First Gore Steel String - Resonances

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Woodsy23
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First Gore Steel String - Resonances

Post by Woodsy23 » Wed Mar 25, 2015 10:28 pm

Trevor,

After completing my first acoustic guitar, a 12 string X-braced, I have now jumped in at the deep end and am attempting the falcate braced steel string with cutaway and live back from the plans in your book. The shape/dimensions are pretty close to your plans with just the upper bout and cutaway slightly modified.

The back and sides are East Indian Rosewood, the top is Engelmann Spruce and all the bracing is sitka spruce with the falcate braces 7mm high. You, Peter Coombe and others have already helped me out with a problem I had gluing the EIR back join. viewtopic.php?f=1&t=6622 and you have provided useful advice on top stiffness viewtopic.php?f=33&t=6596

It is all going reasonably well and I am now I have closed the box and done some initial tap testing to determine the current resonances without the bridge and neck.
This plot is for tapping the top with the top facing the microphone
Boxed - No Br - Front tap 1.jpg
This plot is for tapping the front with the back facing the microphone
Boxed - No Br - Back tap 1.jpg
This plot is for tapping the back with the back facing the microphone.
Boxed - No Br - Front tap - Back to Mike.jpg
I think I am seeing the Helmholtz frequency at 97.5 Hz, the Main Top T(1,1)2 at 185 Hz and possibly the Main Back (T(1,1)3 at 238 Hz. There seems to be some unexpected peaks between the Helmholtz and the Main Top T(1,1)2. The Main Top T(1,1)2 at 185 Hz seems a little low for this stage compared to the values in Table 12-1.

The main thing I need to decide now is whether to thin the edges of the top and by how much. I am worried that, with a low Main Top T(1,1)2, thinning may make it too low.
Your thoughts would be appreciated.
Richard

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Re: First Gore Steel String - Resonances

Post by johnparchem » Wed Mar 25, 2015 10:55 pm

With regard to the extra peaks, this looked like my plot when I was over driving the microphone input; to much gain. Like yours I had the correct peaks with bunches of others.

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Trevor Gore
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Re: First Gore Steel String - Resonances

Post by Trevor Gore » Thu Mar 26, 2015 8:38 am

I think John is on the money. Have a go at cleaning up the signal before you make any decisions. There is no reason that you shouldn't be able to get them looking clean, like in the book. One way of checking levels is to look at a tuning fork signal on the oscilloscope in VA. You should get smooth sine waves when everything is set correctly.

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Re: First Gore Steel String - Resonances

Post by Woodsy23 » Thu Mar 26, 2015 10:03 pm

Trevor and John,

You were both correct in that there was a problem with the signal being fed into VA but it was not the signal level. I have a Line 6 Audio interface so I could see that the levels weren't peaking. However I had forgotten to turn off the amplifier simulation so VA was hearing a steel string guitar played through a Marshall Stack. Doh!

The new spectrum plots are below. The odd peaks have gone but the resonant frequencies are similar to my previous tests.

This plot is for tapping the top with the top facing the microphone.
Boxed - No Br - Front tap 3.jpg
This plot is also for tapping the top but recorded from the K&K Pure Minis I have installed. I forgot to mention that I had installed these in my original post. I assume the mass of the PUs only affects the Main Top T(1,1)2 slightly. There is a peak at 275 Hz which is not in any of the other plots.
Boxed - No Br - Front tap PUs 1.jpg
This plot is for tapping the back with the back facing the microphone.
Boxed - No Br - Back tap 1.jpg
This plot is for tapping the front with the back facing the microphone.
Boxed - No Br - Front tap - Back to Mike.jpg
I am seeing the Helmholtz frequency at 98.2 Hz, the Main Top T(1,1)2 at 187.4 Hz and possibly the Main Back (T(1,1)3 at 235 Hz.

Should I thin the top at the edges? Should I target 180 Hz or 170 Hz for the Main Top frequency?

Your advice would be appreciated.
Richard

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Re: First Gore Steel String - Resonances

Post by johnparchem » Fri Mar 27, 2015 5:27 am

I can not really answer your questions about what you should do. But building a few of falcate braced guitars, including a copy of the Medium sized one you are building, I am finding that my top resonances are dropping around 10 Hz from where you are just closed to when the guitar is strung up. I made my bridge out of walnut as shown in the book.

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Re: First Gore Steel String - Resonances

Post by Woodsy23 » Fri Mar 27, 2015 6:35 am

johnparchem wrote: I am finding that my top resonances are dropping around 10 Hz from where you are just closed to when the guitar is strung up. I made my bridge out of walnut as shown in the book.
That's useful information John. I will also be making a walnut bridge as shown in the book.
Did you thin the top around the edges of the lower bout? If so, by how much?
Thanks.
Richard

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Re: First Gore Steel String - Resonances

Post by Trevor Gore » Fri Mar 27, 2015 8:40 am

Page 4-68 in Design has a table for T(1,1)2 drop in frequency per gram of bridge mass. You don't really want to end up lower than 170Hz, so I'd say by the time you have the bindings on and cleaned up, finish on etc. you'll be in reach of 170Hz, maybe with a little side mass, without any extra edge thinning.

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Re: First Gore Steel String - Resonances

Post by Woodsy23 » Tue Jun 23, 2015 11:57 pm

How time flies. It's been three months since I closed the box and did the initial tap testing. Well, the guitar is now complete except for trimming body resonances. There's no rattles or buzzes and the compensated saddle seems to do the job.

