Australian/New Zealand Luthiers Forum

Kevin, Check out pages 2-22 to 2-25 in Design (if you haven't already). Thanks! No, I hadn't reviewed that section. I wish I would have weighed the back before I braced it & closed the box. Reducing to target thickness would remove about 20% of the thickness. That would of course reduce mass but al...

One last thing I'd like to add to explain why I thought the Martin low & wide lower back braces would get the back to a desirable frequency. This is the plot from one of my earlier builds. It's an X-braced, Martin OM shape with low & wide lower back braces per the Grellier plan. PadaukOM-Plot.jpg I ...

Thanks John.

Do you know what effect a back with a lower resonance than the top will do to the sound?

Unfortunately the back is about 2.8 mm thick. I could attempt to thin it while on the body but I think besides reducing weight, I'll be lowering stiffness too so I'm going to pull it, thin it, & re-brace with taller braces. I made a bad assumption that Martin's old style "low & wide" lower back brac...

Yes, it's a Martin X-brace pattern. Now that I'm checking my notes, it looks like I neglected to do final thicknessing of the back. The back was supposed to be 2.2 mm but I have 2.8 mm written down which was the original thickness when I glued the panels together. This is embarrassing, but I'll chec...

I took some measurements after closing the box. Here are the top taps: AmazonDredTopTaps.jpg I then tapped the back while holding the top against my stomach and leaving the sound hole uncovered to try to measure the back: AmazonDredBackTaps.jpg Did my back taps actually measure the back at 196Hz? If...

Update: I did some brace shaving and sanded the X. Here's where things went: FigSapeleOM-Nov22-2018.jpg T(1,1)1 is off the A note and the other modes dropped too. It looks like I could add a realistic amount of side mass to drop T(1,1)2 by 5 Hz which would get me 2 semi-tones of separation, not the ...

Did another round of taps, results are a little different. 230 Hz is the back. I don't know what 241 is, I couldn't get any visible pattern at that frequency.
Here's the back at 230 Hz. Kevin Looker

The guitar is complete.

I was attempting a live back, I assumed that 239 was the back.

I have a frequency generator, speaker, etc. so I can test it.

Thanks

Looking at the attached plot, you can see that my main air response is right on 110 Hz, or the A note if you prefer. This is an X-braced guitar. If I add side mass my main top frequency will drop from 210 but I need to get it below below 207.65 Hz so that it doesn't land on G#. If I attempt that, an...

Thanks Martin, Is that VA? The plots look smoother like they're filtered which make it easier to read. I think I was initially concerned because my F-long values were always higher than the examples listed in the book but then I realized my plates are all thicker than the ones in the book. Tap testi...

I forgot to mention my plate thicknesses which i think would affect the frequencies.

The first plate is 4.85 mm.

The second is 4.21 mm.

Thanks

I was getting inconsistent & what I thought were questionable results with tapping so I tried to generate some patterns. On this piece I got 81Hz for Fl when tap testing which seemed too high. I then tried to measure with Chladni patterns. I started around 30Hz trying to see Ft. I noticed that thing...

These are my pullers. A pair of inexpensive end cutting pliers - less than $10 here in the US from the hardware store. FretPullers.jpg The tops have been ground down flat. Use plenty of heat when pulling, I use an old soldering gun. After heating try to gently squeeze the cutter jaws under the crown...

Thanks for the replies, John & Trevor.

More good information for me to think about as I try to carefully tweak the guitar.

I guess if this was simple, we'd get bored & take up a new hobby.

Kevin Looker

It may have something to do with having the gain set so high as I did.

I've just started using VA & like it compared to audacity. Being able to immediately see the results of your tap to see if you're getting consistent measurements as opposed to an anomaly is a great feature.

The main thing I changed was holding the guitar body further from the mic. Correct, the bridge isn't on yet. I've already started thinning the perimeter of the top. Would reducing the height of the braces have much impact? The lower legs of the X are not scalloped, just tapered and I would like to g...

Thanks for the replies. I'm using a Shure PG57 into my Lenovo laptop. The mic is mounted to the bench & I'm holding the guitar body (closed box only) about 1m away (maybe a little less). I have discovered that the fan on my computer generates about 90 Hz. I did some more taps & got this. AnigreBoxTe...

I'm attempting a live back, X-braced, L-00 size guitar. I just closed the box & did some tap tests. From what I think I'm seeing, my coupled top response is high at 118 Hz. Is that the back around 263 Hz? Do my measurements look accurate? Any help or insight would be appreciated. Thanks, Kevin Looker

I've used threaded brass inserts which seem to work fine if you can get them into the hole straight. On my last build, I used hanger bolts that engage into a vertical dowel like Collings Guitars does. I assume this is a sound technique because I haven't heard of Collings having failure issues? Kevin...

Thanks guys.

I knew the needle pointer wasn't going to be accurate but didn't know there were cheap frequency counters available. I think I'll buy one which will be a easier than recording tones and analyzing.

Kevin Looker

Thanks Trevor.

I just picked up this off of Ebay. It lists a 600 Ohm output impedance. Does anyone know if I can run this directly into my hi-fi preamp or will I need to somehow change the impedance.

Thanks,
Kevin Looker

Quick question:

What are all of you using for a signal generator?

Thanks,
Kevin Looker

I was never able to sharpen a scraper consistently until I saw the Tage Frid video that's included with the Tage Frid Teaches Woodworking books. Unfortunately, I don't think it's on Youtube. Basically he only files the edge of the scraper then burnishes with the back of a chisel. He goes from filing...