Builders: relationship between top, back, and air?

[gitnoob]

It appears that different builders have different voicing philosophies (understatement?).

I'm curious if any builders would tell us if they target specific resonance peaks for the top, back, and air cavity and what those relationships are.

I was so curious about this, in fact, that I shelled out Big Bucks for Somogyi's books only to discover that he leaves a lot as exercises for the reader.

He does seem to favor the following:

• Back tone should be higher than the top tone
• The relationship between the three resonances should be relatively "evenly distributed"
• Since the air resonance is relatively fixed (for a given soundhole diameter), he appears to tune the top and back to the air
• He doesn't seem concerned about wolf tones, at least for steel-string guitars (I should qualify this: his "even distribution" counters wolf tones to some extent)
• He appears to favor lightly-built tops and backs, implying fairly low resonance frequencies

Is this something builders talk about or is it "secret sauce" stuff?

[Howard Klepper]

Originally Posted by gitnoob

It appears that different builders have different voicing philosophies (understatement?).

I'm curious if any builders would tell us if they target specific resonance peaks for the top, back, and air cavity and what those relationships are.

I was so curious about this, in fact, that I shelled out Big Bucks for Somogyi's books only to discover that he leaves a lot as exercises for the reader.

He does seem to favor the following:

• Back tone should be higher than the top tone
• The relationship between the three resonances should be relatively "evenly distributed"
• Since the air resonance is relatively fixed (for a given soundhole diameter), he appears to tune the top and back to the air
• He doesn't seem concerned about wolf tones, at least for steel-string guitars (I should qualify this: his "even distribution" counters wolf tones to some extent)
• He appears to favor lightly-built tops and backs, implying fairly low resonance frequencies

Is this something builders talk about or is it "secret sauce" stuff?

I won't try to take all this on at once. Many builders talk about this stuff. I will say that I have to wonder if you are reading #3 above re air resonance correctly. It makes no sense to me the way you have it. The air resonance frequency is not fixed by the soundhole diameter; the top, back, and air are fairly strongly coupled and interdependent, so I don't know what tuning the top and back to the air could mean. I also don't know what "evenly distributed" means in #2. The air resonance of a good guitar is usually around an octave or a bit more below the free top's main resonance, and the back about a whole tone above the top. Is that a relatively even distribution?

I don't have Ervin's book, but I have heard that it does not contain a recipe for building a great guitar; people seem disappointed by that. I don't think it was ever intended to do that.

My target for the air resonance of the closed box is around F#2, or a bit lower, FWIW. I used to keep track of top and back resonances, but now I tune for musicality rather than pitch.

I think you might find some worthwhile material on Alan Carruth's website.

[gitnoob]

What I meant by the air resonance being fixed is that my impression is that it's not really tunable once you've decided on the box size, shape, and soundhole location/diameter. While the top and back are still tunable by removing and adding wood.

I've also read Siminoff's take on voicing, and he has a slightly different take than Somogyi, so I wondered how other builders view the relationships.

Somogyi talks a lot about tightly coupling the back to the top -- keeping them "in phase." That concept was driving me nuts, and I bought his book primarily to understand what he meant.

I'm a little bit more clear on the concept now that I've read his books, but not by much.

[KMClark]

The best advice I have is what Mr. Collings said from Collings guitars. "Noone has control over what wood will do or say. The only thing you can do is let it vibrate and not self destruct."

[SteveS]

Originally Posted by KMClark

The best advice I have is what Mr. Collings said from Collings guitars. "Noone has control over what wood will do or say. The only thing you can do is let it vibrate and not self destruct."

That makes more sense than anything I've read on voicing.

Well, not realy. The way you make it vibrate and what pieces of wood you chose are pretty darn important.

[Kent Chasson]

Originally Posted by gitnoob

What I meant by the air resonance being fixed is that my impression is that it's not really tunable once you've decided on the box size, shape, and soundhole location/diameter. While the top and back are still tunable by removing and adding wood.

I questioned that part of the book and can't make sense of what he's saying. You certainly can adjust the main air resonance independent of air volume and soundhole diameter. The stiffness of the plates have a big impact. I've adjusted the pitch after bodies are together by thinning plates or shaving braces.

Maybe he means something different than I do when he says "main air resonance" but I think not since he goes on to define it as the frequency where the box lights up when you hum into it (or something like that).

[Howard Klepper]

It would be true if the guitar had perfectly rigid back, sides, and top, like a Helmholz resonator.

I've found that a little bit of thinning around the perimeter of the top can drop the main air resonance quite a bit.

[Brackett Instruments]

The helmholtz frequency is determined by the size (and probably shape) of the box, and soundhole. I believe alot of high end classical guitars have the soundholes tuned for some kind of maximum output. The builders keep enlarging the soundhole until they go too far, then use soundhole binding to get it back right.

The soundhole/waist relationship has alot to do with a guitars presonality too, but I don't have the vocabulary to accurately describe it. The shallow waist on a dread has alot to do with making a dread sound like.....a dread. The deeper waist is part of the reason most slope shouldered dreads sound a little more.....refined.......or a little less.....dreadlike. It's also part of the reason a (17") Jumbo, even though it's bigger than a dread isn't usually louder, or bassier, but actually more balanced.

