Official Luthiers Forum!

I remember seeing articles where a top is tuned to a certain pitch (mainly classicals) with the back being tuned slightly differently.
Recently there have been a few posts about tuners for this purpose, and wondered if if ayone would like to share what pitch(es) or range of pitch/frequencies they aim for with acoustics, and if this varies for different body sizes, main body air resonace/frequency?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Hi Collin,

I read "The Art of Tap Tuning" by Roger Siminoff and he goes into that issue in great detail. Though he does go into a fair amount of physics, he makes the application quite practical.
I haven't done enough of it myself to comment personally about your question.

Beth

Attached is my current database (steel string) of air, top, and back frequencies, with soundhole both open and closed.
Al Carruth has some great posts in the archives on this topic.

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David Malicky

Nice chart David, I like the fact you are dealing in frequency and not scale notes with cents flat or sharp. What FFT program did you use? I have been putting together a similar spread sheet on my builds and it is very interesting to me to see another persons findings.

I read somewhere on the internet that the free air top frequency should be below 200 Hz. but I can not remember where I found it. The left brain luthiers yahoo web group is a great place to find this kind of information.

Fred

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

Thanks Beth for some potential reading, I've often considered buying that Book.

David, that's a great start, obliged - did you use a program for the spectrum analysis to get the frequencies, where the tuner was not used? I'm trying Audacity, and will take the time to search Al Carruth's posts.

Fred, thanks for the suggestion about leftbrainedluthiers, I've joined - and boy, did it make my head nip searching that!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Glad the chart is useful. Fred, I'm using Audacity... easy, accurate, and free. Yes, it's interesting to me as well to see where the resonances fall for various guitars, and how their sound relates to that.

Colin, Here's a quick start for Audacity, if helpful (about 2/3 down the page): http://www.luthiersforum.com/forum/view ... 01&t=24630
More tips:
- For completed guitars, I play first and write down impressions, then measure. For guitars in-process during sanding or shaving iterations, I tap soundboards or bodies, listen and write, then measure. I think this helps develop one's 'ear', and helps to avoid biases that the reverse order could generate.
- Built-in laptop mics can give inaccurate peaks. I usually use a Blue Snowflake, but any halfway decent mic should work.
- Mic, hold, and tap the guitar in a consistent way for each test, and do some validation on each before relying on the data:
..- Place the mic near what you want to measure to minimize room artifacts and other resonances. (But not in front of the soundhole; this would block some of the air column and lower the 'main air' freq.) I place the mic on desk about 8" off the lower bout: this picks up both the 'main air' and 'main top' about equally.
..- I rest the tail lightly on my knee, left hand around neck to dampen strings. My knee does dampen the body some, but I find peak freqs are within 1 hz of holding the body free (which isn't as consistent for me). The top, back, and air resonances are all coupled (each one affects the other). Some recommend damping the back when measuring the top; that may work for them, but realize that damping the back will affect the air and top freqs, often a great deal, and potentially inconsistently depending on how the damping is applied. [Edit: when starting out, it can be helpful to dampen the back while measuring the top, and v.v., just to be sure of which peak is the top v. back since they are often close together. Then remove damping to get the actual resonance in Hz, which will shift a little from the damped case.)
..- Experiment with different tapping methods. A soft mallet on the bridge is the usual recommended method. I use my finger since I always have it with me. If using a finger, validate it first vs. a mallet: the soft flesh and longer impact dampens the high freqs. In my case, it dampens above ~400 Hz, which is ok for my purposes. For the top, I tap on the bridge 3x in 3 places (left, middle right) and FFT the average of those 9 taps. For the back, I tap in about 4 places around the lower bout. I usually find 2-3 back resonances in the 200-250 Hz range. Tapping over a brace results in much stronger resonance.
- I find tap testing a great tool to accompany voicing trials, for tops, backs, bodies, or completed guitars. I shave or sand a bit, tap and listen, then tap and FFT. I record impressions and #s for each step in a table. Sustain can also be tracked, though it's harder to do consistently. It's interesting to compare the frequency changes to what I think I'm hearing; that is a large topic of it's own. The FFT shows which wood affects which resonance, and how the resonances are coupled: for example, how lowering the 'main back' lowers the 'main air'.

Good luck!

David

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David Malicky

David, thanks for the tips, and for directing me to your other post.
I struggled with Audacity when trying it yesterday (with a creative external mic), and you have clarify how best to use it.
What a tool!

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Audacity has a new version, which may have changed some of the steps to get the frequency spectrum, but I downloaded the new version today and tried it and it was pretty cool. Thanks, David!

I just spent some time with Audacity checking out my latest build and I like it a lot. I have used Strobosoft up to this point and for tuning tops and backs I will probably still use it for this. The ability to save the taps in audacity over the build and go back and revisit what you have done will be a valuable tool for me also.

Fred

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

I record my taps with my little Zoom H-1 and download to the computer in the house. Then I look at the wave analysis using either Audacity or Adobe, Audition which can do the same thing, except it's not free.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Do you notice any difference in the spectrum analysis?
Do you have a preference?

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Actually Audacity is fine. Audition, now named something else, and a part of Adobe's Creative Suite, is their version of ProTools. The limited edition is about $200, and the full edition is something like $1600. Unless you are doing audio and video editing or mastering, there is no need for it. Audacity is fine. The Analysis is very similar, and you get the same results, thought the windows and graphs look a bit different. Actually Audacity will do most of the editing too, but wasn't available when I needed an editing function.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars