Using Audacity to determine resonance frequencies???

Construction and repair of Classical Guitar and related instruments
Supramisfit
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Using Audacity to determine resonance frequencies???

Post by Supramisfit » Tue Apr 16, 2019 1:40 am

So I just closed the box on my first build and I'm trying to gather and log as much data for future reference. I'd like to determine the main resonance frequencies and could use some help. I took a sound recording of me tapping the bridge area (bridge not installed yet) and input the recording into Audacity but I'm not quite sure how to decider the information. Here is a screen shot of the Audacity graph if anyone can provide some info. Thanks.
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RockyWestTexas
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Re: Using Audacity to determine resonance frequencies???

Post by RockyWestTexas » Tue Apr 16, 2019 11:50 am

Most of your questions will probably be answer by a YouTube video. Search for "Tap testing of acoustic guitar body using Audacity". Write back if you need further information.

Rocky

Alan Carruth
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Re: Using Audacity to determine resonance frequencies???

Post by Alan Carruth » Tue Apr 16, 2019 4:11 pm

Try sampling at a much lower rate than 44,100. This will cut the upper frequencies off, and may give you better resolution in the low range. Right now all you're seeing is the ghost of the 'main air' and 'main top' modes near 130 and 260(?) Hz. Both of those seem high.

Paul Micheletti
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Re: Using Audacity to determine resonance frequencies???

Post by Paul Micheletti » Tue Apr 16, 2019 5:04 pm

You are using the sample size 1024 which is the default. It produces these flattened humps. Try changing it to 2048 which is the setting I've settled on.

I have no idea why the sample size setting changes the results significantly. I don't really trust this for specific info as much as relative info. I measure each instrument and develop targets based upon which instruments I like best. And my targets are more the shape of the envelope rather than specific tap frequencies.

Dave M
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Re: Using Audacity to determine resonance frequencies???

Post by Dave M » Tue Apr 16, 2019 5:08 pm

Just in case you are not aware: many people use Visual Analyser which is free software. I have certainly found it easier to set up than Audacity.
Dave

Supramisfit
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Re: Using Audacity to determine resonance frequencies???

Post by Supramisfit » Tue Apr 16, 2019 6:07 pm

So I've played with the Audacity program a bit more and here is a new screen shot. Hopefully I can learn something in the process of all this and if I can begin to understand the data and how it relates to this particular guitar maybe it can prove useful to make any changes to improve the instrument. I will try the program others have suggested also for comparison.
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Stephen Faulk
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Re: Using Audacity to determine resonance frequencies???

Post by Stephen Faulk » Wed Apr 17, 2019 12:33 am

I found the video interesting because he shows how the program works, but confusing about what he's trying to look at and why.
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Trevor Gore
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Re: Using Audacity to determine resonance frequencies???

Post by Trevor Gore » Wed Apr 17, 2019 1:27 am

Supramisfit wrote:
Tue Apr 16, 2019 6:07 pm
So I've played with the Audacity program a bit more and here is a new screen shot. Hopefully I can learn something in the process of all this and if I can begin to understand the data and how it relates to this particular guitar maybe it can prove useful to make any changes to improve the instrument. I will try the program others have suggested also for comparison.
I think you need to learn a bit more about how to set up a spectrum analyser to match your usage.

Certainly, Audacity is far from the best tool for this job because it is primarily for recording and wave editing. Other tools are available and the one I prefer is Visual Analyser. Instructions on how to set it up are here. You will need a good book :wink: to help you interpret the plots and learn how to use them.
Trevor Gore: Classical Guitar Design and Build

Supramisfit
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Re: Using Audacity to determine resonance frequencies???

Post by Supramisfit » Wed Apr 17, 2019 1:50 am

Trevor Gore wrote:
Wed Apr 17, 2019 1:27 am
Supramisfit wrote:
Tue Apr 16, 2019 6:07 pm
So I've played with the Audacity program a bit more and here is a new screen shot. Hopefully I can learn something in the process of all this and if I can begin to understand the data and how it relates to this particular guitar maybe it can prove useful to make any changes to improve the instrument. I will try the program others have suggested also for comparison.
I think you need to learn a bit more about how to set up a spectrum analyser to match your usage.