I have just done a tap test with the guitar strung and set up. Here's the result.
All Complete - Front tap 2.jpg
I am seeing the Helmholtz frequency at 96.5 Hz, the Main Top T(1,1)2 at 169.6 Hz and the Main Back T(1,1)3 at 229.5 Hz. So, looking at Table 22-1 of the white book, my top T(1,1)2 is about right. The Helmholtz is 6.5 Hz higher than in the table and the main back T(1,1)3 is just over 5 semitones above the main top. I can't hear any problems with any notes but that may be because I am not experienced in looking (listening) for them. Anyway, there's no major clunkers and I am very pleased with the sound (with crappy strings on). It might sound even better with the set of Elixer Nanowebs I have to put on.

Trevor, it seems that is was a good call not to thin the edges of the top. Do you suggest doing (or trying) anything about the Helmholtz or Main Back resonance? I'm worried that if I try to lower these, the Main Top might also drop too far below 170 Hz because of coupling/repulsion. You said I wouldn't want to go below 170 Hz. I could try adding some mass to the back to lower the T(1,1)3main back and see how much it affects the main top and Helmholtz.

Any other comments on the spectrum?
Richard

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Re: First Gore Steel String - Resonances

Post by Trevor Gore » Wed Jun 24, 2015 7:52 pm

Woodsy23 wrote:Do you suggest doing (or trying) anything about the Helmholtz or Main Back resonance?
Put the new strings on it and play it for a while, Richard. If there are no problem notes, stick with what you have.

A good set of numbers would be 95, 170, 214. Dropping the back stiffness will drop all the frequencies. So if you really need to, reduce the back stiffness a touch to drop the main air resonance a little, but not so far as to drop the T(1,1)2 very much.

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Re: First Gore Steel String - Resonances

Post by Woodsy23 » Thu Jul 02, 2015 10:47 pm

I have put new strings on and I'm very happy with the sound. I can't hear any problems with any notes.

While I was changing strings, I did a quick measure of the bridge rotation during string unloading and then did some top deflection measurements. I had previously posted about the deflections I could expect in this thread. viewtopic.php?f=33&t=6596&hilit=deflection. I had constructed a measurement similar to Jeff's and Trevor's as shown in this thread. viewtopic.php?f=33&t=5464&hilit=deflection. I used a series of weights from 0.13 kg to 1.3 kg, plotted the results in Excel including a linear "trend Line" (like Dave M in the previously mentioned thread. The stiffness of the top was less than I thought it would be (greater deflections) so I repeated the test three times.

The bridge rotation I measured was 1.4 degrees. This seems believable as there is little bulging of the top in the lower bout, less than in the 12 string I made.

The stiffness I got was between 4.5 and 5.2 kg/mm (45 to 52 N/mm). Unfortunately, I can't measure the uncoupled top frequency at the moment as I haven't got anything suitable to plug the hole with. But, assuming the uncoupled top frequency is 150 Hz (20 Hz lower than the coupled value of 170Hz), the monopole mobility I am calculating is between 18x10^-3 s/kg and 21x10^-3 s/kg. I did not expect it to be that high.

I should try to determine the uncoupled frequency so I can calculate the monopole mobility more accurately. Also, I did the test without strings on. I assumed the string force would not make much difference to the deflection measurements (laws of superposition) but maybe that is not the case.
Richard

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Re: First Gore Steel String - Resonances

Post by Trevor Gore » Fri Jul 03, 2015 9:27 am

Woodsy23 wrote: Unfortunately, I can't measure the uncoupled top frequency at the moment as I haven't got anything suitable to plug the hole with.
Handy things that work as plugs are tapered yoghurt/dip/custard containers similar to those by Jeff's left hand in this pic. The uncoupled top resonance is typically between 10-20Hz down.
Woodsy23 wrote:Also, I did the test without strings on. I assumed the string force would not make much difference to the deflection measurements (laws of superposition) but maybe that is not the case.
I've not found any significant difference. I usually do the deflection tests with the strings at pitch and the tap test with the tension dumped (so I can get the plug in).

Richard, having built at least a couple of instruments now, can you say what (if anything!) you found most beneficial about the techniques in the books? Thanks.

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Re: First Gore Steel String - Resonances

Post by Woodsy23 » Fri Jul 03, 2015 1:41 pm

What did I find most beneficial about the techniques in the books? That's an easy one to answer. EVERYTHING!

As any beginner, I found the books to be totally comprehensive. Of course, the unique aspect of the books, as everyone who has read them knows, is the information on determining the target plate thicknesses and tuning the resonances. There is no other way for a beginner to work this out. Even attending a guitar building course will only teach you a little about these aspects.

On this matter of thinning the plates to the optimum thickness, I have wondered whether experienced luthiers could pass on their skills in this area by posting sound clips of their tap testing with some commentary on what they are hearing. Could that work? The internet makes that more achievable these days.

I take some pictures of my latest build soon and post them in the "Book Builds" thread.
Richard

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Re: First Gore Steel String - Resonances

Post by Woodsy23 » Fri Jul 03, 2015 3:54 pm

Trevor,

Thanks for the tip about plugging the sound hole. After rummaging through the recycling bin I found a container of the right size, plugged the hole and measured the top resonance. It came out at 153.5 Hz. That gives a monopole mobility of between 19x10^-3 s/kg and 21.5x10^-3 s/kg.

Enough about numbers. I'll just enjoy playing it (at least trying to).
Richard

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Re: First Gore Steel String - Resonances

Post by kiwigeo » Fri Jul 03, 2015 7:33 pm

Trevor Gore wrote:
Richard, having built at least a couple of instruments now, can you say what (if anything!) you found most beneficial about the techniques in the books? Thanks.
That's a difficult question to give a one line answer to. One thing I like about the books is that even if you're mathematically challenged like I am you can still get enough out of the texts to build a much better guitar. A good example is the Chapters that deal with the calculations for the compensated nut.....three alternative ways of doing this are presented ranging from a simple method for those who are hopeless at maths to the complex method which involves alot of maths.
Martin

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