Official Luthiers Forum!

This is a follow up to a thread & did a while ago while building an OM. In the earlier thread I measured tap responses of just the box, no bridge, no neck, no string tension. This was that thread viewtopic.php?f=10101&t=39159

The guitar is done & I've taken measurements of the completed guitar - fully assembled with strings. Here's what I got

So would you interpret it as:
131 Hz frequency of the air
157 Hz top frequency
197 Hz back frequency

Thanks,
Kevin Looker

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I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

131 Hz would be an extremely high main air freq. The bare box in your prior post had a very normal main air of 104, so we'd expect the completed guitar to be a little lower than that, around 100. The FFT above has no content there, which makes me wonder if either the mic was blocking the soundhole, or something was amiss in the recording (maybe a laptop mic this time?)

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David Malicky

Yeah, the plot is a little funky. It's on my new laptop (Windows 7) which required a new Audacity install. I'm using an external mic but I had trouble getting the gain set - it was clipping so I had to turn it way down. There's a chance that the computer is possibly combining the external mic with the mic in the laptop.

I think I'll take more measurements with my old setup to see how they compare.

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

There's very definitely something wrong there. If you're on Windows, I would strongly recommend using Visual Analyser. If you really want to use Audacity, make sure (as you have to in VA, too) that you have the FFT sample size and frequency sampling set correctly (16384 and 11025Hz respectively) as well as checking your inputs.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Looks like a mountain with a lot of spruce trees!!!!!!!!!

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MK5acoustics.com

Well this is embarrassing.

Every time I do one of these spectrum threads I screw something up - the batteries in my "good" mic need replaced. I was able to use my other mic which I believe is accurate.

Here's what I think is an accurate plot.

Air 101 Hz
Top 201 Hz
Back 220 Hz

Do these look realistic?

Thanks,
Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

That makes more sense, both in the waveform and the numbers you are getting. The air and coupled top frequency are nicely centered between scale tones but the 220 is directly on the A3 note.

Fred

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Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

Definitely looks more like. Try linear scaling on the frequency - you might find it easier to read. An octave separation between the coupled main air and main top is generally not a good idea, even when between scale tones.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks for the replies.

So If I understand correctly, we have two issues (besides the top & back being closer than 4 semitones):
1.)The back frequency lands directly on a scale tone (220 Hz = A3)

2.)The air & top frequencies are an octave apart (101-201 Hz)

In trying to adjust these frequencies, I'm a little confused.

Adding side mass will lower the top frequency, but not affect the air or back or have minimal effect on them?

I want to lower the top frequency & raise the back but since they're coupled, they'll follow each other to some extent.

If I start messing around with adding stiffness to the back and reducing stiffness of the top, will I be able to get the frequencies to effectively move in the desired directions?

Thanks,
Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

First thing is how does it sound? If you can't hear any wolf notes, is reasonably balanced etc. etc., no need to do anything, which also suggests you haven't got a particularly responsive (high monopole mobility) guitar. If you have built a responsive guitar, you may need to do some tweaking, but if the top is at ~200Hz, I think you could get quite a bit more responsive.

If you want to experiment a bit, the first change I'd want to do is try to get the top resonance down to 190Hz or even 180Hz, preferably by removing stiffness. That, I suspect, is going to be hard for you to do, because dropping 20Hz by bracing shaving is a long way to go, which then suggests that the top was left too thick and at this stage it's really hard to do anything about that. So, aim for 190Hz, maybe using some side mass to get the last little bit and see if it sounds better there. The back will follow a little bit, but not much. You'd be better off with a stiffer back, but I suspect it's not doing very much harm.

My medium sized guitar (plan in the back of the book) is a modern take of a 000, and that model sounds great at 95, 180, 226. Those numbers on a high mobility 000 would be killer.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks Trevor.

The guitar actually sounds pretty good when played with a flat pick, good separation, crisp & powerful. It's not very responsive to the light touch though.

I built this one with the intention of experimenting so I'm going to give it a try.

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

You may find it easier to drop the main air pitch a bit by lowering the 'main back' resonant frequency. Try adding some mass to the center of the back, or over the middle of the brace just below the waist, and see what happens. If that works, shave enough wood of that brace in the center to drop the back pitch the same amount. One big advantage here is that you're not taking the chance of weakening the top.

Having backs relate at a minor instead of major third also works fine.
I've put out a few like that recently. In fact, i just did that to a Parlour guitar last night and it opened the bass tremendously, without touching the top. I would not loosen your back any more than it is though. There's a very good chance (read near certainty) that dropping your top via brace shaving to 190 will both solve your air/top octave problem and knock your back off of 220.
I would try that first, then measure your bridge rotation. If you're still in the safe zone, I would consider going to 180. Final results of 180 top 215 back will sound great, if you don't go past structural stability getting there. Trevor can probably tell you more about how to measure that. Got the books?

Yeah, I have the books. I just have read & re-read them over & over again before I get a solid understanding.


Thanks,
Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

Yeah, I hear ya there.
I like looking at all the funny pictures with the squiggly lines and brackets and stuff....

If we stare at them long enough they'll make sense...

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Mountain Song Guitars www.mountainsongguitars.com

I hate to admit it, but I have a degree in mechanical engineering & it takes a lot of effort.

The best part is that I miss the point or get it wrong most of the time.

I think it's time for the "Gore for Dummies" series.

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

Yes please!

That's what being taught in imperial units does for you! Makes anything incomprehensible!

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Kevin: Thanks for being so candid. You are not alone in your struggles. Trevor's book contains lots of useful data that is quite easy to grasp...........................but there is some that is twisting my neurons into a knot. Carry on and please keep your efforts and results coming.
Tom

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A person who has never made a mistake has never made anything!!!

I have to agree - it's not easy stuff and I'm a EE, with strong control systems and RF background, so I get the math pretty good, but the acoustics are a challenge to get my head around.

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Steve Smith
"Music is what feelings sound like"

Hmmm....

Perhaps my lack of math is actually a bonus. I was able to put the principles to immediate use without so much as consulting a calculator...

The principles are can be applied without understanding the theory and I don't think you have to understand the theory to benefit. I just like to understand the derivation and I do think it helps you get farther down the road. That said I built my first falcate braced guitar simply by using the build book without much of the theory and the folks who have played it seem to like it.

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Steve Smith
"Music is what feelings sound like"

I haven't even tried to follow the specifics of the mathematical modeling & derivations.

From my experience in engineering school, you will be given the final equation(s) & trying to get a complete understanding of how all the components affect each other is where the challenge occurs.

Kevin Looker

_________________
I'm not a luthier.
I'm just a guy who builds guitars in his basement.
It's better than playing golf.

The cool thing is that you don't need to be able to do the math to understand and use the principles....
Though I'm sure math chops would get you further...