a base pattern and to some voicing once the guitar is built. As Alan points out , once the top is glued to the guitar they change.

Wayne Henderson voices his tops without a plate. It takes them to the point that they stop ringing , and then sets the plate. So as you can see there are many ways to do this. Alan Carruth has a way of explaining things very well. You have to start somewhere. So have at it and find your process.

here is a link you may find interesting.
http://theunofficialmartinguitarforum.y ... NG-LIBRARY
this is a bracing library of Martin guitars through the ages. Interesting to look at and study.

The only thing I would say is that you have to be clean with the joinery.

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PostPosted: Mon Sep 01, 2014 9:13 am 
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bluescreek wrote:

The only thing I would say is that you have to be clean with the joinery.



I strongly concur with John on this point. Loose joints that have glue as a filler are a sound/tone dump. I think newer builders don't even have this on their radar when they start out. I try to check every joint with out glue before going for the sticky stuff...!
Tom

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PostPosted: Mon Sep 01, 2014 10:08 am 
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Mahogany
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Absolutely, Tom. I had a local shop thickness sand some tops for me (#3 and 4, I think). When I glued the braces on 3, I noticed some small gaps because the top wasn't perfectly flat, it had a couple of low points, like corrugations. By the end, after heating, messing up the seam, etc. I tossed that top and learned a valuable lesson. Now I mark my tops all over with pencil and sand with a flat "sanding beam" kind of thing until the pencil marks disappear evenly. I really kind of enjoy the joinery part and pay a lot of attention to it.

John, you sold me my first kit, a 00 with some figured hog. I built it kind of deep, and it is my daily player. It has something of a fan club, and I've kind of stopped noticing the little mistakes that kind of bugged me at first. Even have grown to like the overly chunky neck ;) . Thanks for helping to get me into this!

Going into the garage to glue the top. Wish me luck! Will report back in a day or two...


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PostPosted: Mon Sep 01, 2014 1:55 pm 
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Brazilian Rosewood
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Trevor Gore wrote:
" Most of us have found that free plate frequencies are not a good predictor of closed box frequencies, if those are what you're interested in. "

Yup. Several years ago I dug up the data on about twenty guitars of the same size and shape, looking for any sort of correlation between the 'free' top ring+ mode pitch and the frequency of the 'main top' mode on the assembled instrument. There was none. However, there was a correlation between the mode shape and the frequency shift between the 'free' and assembled mode pitches: the 'better' the free plate shape the less of a pitch shift there was. It seemed to me that there was also a correlation between the mode shapes and the quality of sound of the assembled instruments, but:
a) I'm biased, and
b) what's 'quality'?

Some years back I made what was supposed to be a matched pair of Classical guitars. After a lot of effort to control everything as closely as possible, they ended up not sounding the same. They were both good guitars, and very similar, but everybody who tried them preferred one over the other. The one they liked had the better mode shapes. Since they didn't know which was which, this is a reasonably good test, although, of course, one would like to repeat it. Up until then I had been controlling for frequencies, and those matched well. Since then I've been looking at mode shapes more, and getting better results.

I'll note that 'better' mode shapes tend to give 'better' tap tones; ones that are clearer and have lower loss.

A short digression. The Ford Model T car used magnetos in the ignition system. There was a separate capacitor for each cylinder, with a set of points that worked like a buzzer to produce an AC current that could be stepped up to fire the plug. To set the gap on the points you rotated the engine to the correct position and listened to the buzz, adjusting the setting until you got a specific note. I had read years ago that the cars actually came with a 'C' tuning fork, and a recently referenced article on line suggested C# as the correct pitch. At any rate, the pitch was just an indicator of the gap, which was what you were really trying to adjust. I strongly suspect that this is where the phrase 'tuning up a car' came from.

Back when we started looking at Chladni patterns we thought that the pitch might be a useful indicator, and 'tuning' the plate seemed a good term for what we were doing. At this point we know the 'free' plate pitch doesn't convey a lot, but we're sort of stuck with the term.


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PostPosted: Mon Sep 01, 2014 5:57 pm 
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Mahogany
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Top glued to the rim, 205 hz. Trimmed the overhang/excess, dropped to 195-200. Whether or not that matters is the relevant topic of discussion.

Someone asked earlier in the thread about size. What we have is essentially a deep (4.5 at the tail) Koa/Adi 000-18.

More importantly, it definitely sounds like a good drum to me. Nice and crisp, good overtones....

Thanks for chiming in, Alan! I'm always hoping for your input when I head down these technical rabbitholes.


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PostPosted: Mon Sep 01, 2014 7:24 pm 
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Nice one Tom.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.


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PostPosted: Fri Sep 05, 2014 9:12 am 
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More data points:

As above, 205 hz, top attached to rim
Back, carved, free, 255 hz (.105 thick, traditional Martin-style braces)
Back, closed box, 225 hz
top, closed box with 1/8 to 1/4 inch overhang on back, 193 (an hour after gluing with Titebond)
top with box all trimmed up (and maybe glue fully cured) 204 hz

All these numbers were within the realm of what I expected, minor surprise in how much back freq dropped after I closed the box compared to past builds.

Takeaways? Not sure. I'm pretty comfortable it's not too overbuilt, and the freqs are in the range (I think) of some degree of wolf note margin of safety, according to past conversations with Alan. Interestingly, either trimming that overhang made a huge difference in the drum, or having a few extra hours of cure time did, but it went from muffled to decidedly not muffled.

Makes a flippin' LOUD drum, but still going to sand a few thousands off the top.

I'm thinking really dark Katalox binding to work with some dark streaks in the Koa, with a (really amazing) birdseye redwood tail graft to match the rosette....


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PostPosted: Fri Sep 05, 2014 11:18 am 
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Brazilian Rosewood
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What size body?


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PostPosted: Fri Sep 05, 2014 11:50 pm 
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Mahogany
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It's an OM, 4.5 inches deep at the tail.


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PostPosted: Sat Sep 06, 2014 5:48 am 
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Brazilian Rosewood
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tjp wrote:

Interestingly, either trimming that overhang made a huge difference in the drum, or having a few extra hours of cure time did, but it went from muffled to decidedly not muffled.


Likely drying time. Next day my boxes always sound better than after unclamping. Same with tapping a loose top with newly glued bracing...


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