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Hi guys.
Am new here. Was just browsing and noticed some threads about tapping and what to adjust for. So wanted to share some results of my current build, see what the guys have to say about it. I don't have the Gore books, but have read up on what is being said about this method. Sounds interesting.

So here is what I've got:

Steel string, Dread size, 4.9" depth.
Lutz top, .1" thickness, baked. 28' radius down to bridge, then flat.
Martin X style bracing, .25" thick, scalloped, parabolic.
Osage Orange sides, .95" thickness, sound port.
Osage linings, solid.
Osage back, .075" thickness.
No bridge, no finish, no neck.

So this is what I'm getting when tapping at the bridge area. Back monopole seems very active, but not sure what it all means.
88hz Heimholz
173hz Top monopole
205hz back monopole
320hz cross dipole

Also have peaks at 340hz, 380hz, 440hz.

i don't know anything about this, but shouldn't the sampled range get at least to 5Khz? yours stops at 650hz

You can't control what happens in that range, so no real need to plot it.

Your top 173hz would be in the neighborhood, if the bridge was on and it was strung up. As it stands, when you add the bridge mass, the freq is going to drop below Gores target freqs, which are very effective IMO. So not exactly good news.

As well, your back is below his target dread range, and unfortunately close to 207hz which is a scale tone. So also not good news. I think .075 is a little thin for dread backs.

So, your best bet is to use the very very lightest bridge you can get your hands on. Use a rectangle bridge 1"x6", that's plenty of surface to hold. Try rosewood or even better, walnut. Dye it black if you hate the look. Use light bridge pins as well. A few grams weight savings in the bridge area goes a long long way.

However, don't despair. Though your numbers are low to Trevor's specs, it's quite possible that you will have a great sounding guitar anyway. Someone told me that sometimes Somogyi's top monopoles go as low as 128hz, though I have no real verification of this.

What concerns me about going below 170hz is really more about long term structural strength, but I have a lot more investigating to do in that regard.

Anyway, I hope you found some of that helpful, I would love to hear how it comes out!

Cheers,

Ed

Ed beat me to it but here is what I had ... In tonal qualities of a steel string guitar in Trevor Gore's book, he say "We pitch the main top resonance at either 170Hz or 180Hz with a main air resonance of 90Hz or 95Hz" 88 - 173 seems pretty close. The bridge may drop the top lower. He likes the back 4 semitones higher than the top. You top is basically an F so you would want to see an active back around 220Hz. Too close of coupling between an active back and a top can drain energy from the top is some frequency range. Everything I am saying is just book learned, I have been documenting my guitars so that I can start to make some of my own correlations.

nyazzip after saying you do not know anything about this why do you think he should show that graph out to 5K. It would be so compressed it would be hard to see all of the interesting tap resonances.

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i replied because no one had answered the post. it is news to me that the tap measurements traditionally cut off at such a low frequency, but now i know....interesting.

The things is, while it's interesting to see what is going on up there, you can really only directly control with certainty the monopole of the back, and the monopole of the top. The air chamber resonance will vary somewhat depending on the stiffness of the plates. Since they are all coupled together as a unit, influencing one influences the others. Loosen the top and the back monopole will drop slightly as well.

You can influence the freq range of the dipoles by changing your bracing style I *believe*. For example, you can imagine how using tapered bracing would raise the freq of the cross dipoles anyway, but I don't know how to set them to specific pitches like you can the top and the back.

Anyway, my point is since you can't control the higher freqs, measuring to 650 is plenty to give you the data to control that which you can....

I wonder if I could get a little more primitive, but I think it's in-line with this thread. I'm still working on getting Visual Analyzer results, but I've got a couple of Chladni patterns to show and ask questions about. I just put the bridge on a guitar (classical with seven fan bracing) and thought I'd check out the monopole. Using tea leaves in a practical sense, I generated the two patterns below. As I dialed through the range of the monopole, it appears I got my strongest response on the top at 207 Hz although things were moving at about 180 Hz, but they were really bouncing at 207. I noticed as I was doing this that the back was vibrating pretty strongly as well, so I flipped the guitar over and placed the speaker beneath the soundhole, and got a strong response on the back at around 211 Hz (stronger than the top based on my visual observations of how well the tea leaves were moving about). Based on the discussion above, I'm concerned that the back might interfere with the top. Guess I'll know more when I actually plot the frequencies and string the guitar up. I'm also a little concerned that my speaker may not be adequate for the task (have to hold it fairly close to the soundhole), but I'm wondering if anyone would care to comment on these patterns?

