Neil Miller
Forum Replies Created
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The nut just keeps the strings in position, so it shouldn’t need any adjustment as long as there’s still sufficient string down-pressure on the zero fret.
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The divots in the zero fret almost look intentional, though that doesn’t make sense since one of the big advantages of a zero fret is that it can be levelled together with the other frets. Are the frets above the zero fret also worn? If so, you need a thorough levelling. If not, I would replace the zero fret with an in-tact fret and level it to match the other frets.
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My initial interest in testing billets was to identify the wood with the highest stiffness to density ratio. I find there is a wide variation even within the same species and supplier. So being able to test in the billet stage gives me the ability to pick out the wood that will produce plates with adequate stiffness but the least mass. A significant issue I still struggle with is the fact that transverse stiffness varies much more widely from billet to billet than does longitudinal thickness, and in an instrument without any transverse bracing it seems like that should be a critical factor to consider. Yet the sound radiation coefficient used by Gore and Nicoletti for guitars doesn’t incorporate it.
Having these data also helps me adjust initial targets for carving thickness, but final my thicknessing happens with measuring deflection and listening to tone in the white.
That said, as noted in my original post, I’m still trying to figure out the best methodology from both a standpoint of practicality and accuracy and I’m eager for feedback and ideas. I was glad to hear Martino confirm in the recent webinar that while my frequency testing methods may not be completely appropriate for thicker billets, they should at least be consistently off from billet to billet, so as long as I stick to the same methodology, I should still be able to measure relative properties between billets.
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Testing rough-sawn samples is attractive since it allows me to visit a supplier and sort through multiple samples in order to select the best candidates. I tested 7 Englemann samples before and after planning and sanding them smooth. Density calculations increased 1% and stiffness calculations increased 3-4% after cleaning them up, probably because the rough-sawn surfaces led to over-estimated thickness measurements. One summary measure of wood quality, the Sound Radiation Coefficient, has longitudinal stiffness in the numerator and density (cubed) in the denominator, so the final quality statistic for the before and after measurements was virtually identical.
One drawback to measuring samples in the rough was that the cross-grain stiffness was much more difficult to measure. While longitudinal and twisting peaks were readily apparent in the rough-sawn frequency analysis, it was often unclear which peak represented the main cross-grain frequency. Once the samples were planed and smoothed, the cross-grain mode showed up much more clearly and consistently.
Conclusion: Rough-sawn samples produced data which were very useful in initial triage, though for a fuller assessment of wood quality, samples should be rechecked after planning them to a consistent thickness and smooth surface.
I hope these observations are useful. I’m eager for feedback, and happy to share more detail on my methods and data.
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In the past, I’ve had trouble comparing stiffness calculations derived from physical deflection to those calculated from frequency testing, but I haven’t had this range of material. With a multi-species data set, I found a clear correlation between the longitudinal stiffness of the two methods, though deflection produced E long values 1.5-2 Gpa lower than frequency. Within individual species, however, the comparison broke down, presumably because one or more of the methods wasn’t precise enough to parse the smaller within-species differences.
Plotting stiffness against density I think I found the source of the inconsistency. The relationship with frequency-based stiffness was close and linear across and within species, with the exception of the redwood. Deflection-based stiffness had much more scatter, and no apparent relationship within samples of the same species. This observation didn’t surprise me. When measuring deflection, I had to recalibrate the apparatus for each sample, and even then I often wasn’t happy with the consistency of measurement.
Conclusion: Frequency and deflection produced similar results, but frequency-based stiffness measurements were more precise and therefore preferable for comparing samples. Furthermore, with frequency analysis, I could measure cross-grain stiffness, which varied 90-180% within species versus just 9-14% for longitudinal stiffness. Finally, frequency analysis was much quicker and didn’t make my back hurt as much as repeatedly lifting a 50 lb. weight onto a spruce blank!
