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Thanks to your tips, I've loved the Gore Gilet book. Now, I've been tapping a few guitars, measuring their T(1,1) air top and back resonances. I was wondering what your target values are. Right now we're building a large bodied steel-string guitar with forward shifted scalloped X-braced top and a radial Gore live back design.

I'd love to hear your target values and maybe even the 'why'?

Thanks so much, I've learned so much!!!

Hi Joost. Have you seen the recommended targets in the book? I can't remember the page off the top of my head, but they're listed here and there throughout Build, and all in one place in the index of Build. Probably Design, too. Or are you asking to see if we've developed our own?

Yes I did find that in Appendix V of the design book. The Build book was sold out, so don't know if there's more in there. However, there are no back resonance targets for the large bodied steel string guitars. And yes, I was curious if you guys had developed your own targets through experience?

I don't have specific frequency targets for these things: I'm more concerned with trying to get the best out of the top that I can, and then try to get the back to work with whatever that is. My experience suggests that it's quite possible to get a 'good' guitar from a wide range of resonant frequencies. That said, higher or lower frequency placements are reflected in the character of the sound, so you need to think about that in relation to the customer's notions of what sounds 'good'.

If you are doing a Gore radial braced back and you want it to be live, the goal is 3 semitones above the top resonance.

Sometimes I wish someone with a Spectrum Analyzer program and a mic worked at a well stocked music store and found themselves locked in over night with nothing better to do than tap away and record the results. Just saying.

As a builder who does have target resonance goals as well as running a busy repair shop with the opportunity to measure the resonances of many guitars, I find that the majority of factory guitars have tops in the 170-190 hZ range, backs are around 210-240 hZ, and air is usually around 95-100 hZ. Guitars with a lower top resonance have more warmth, all other things being more or less equal. I once measured a Taylor with a sinker Redwood top that sounded like a baritone and had very weak high frequency respsonse. The top was 140 hZ.

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It will probably be alright.

Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out.

So, three is the number, although I understand forty two unifies things pretty well.

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George :-)

But what is the question?

From my understanding of the Gore\Gilet Books, If trying to voice an active back three semitones has been shown to allow for good coupling between the back and the top without overly impacting the response of the top resonance. If the back is voiced higher one moves toward a reflective back (less coupling). If I remember correctly the worst response is if the back is tuned lower than the top.

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http://www.Harvestmoonguitars.com

Keep in mind that Gore does his measurements in a particular way. If you measure things differently you might get different numbers on the same guitar. He and I have discussed this on line on several occasions, and tend to agree to disagree about some of it. There's nothing wrong with either method, but you have to understand that the numbers you get depend on how you get them.

Basically, the closer two resonances are to each other in pitch the more tightly they will couple, all else equal. Coupled resonances tend to shift in pitch from what the same system would be uncoupled. Thus, for example, part of the coupling between the top and back takes place through air pressure changes in the box. If you block the hole it eliminates the Helmholtz type 'main air' resonance, which is very strong and couples to the top and back, so it changes the pitches of the top and back tap tones. In this case they're much closer to their 'true' values than they are with the hole open, but they're certainly different from what they will be when you play the guitar. Are the coupled pitches what count, or the uncoupled ones?

A lot of the power in the low end comes from the air flow at the sound hole near the 'main air' frequency, and the more tightly the top and back plates are coupled the more flow you're likely to get. Thus having the 'main top' and 'main back' pitches as close as possible would maximize power in the low end. Note that in terms of coupling strength it doesn't matter which plate is higher in pitch: in theory if bass power is what you're after, the back can be either higher or lower in pitch than the top. Gore seems to prefer guitars where the back is higher in pitch, and that certainly is the norm, but I've seen instruments where the back was lower, and some folks really liked them.

Having the two plates closer in pitch makes the 'thuddy G' type of wolf note more likely. I can say from experience that you certainly do NOT want the main top and main back resonances too close in pitch: I've seen some really weird things happen when that's the case. How far apart do you need to get them to be 'safe', and how are you measuring the pitches?

In the end you can use the things that Gore says, or that I say, or anybody else for that matter, as points of departure, but in the end you have to decide what the 'right' values are based on how you do things.

I don't do the Gore method (mostly 'cuz I can't afford the books!), but I can pass on a funny "active back" story. I was playing my latest build and was just amazed at the sustain. The tone seemed endless. Then, like magic, GONE! I varied the way I played, I checked the tuning, I was plain stumped.

Defeated, and convinced I had heard things I must have slumped in my chair. MAGIC! It was back! I straightened up to play better, GONE...

Yeah, most of you probably figured it out by now. Every time my generous gut touched the back of the guitar it was like throwing a blanket over it. Pull the flab away, and bingo... color, overtones, sustain, etc.

So from now on I will only make guitars for skinny guys who play seated and slouch!

Working on it

A kid came by here once looking for help setting up some sort of science project. After some talk he settled on looking at the 'main air' resonance of guitars, since it's easy to do and has a lot of bearing on the tone. He went down to NYC and worked his way through two or three of the big guitar shops one day, so he must have gotten a pretty big sample. He won a prize in his science fair, too.

The really easy way to check the 'air' resonant pitch is to sit down with the guitar in your lap and pluck the low E string. You pinch the string up near the nut between your thumb and finger, and move the pinch point up and down until you get the loudest 'thunk'. That's the air pitch. You can get a pretty good idea of where it is from the position of your hand between the frets, but a tuner is a real help, since your fingers don't make a really hard stop.

I would like to see if there is some relationship between the air, top and back resonances for manufacturers that have a recognizable sound. Do guitars from Taylor with the same sound have the same resonances or spread in resonances? Does a Gibson sound like another Gibson with the same resonances? Is it really the tone woods that have a particular sound or is it the resonances due to the mass and stiffness for the thickness that the builder is used to using? Might have to do some deflection testing on the guitars also. Think this is were you will be gently asked to leave.

FWIW, a while back I was at Guitar Center tapping the top and back of various guitars. Mostly I noticed the top and back seemed to be a major or minor third apart. The Yamaha was a fifth. I preferred the sound of the third. I did not try to determine which note it was.