Coach71, I would like to see some pictures of your osage orange when you get a chance.

Thanks,
Marty

After taping on a 20gr bridge, the top monopole dropped to 152hz. So I'm presuming I need to go down further with the back as well (It's still 80 grit rough sanded). I've no issue with that.

I would not make the back thinner. I think it is already too thin. I do not think you need to lower its frequency either as it was too low. If you do decide to lower it wait until the guitar is finished and take a bit off the braces.

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What John said...

Guys can I butt back in on this conversation with a few questions?
After fooling around with Chladni, I started playing around with VA (wasn't having much luck with Audacity), and made plots of top and back of guitar. I'm using Visual Analyzer but don't like the scale settings or can't figure out how to adjust them.
Coach71, what analyzer are you using - I like your spectrum plots much better than mine (see below). Next question? Is there value to tapping the back, and if so do you place the mic near the soundhole to record? Below are plots for top and back. Any comments from anyone who has looked at the things would be appreciated. The guitar is a classical (7 fan bracing), bridge attached. Ambrosia maple back and sides, WRC top. Just realized as I posted these that the top is plotted in log scale and that back is plotted in normal scale. Sorry about that. Thanks for putting up with me on this, but maybe a chance to learn a thing or two.

Coach71, thanks for picture of the osage orange.

I use VA and followed the instructions from an Australian forum. http://www.anzlf.com/viewtopic.php?f=1&t=3696&p=44370 .

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There is definite value in tapping the back. About 8"away from the center of the lower bout on the back for the mic.

I'm not really sure what the freqs should be for classical, sorry...

For a classical get the top between F# and G or G and G# 190 or 202.

It is hard for me to tell where your top and back are.

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John and Meddlingfool, thanks for the responses. John my top came in at around 210 and back at about 225 (although I need to re-do the back). When I first did the back I held the mic at the soundhole; I'll do a Meddlingfool suggested and hold above the lower bout on the backside. I've got another almost ready to check (needs a bridge), but this is much easier than the Chladni setup, and it doesn't hurt my ears.
Thanks, again.

Looks like top is around 220 so room to come down, and back looks around 225 which is probably a bit low for the major third relationship, but I have found a minor third works just great on smaller guitars, however all my experience is steel string-centric!

This guitar has a flat top and is almost 5" in depth. I presume these 2 factors contribute to lowering the frequency of the top monopole. So suppose there is adequate structural integrity, is there any reason why a top at 152Hz and back at 200Hz would not couple sufficiently?

I do have a fairly lightly build Walnut dread at 160Hz top, 200Hz back. Very good sounding guitar and structurally fine. Top thickness is at .1", back is .08".

I don't think there's any particular reason why it couldn't sound good. Only one way to find out!

mkelyvrod wrote:
" I flipped the guitar over and placed the speaker beneath the soundhole,"

How close was the speaker to the soundhole? If it's less than a soundhole diameter away (and two would be much better!) it's altering the pitches of things. I'd drive the back directly with the speaker, still making sure that the sound hole was clear. You can get the speaker as close to the back as you need without making too much difference, and put in much more power that way.

Thanks Alan, I was at best only a soundhole diameter away. I will take your advice and work directly above the back.

So, with finish and 18g bridge already glued on, I'm getting these:

89hz heimholz
168 top mono
209 back mono
318 cross diapole

Looks like the top mono will need to be worked on. Anyone offhand know if putting on strings raises or lowers the top mono,how much? I have a 4mm saddle slot, so some weight will be added there.

To raise the back, I can add some stiffness via a brace. Heimholz I can raise via increasing soundhole.

Well, dang. Stringing it up will drop it. If the numbers were of a strung guitar, you'd be pretty close to on the money, just on the underside. String it up and see what you get, I'm betting 161-163 for the top, and only a very subtle change in the back, often times upward a HZ or two. String it up and measure, that's when it counts....

Coach71,
can't really help, but I'm interested in your results and the idea that you want to modify your finished results by adjusting things. Hopefully Trevor, Alan and/or meddlingfool will respond as they seem to be up on what this really means. I do have a couple of questions, however: 1) what targets are you trying to get to (seems like you have some in mind), 2) how would you increase the soundhole, and 3) what graphing program are you using?
Thanks,
Marty

Here's what i'm getting strung up to pitch, using plastic pins.
89
162
206
314

Well, those top and back numbers are all about 8hz low for the Gore 'ideal'. The question of greater significance is, how does it sound?