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Thanks,
This looks like just the stuff…
Neil
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I’m not trying to push any one method, just trying to understand the options. The T. Gore books talk about both, so it seems reasonable that they should produce similar results. I’m curious to know if others have produced results similar to mine, or if perhaps I’m doing something wrong…
I will differ with you on the issue of brace selection. Brace wood contributes significantly to both the mass and stiffness of a top. Why should we give it any less attention than the top plate? Too often suppliers throw the left overs in the brace wood pile, and I’ve found HUGE variation in both density and stiffness from the same source.
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Update: Turns out it’s not impossible to raise the frequencies. Removing the 35g temporary bridge raised the air resonance 2.1 hz and top monopole 2.6 hz, so a lighter bridge could help a bit.
Filling in the scallops on the lower portion of the X-braces by splicing in spruce raised the main resonances 3.7, 8.6 hz respectively, but after crowning the tops of the splices it dropped down to about half that, making me wish I had glued in even taller splices. So, I’m still at 85.3 hz and 162.4 hz.
In addition, the top monopole appears to be very weak and filling in the scallops appears to have boosted the cross dipole significantly (based on tapping on the antinodes, I don’t have Chladni equipment). Here’s the analysis (red = scalloped, blue = rough splices, green = trimmed splices.
I went ahead and glued up the box with the trimmed splices. I’ll use the lightest bridge possible and hope for the best. It’s all rather mind boggling!
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Muted top monopole on this D28 copy is higher than unmuted. I’m wondering if there might be some strange coupling going on. I can’t identify a clear cross dipole and the back is not active, but I think it shows up (220hz) when muted. It’s a sitka top with Gore-style X-bracing (no scalloping).
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Thanks for this great feedback! Your observations push me further in the direction of thinking deflection measurements just aren’t practical for my needs.
With regard to frequency testing, if the stiffness measurements are over-estimated, can I assume that this is consistent from sample to sample? My main objective is to compare samples, so if they’re consistently off, I can still achieve what I need.
Do you feel the alternative method you propose is more appropriate for thicker billets than the method in Nicoletti’s book? If so, is there somewhere I could get more detailed methods? I assume when you say to strike the “extremity” you mean striking the end of the beam with a blow parallel to the long axis, correct?
I’m VERY eager to see what you develop for thicker beams. If you have pre-publication information, that is welcome as well. I’ll do all I can to attend the December meeting.
Neil
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I’m not clear how he’s checking the temperature. IR of the wire before inserting?
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No apologies for the novel. It’s great information.
One question I have with the Assilex is how to know when it’s shot. Since it doesn’t clog up, and wipes clean, it’s hard to tell when it’s worn out.
Neil
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Not sure I follow your point #2. Can you elaborate?
The problem does seem to be accentuated when the player uses a capo.
If/when I get the guitar back I’ll try to share some recordings.
Neil
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Thanks for the correction. My recollection is that the tone came at F3, but you’re right, that’s a half step away from the top resonance. All the other evidence fit so well that I misread my frequency chart.
Can you think of any other explanation for why an atypical buzz would occur at the same frequency on 2 strings, and go away completely when the top plate is damped??
If I still had the guitar, I would want to recheck my frequency analysis…
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Thanks Al,
I use the FNF liquids, but not powdered stain. I’d welcome a 1-on-1 if you’re game. Whatsapp works well (+1-253-376-0116). I’m on Pacific time.
Looking forward to talking!
Neil
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Nice plots. Have you discovered the overlay feature in REW? It’s a fantastic feature for comparisons like this.
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I didn’t repeat the test. When you say you find deflection is less “reliable” what standard are you using? Do you mean less “repeatable” or is there another standard against which to measure them? These same tops were tested by Pacific Rim Tonewoods, and I found a similar (weak) correlation between theirs and my deflection-based E, but absolutely no correlation between theirs and my frequency-based E (r2 = 0.087).
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The units are gigapascals for flexural modulus on both axes. One calculated from frequency and the other from deflection. They’re within a reasonable range for spruce, just no correlation between the two methods.
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Never mind, I just found the answer in Giuliano’s book!
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What software is this?? In addition to these guidelines, do you have recommendations for software settings we should be using for frequency analysis?
