Spectrum results

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AKEric
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Posts: 14
Joined: Fri Sep 28, 2018 1:29 pm

Spectrum results

Post by AKEric » Sat Nov 02, 2019 9:30 am

My guitar is 99% complete. It’s a OM sized, 14 fret, live back, falcate. I did the spectrum with no strings so I could get to the inside easier if necessary (I’m not sure if taking spectrum analysis with strings on or off makes any difference). Bridge with intonated saddle and 6 bridge pins = 25g. Not the best, but I can live it.

Spectrum results:
Air = 95
Top = 175 (I think I thinned the top too much way back when)
Coupled = 217

Close for the first try, but the 175 is bad, being on a scale note (F). Ideally, I’d like to drop the Air & Top readings 5hz each to 90 & 170.
Experiment: I added 380g of side ballast. I was shooting for 350g but the pieces of bar stock I cut ended up heavier than planned. I plan on cutting one down to hit the 350g mark.

Second analysis with 380g of side mass:

Air = 95 (No surprise it didn’t drop).
Top = 169 (Missed by 1hz. The extra 30g of side mass?) Trevor, you’re incredible.
Coupled = 113 (Good 4 semitone separation).

I have yet to put on new string to see how it sounds (I still want to get at the insides), but I like these results.

Is the T(1,1)1 being 5hz off from the desired 90 a big problem?

I don’t have the rig (yet) for measuring the specific mobility of the back, but these results would seem to indicate that my back is at least somewhat “alive”.

Build section 22.3.1.2 & 3 (pg. 22-8) indicates I could lower the T(1,1)1 & T(1,1)2 at the same time by shaving the back’s lower bout brace to reduce it’s stiffness. I would prefer this to the side masses because the guitar is already heavier than my other guitars because I used ash for the linings to add more robustness per the Books. The 350g makes it quite a bit more. The photo is the current condition of the back brace (accomplished before gluing the box together).

Any input regarding taking that brace down some more to achieve a 5hz drop in T(1,1)1 & T(1,1)2? This assumes I take this route instead of the side mass route. The down side here is the (T1,1)3 is already close to specifications. I have tried adding poster putty to the back, but got somewhat muddied results on the spectrum.

Any input is greatly appreciated,

Eric
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Trevor Gore
Blackwood
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Joined: Mon Jun 20, 2011 8:11 pm

Re: Spectrum results

Post by Trevor Gore » Sat Nov 02, 2019 10:32 am

95,170, 214 is a great result. Carving the back brace to reduce the T(1,1)1 and T(1,1)2 sufficiently will bring the T(1,1)3 down way too far and cause acoustics problems that you don't need because you no longer have the 4 semitone separation.

Stringing up will possibly drop the T(1,1)2 a little because of the added mass of the strings acting on the saddle and because the saddle with the strings passing over will possibly be more effectively "joined" to the top. This will mean you will need less side mass to get the T(1,1)2 down to 170Hz.

So string it up, correctly size the side masses and you're good to go.

AKEric
Wandoo
Posts: 14
Joined: Fri Sep 28, 2018 1:29 pm

Re: Spectrum results

Post by AKEric » Sun Nov 03, 2019 5:08 pm

Thank you so much for the advice Trevor, you really sent me in the right direction, and your advice was spot on. This is all starting to make sense to me. I strung it up and fine tuned the relief, nut slot depth and action. Ended up with 13 mm string height at the bridge. I averaged 10 plots on the analyzer using zero side mass:

Air: 94
Top: 172
Coupled: 215

The guitar sounds very good to me. I think I'll forego the extra 100+ grams of side mass and call this one done and not worry about being over by 2hz on the top. I'm looking forward to the next build, your process is amazing.

Again, thank you for being so accessible with your advice. I can imagine answering similar questions can get a bit trying.

Eric

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Trevor Gore
Blackwood
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Re: Spectrum results

Post by Trevor Gore » Mon Nov 04, 2019 9:24 am

Good to hear it all worked out. Well done!

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