Like Howard, I don't tune my plates to a specific pitch, but more of a target tone. However, I want the top and back to be coupled, before the installation of the bridge, and strings. The weight of the bridge will lower the tops frequency a little, and so will string tension, along with a little playing time. This moves the top and back far enough apart to prevent wolf notes. I believe alot of builders do this. A guitar voiced this way really changes the first five minutes the first time it's played. I need to record it sometime.

I don't read books on the subject so I can't comment on any of them. There's a short voicing video on my website, or by looking under Brackett Instruments on youtube. It isn't scientific, or meant to be a "how to" video, it's just what I do to a top before installing it on the ribs. I do my final voicing of the top after it's installed though.

[KMClark]

Originally Posted by SteveS

That makes more sense than anything I've read on voicing.

Well, not realy. The way you make it vibrate and what pieces of wood you chose are pretty darn important.

Yes thats what he meant by vibrate and had more to do with integrity.. I know what Mr. Collings meant. Short but sweet. Anyway!! Keep mind reading wood my friend.

[Burton LeGeyt]

I have not read the Somogyi books but I do agree with some of the things he recommends. I try to get my back just slightly higher pitched than the top plate. I think that this allows them to couple well (as they are close together), and reinforces a good depth in the bass tones. If they are the same there will be a wolf tone at that frequency. I like to have them less than a whole tone apart.

I tune my free plates to where I want them and record those pitches and how well each pitch is "tuned" using the Chladni method. I know that does differ from Somogyi who prefers to tune with the top plate glued to the rims (as I understand at least). I often alter the overall stiffness depending on what I am trying to accomplish with the guitar. I haven't been attempting to tune each instrument to a specific pitch, I have been focused on tuning the top plate and then tuning everything else off of that.

This stuff does get talked about quite a bit, there are certainly the two extremes, builders who rely on instinct alone and others who are constantly trying to understand the science. Obviously, most people fall somewhere in the middle. Most of what I have been able to grasp has been from Alan Carruth who is always working to enlarge our understanding of what is happening with the guitar. In my opinion other stringed instruments have had more attention paid to what is going on. I think it is because they are more more rigid in what constitutes an exceptional instrument. That, coupled with the fact that the shapes are more standardized, means that it is easier to observe patterns in great instruments. Steel stringed acoustic guitars generally all sound a bit different and are shaped differently. That makes everything a bit harder in terms of the science but makes it more enjoyable for the creative minded builder.

[Gimar Yestra]

The way I see voicing the top and back at this moment is:

By tapping the top you get and idea of the sound frequencies the top can produce, I guess you want the back to be able to reflect the same-ish range of sound flavours. I dont know if this is correct, im sure there is much more to it. I'm very unexperienced with building acoustic guitars, and I'm pretty sure my philosophies will change alot of times over the next years or builds.

I guess in the end It's about developing a feel or intuition about the wood your working, just like a cook might cook his steak medium rare by his intuition, rather than calculating the mass of the steak and how long it should cook to get a medium rare steak...

weird comparison but it makes sence to me, though I could be miles off.

[Bruce Sexauer]

To me all of this kind of thinking is complete nonsense. For a time I avoided saying so because I didn't want to undermine my credibility, but at this point enough people have experienced my work that I am not worried about it. Some builders (and some customers) need to have a framework to hang their rationalizations on, and if that's what it takes, more power to them. I do not mean to be harsh (as it sound even to me), but rather that it is a sort of dogma as religions tend to develop; some folks need the dogma, others do not. I do not believe the human intellect is powerful enough to cross the gap between engineering functionality and true greatness, but that it can only consistently be accomplished through experience and intuitively artistry. I do believe that any input or analytic device is useful as it adds to "experience", so these exercises are not actually a waste of time, merely misunderstood. This is a BIG IMO, of course, and entirely controversial. I offer my own body of work as an example as I have never been much for measurement. If you have not actually played one of my guitars, please do not judge my words as I may appear (to you) to lack credibility. Thanks for that, please.

I had a look at this topic because you don't see much written about the relationship between the top and the back as coupled by the air column, and this is actually a huge consideration, IMO. There are two divergent schools of thought on it, and the other one views the back very differently than my school, thinking of it primarily as a reflector. Different religion, sort of! And just like religion, it is questionable whether the belief system effects the results, or whether we are all on the same field, and some merely have the right and others the wrong idea. It appears that good guitars can be made both ways, though I don't see how or why. Judged by my standards, great guitars (in my experience) come from only one school. I find these ideas/dichotomies fascinating.

[gitnoob]

I don't want to misrepresent Somogyi, but I get the sense he agrees with you in a way.

He builds a mental framework, and he uses a lot of engineering analogies. The framework does seem to help guide him in terms of design decisions -- where the bridge should go, how the braces should be shaped, etc. But ultimately he appears to go by an intuition he's developed over time, and he keeps track of what worked well and what didn't.....

[Jeff M]

The only thing I know is the top and back weigh more than the air.

[Bruce Sexauer]

At 14 pounds per square inch the air actually weights quite a bit!