Certainly, Audacity is far from the best tool for this job because it is primarily for recording and wave editing. Other tools are available and the one I prefer is Visual Analyser. Instructions on how to set it up are here. You will need a good book :wink: to help you interpret the plots and learn how to use them.
Well that is definitely a true statement and why I'm here asking the questions. This is all completely new to me.

ernandez R
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Re: Using Audacity to determine resonance frequencies???

Post by ernandez R » Wed Apr 17, 2019 6:24 am

Supramisfit wrote:
Wed Apr 17, 2019 1:50 am
Trevor Gore wrote:
Wed Apr 17, 2019 1:27 am
Supramisfit wrote:
Tue Apr 16, 2019 6:07 pm
So I've played with the Audacity program a bit more and here is a new screen shot. Hopefully I can learn something in the process of all this and if I can begin to understand the data and how it relates to this particular guitar maybe it can prove useful to make any changes to improve the instrument. I will try the program others have suggested also for comparison.
I think you need to learn a bit more about how to set up a spectrum analyser to match your usage.

Certainly, Audacity is far from the best tool for this job because it is primarily for recording and wave editing. Other tools are available and the one I prefer is Visual Analyser. Instructions on how to set it up are here. You will need a good book :wink: to help you interpret the plots and learn how to use them.
Well that is definitely a true statement and why I'm here asking the questions. This is all completely new to me.
Lots of learning...

I was thinking Garage band had a spectrum analyzer of sorts. I didn't find it but the simple integrated tuner told me my main resonance, fresh shellac and no bridge, that it was humming right along at 25 cents short of E flat while tapping directly in the center of the bridge, mostly the same all over the top and back except for a few places showing D outboard edge of lower bought. This one is braced, (Kashi-esc) so that the bridge ties all the sound bars together so thinking it will tighten up once it is glued on. Also the fresh shellac will cause the pitch to rise as it dries.

Guess i will need to get more fancy to see exactly what the top plate is doing.


HR
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Beowulf
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Re: Using Audacity to determine resonance frequencies???

Post by Beowulf » Wed Apr 17, 2019 3:00 pm

How does stringing the instrument, i.e., loading the top but damping the strings affect the resonances?
1971 Yamaha GC-10 (Hideyuki Ezaki)
2017 Yamaha GC82S (Akio Naniki/Naohiro Kawashima)

johnparchem
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Re: Using Audacity to determine resonance frequencies???

Post by johnparchem » Wed Apr 17, 2019 3:39 pm

I find loading the top drops the top resonance 5-8 Hz, the air resonance does not move much.

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Beowulf
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Re: Using Audacity to determine resonance frequencies???

Post by Beowulf » Wed Apr 17, 2019 4:07 pm

johnparchem wrote:
Wed Apr 17, 2019 3:39 pm
I find loading the top drops the top resonance 5-8 Hz, the air resonance does not move much.
Thanks John. I have used Audacity to compare the air and top resonances of my GC82S and GC-10...most interesting to compare. The GC82S has a 91Hz air resonance (between F & F#) and the GC-10 is at 94HZ (between F# & G). The top resonances are 208HZ and 200HZ respectively, both around G#3. I do not notice any wolf notes or differences in level or sustain around those resonant frequencies. I'm sure that Audacity is not as sophisticated or accurate as Visual Analyser, but it seems very simple to use.
1971 Yamaha GC-10 (Hideyuki Ezaki)
2017 Yamaha GC82S (Akio Naniki/Naohiro Kawashima)

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guitarrista
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Re: Using Audacity to determine resonance frequencies???

Post by guitarrista » Wed Apr 17, 2019 5:36 pm

From the two graphs you have, it seems the first peak is at 119Hz which is midway between a A# and a B; the second is around 250Hz or so (lower res from the first graph so hard to say exactly) - which is about a B. I don't know if this makes any sense to a luthier.
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Supramisfit
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Re: Using Audacity to determine resonance frequencies???

Post by Supramisfit » Wed Apr 17, 2019 6:21 pm

The peak at 119hz seems so high to me when I hear about what other guitars measure at. I'm curious as to what construction methods lead to this type of resonance and what are the sound characteristics of a guitar in this frequency range? The body of this guitar is slightly smaller than your "standard" guitar and I don't think it's overbuilt but I could be wrong about that. It sounds louder and more responsive to tapping the top than any of my other guitars. My other guitars peak frequencies measure much lower

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