Collecting data on good examples

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Craig Bumgarner
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Collecting data on good examples

Post by Craig Bumgarner » Fri Apr 26, 2013 12:26 am

I'm going to an annual gypsy jazz festival where the Selmer style guitar I build will be in abundance, many of which are some of the best in the world for this style music. I'd like to be able to collect data on the best of these guitars without too much intrusion. It seems like a great opportunity

I've been reading Trevor and Gerard's books. I have set up Visual Analyzer for frequency response curves and been doing lots of tapping. Have not built a guitar using these methods yet and though that would certainly help in my understanding of how to test good examples of finished guitars, the festival is coming up soon and I hope to be able to do the best I can even if I'm not all the way there in my understanding.

Using the framework laid out in the books, what might the best approach to collecting data? It is not likely I am going to have more than 10-15 minutes with any one guitar and it is unlikely I will be permitted to unstring most of them.

I have a laptop, a good mic, VA and Audacity. I have a Hacklinger gauge for plate thickness. Mirrors, lights, etc.

Selmer guitars have a floating bridge and tailpiece like an archtop and in the past I have used a deflection test that replicates the downward string pressures (20 lbs) of the floating bridge and dial indicators to measure deflection. This is very useful, but cumbersome to set up. If someone has a better idea on a deflection test that can be easily applied to an unstrung guitar, I'm all ears.

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Mike Thomas
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Re: Collecting data on good examples

Post by Mike Thomas » Fri Apr 26, 2013 7:51 am

I'm sorry that I personally don't haven the expertise to give you advice on this, but others on the forum are eminently qualified, and I'm sure they will chip in. I would like to say, however, that I have been dying for someone to take a serious look at Selmer and Selmer style guitars. How valuable it would be to have the data for highly regarded examples of both the petite bouche and the grande bouche versions. I really hope that you are successful in this.
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Trevor Gore
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Re: Collecting data on good examples

Post by Trevor Gore » Fri Apr 26, 2013 11:26 am

Craig, just tap the guitars in the normal way and save the files. If you do that in the latest versions of VA you should get .txt files and either a .wmf file or a file in VA format all of which can be stored and used later. Additionally, you can create a table in a spreadsheet and log the values of the main resonances (best found by looking them up in the .txt file, by searching through and looking where the numbers "change direction"). I'm sure everyone would like to see the results when you have them!

Jeff has built a Selmer or two and may be able to add more about things of interest specific to that type of design.

Craig Bumgarner
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Re: Collecting data on good examples

Post by Craig Bumgarner » Sat Apr 27, 2013 12:07 am

Okay, thanks Trevor. I did some tests taps this morning on one of my older guitars, strings off, strings on. I was a little concerned that with a Selmer type guitar, floating bridge and tailpiece, that I might get significantly different results if I could not tap in the center area of the top made inaccessible by the strings. No big difference though.

I'm just getting started with this, but one things I have noticed so far about the Selmer style guitars I have tested is the main peak which I take to be the monopole resonance is much higher (~ 260-280hz) than the targets in your book (170-180). The coupled Helmholtz is similar at ~ 95-100. But the high monopole made me wonder if my test regime was flawed somehow. So I tapped an old dreadnaught I have and it is right in the range you suggest for dreads in your book.

I've tapped three other Selmer style guitars, one of mine and two by other builders and the FRC are similar for the air and monopole peaks. At this point I have no real idea what the peaks beyond the monopole peak mean. I'm guessing some Chiadni testing will help me identify modes with FRCs, but I'm not set up for that yet.

Here is a FRC of a Selmer body I just boxed last week. No neck, so strings, no bridge. Cedar top, old school ladder bracing (four), small oval sound hole. Fairly typical set up. I go a little heavier on the top and lighter on the bracing than most builders which gives me a slightly lusher tone without diminishing volume or dryness in rhythm chords. The high monopole is, I'm guessing, not really surprising. These are bright punchy guitars which have little use for bassy resonances which tend to only muddy up the chords when being played fast and furious. I'm looking forward to applying principals discussed in the book to making mine better.
FRC #11, closed box, 3, USB.jpg
FRC #11, closed box, 3, USB.jpg (84.56 KiB) Viewed 7787 times
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kiwigeo
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Re: Collecting data on good examples

Post by kiwigeo » Sat Apr 27, 2013 3:32 pm

The peaks beyond the main top resonance are a combination of monopoles, dipoles and more complex modes. One thing I've noticed is that up towards the high end of the spectrum the modes are more heavily influenced by minor components of the bracing under the top.

Photo is of a falcate braced classical built Spanish method. Top is Englemann and its being excited at 683Hz.
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Trevor Gore
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Re: Collecting data on good examples

Post by Trevor Gore » Sat Apr 27, 2013 6:50 pm

Craig Bumgarner wrote:...one things I have noticed so far about the Selmer style guitars I have tested is the main peak which I take to be the monopole resonance is much higher (~ 260-280hz) than the targets in your book (170-180).
Yes, that would be correct. The Selmer style guitars tend to be very stiff, hence the high pitched main monopole. Other than that, I can't tell you much! I've never built one, but I know a few people who have. I love the music that gets played on them, though. If you haven't come across him yet, check out one of our (ex) locals, Ian Date, http://www.iandate.com/.

When you're tapping out other guitars, make sure they're strung, etc. because the bridge mass will be part of the "equation" affecting what you hear.

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