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Today, just on a whim,I placed my guitar tuner inside the body of an almost finished guitar...(no Strings,bridge or frets yet)....I tapped the area over the bridge plate and the  tuner indicated that the sound was in the key of "G"...Now, this has got me wondering, what would it indicate if I tapped it and used a tuner before I glued on the top....Can you tune a top to "E" , "A"....etc.Or is it just a fluke that it happened to show "G" ? Are there any lessons to be learned here?....Larry

Larry:
    Find a copy of the Roger Siminoff book on "The Art of Tap Tuning". Your question opens up more than a can of worms, its more like a whole barrel full. Mr. Siminoffs' book goes into great depth on tuning various parts as well as the whole assembly to specific frequencies...   Good luck,   Mikey

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Larry buddy just think how well it will sound playing Freebird

Jody...the guitar is EIR ,Englemann dreadnaught. I tap tune my tops to a sound that I like .....Maybe that sound is in "G".....The thing of it is, now I am thinking , If a guitar turned out to be a great sounding guitar and you knew what the unglued top was tuned in......well,  can't that sound be reproduced in future builds....I know that there are a lot of things that will alter the sound...but it gives me something to aim for instead of hoping that it comes out right...Does this make sense or am I after having one too many coffees.....Larry

Hesh
" If I leave here tomorrow ,
Would you still remember me"
"LA, LA La"....
We need some music notation in here
Hope you are right..

  Larry, well , as some one said earlier this is a huge can of worms , but from what I have   read and experienced from other builders , I think your idea is right on , what amount of experience it takes to be able to control the sound is up in the air , But I know  a master builder can manipulate the sound in subtle ways. the reason I guessed dread naught was the key of G is most popular in bluegrass music , and dreadnaught guitars are most popular there too . what I found most interesting was  even though you didnt tune  to  precise tones , your box,  ended up tuned to G . this made me wonder if it was  the body  design  , or  perhaps the size of the soundhole which gave you your G tuning.  Jody

 

When you tap a guitar top like you describe, you are getting the main air resonance mode. I believe that is mainly caused by the volume of the box or it would be if the top, sides and back were completely rigid. Now, stick your hand in the sound hole without touching the sides of the hole and tap again. That pitch is the top resonance mode. Tap the back with your hand in the sound hole and that is the back resonance mode. A top resonance pitch of F# seems to be a good value for the top mode but it is not essential. I've liked the guitars with and F# top mode better than the others.

The top and back seem to work better together if the back is a semi-tone higher than the top so a G is good for that.

This is a topic where builders will disagree. Many of them have years of experience and many many guitars under their belt. So do be surprised if there are other posts that claim the I'm full of it.

That should be "don't be surprised"

Mike, are you sure it is exactly the main air mode one gets when taping the bridge. Because if you tap the back, you get a different note (usually higher pitched) and if you tap the endblock etc. Why would only the bridge trigger the main air mode.

A luthier that helped me a great deal since I started building told me to find the main air mode by singing * in the hole* and paying attention at which frequency the guitar moves the most.
For the life of me I cant sing better a mule does and I kept forgetting to buy a small speaker and stick it in the guitar while coupled to my tone generator.

Btw, on many classicals, the top main mode is around G3 196 Hz and the air mode should be at least an octave lower, usually 98 Hz G2.

If you go lower you get deeper bass but lose some treble sustain. Smaller instruments are inherently "higher pitched".

The top main mode is given by the usual suspects: stiffness and weight. Stiffness pushes the resonances up, weight pushes them down. So when one shaves the spruce or the braces these 2 factors work together. Empirically stiffness is stronger than weight, the thinner you go the harder it is to keep them in balance.

These 2 working together, or actually one against the other, allow us to construct an instrument that sounds like a geetar even if our plates are all over the shop: from 0.7mm to 3.5mm, and bracing equally wild...

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when I first heard that most guitars fall somewhere around G, I thought
BINGO, who needs experience feeling wood, who needs deflection testing
and all the rest.  One just needs to shave the top and braces until he
moves the resonance where he wants and there you have a great guitar.



Wrong, you have 1 million different ways to shape the plate tapering and thickness and bracing and all that.

Here an experienced builder needs to feel other balances: left and right, and between bracing and top. There is not a right way and a wrong way, just different ways to get a different sound, and none is exactly free or easy.

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 LOL here we go again!    LOL  Jody

well what did I say wrong, or funny? 

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Notes in the key of G:... G A B C D E F#. Ok which one do I tap-tune to? Don't shoot me I just found the subject title a bit out of wack.

[QUOTE=AlexM] Mike, are you sure it is exactly the main air mode one gets when taping the bridge. Because if you tap the back, you get a different note (usually higher pitched) and if you tap the endblock etc.
[/QUOTE]
That is not my experience. If the sound hole is unobstructed, I get the same fundamental pitch regardless of where I tap on the body. The overtones can be quite different but the fundamental is the same.

When I stick my hand in the sound hole, however, the back and top sound quite different.

 alex , you didnt say anything wrong or funny , its just that tap tuning itself  ,and  various prescribed ( or described) methods are highly debated in the realm of lutherie , the spectrum runs from some luthiers swearing by it ,  up to  cumpiano calling it a fools endeaver ... just  pick  where in that spectrum you  want to be and fire away! someone somewhere will refute you!  LOL   Jody 

Oh, ok 

well, I don't actually believe in *tap* tuning, but more in resonance tuning, and just as an extra control step. Every time i tap on a guitar I hear a different sound, and i probably suck at picking the fundamental of it (see the discussion with Mike above)

But the resonance changing with stiffness and weight is simple physics, no mojo or voodoo here, no need to have good ears. Except the odd funny piece of wood, the main mode of a piece of wood will get lower when thinning it, go back up when bracing it with a light but strong brace, or not move at all (even get lower, think bridges) if that brace is very heavy. If you test and record is just as good info as deflection testing.

check byersguitars.com

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 Alex, I thank you for sharing this little bit of wisdom ! Jody

 can you give me a general idea how much bracing will raise the pitch ? thanks Jody

Jody that is really a too subjective question to quantify. Each top will have different cross and lateral stiffness and each brace will be different as well. Ears to the wood is the only real way. Do a bit, tap again. repeat till you are there.

 gotcha thanks  Jody