Do you build your tops to a certain pitch or body resonance?

[Dofpic]

I was curious when you build your guitars do you try and get the guitar to a certain pitch without strings when you tap on it? I was playing around with my four instruments. I tapped the top with my finger and damped the strings and these are the pitches i got. One was at low F. One was between low e and f. one was f# and one was g. What does this mean? I once heard with a player say they like guitars with a body resonance around e flat or d or f. does this mean at a certain body resonance that you will get more or less overtones, certain notes on the fretboard will stand out more etc. Also am I doing this correct or is there a correct way to get the right pitch?

[erictjie]

i heard that for flamenco blanca the desire tapping sound is F#. i am interested to know what luthiers will say too

[James Lister]

I tapped the top with my finger and damped the strings and these are the pitches i got. One was at low F. One was between low e and f. one was f# and one was g. What does this mean?

It means you've built four guitars with different body resonances.

There isn't any one "ideal" body resonance for classical guitars, and there are great guitars made with body resonances anywhere from below low E up to G#. The body resonance does perhaps determine the overall character of the guitar - if you like deep, warm basses, you're likely to prefer a guitar with a body resonance below F#, but if you like bright, strong trebles, you'll probably prefer something higher, but there are no absolutes.

As a maker, I think it's important to work out what sound you're looking for, and then try to be fairly consistent in how you build (at least for some time), and the body resonance is one measure of your consistency. If it's varying by more than a tone from guitar to guitar, then you're not really there yet. Of course, if you don't know yet what you're looking for tonally, then you probably need to keep changing things around for a while to see what happens.

James

[Alan Carruth]

James is right. FWIW, what you heard was mostly an _air_ resonance, although the top gets into it as well. The low-pitched 'Helmholtz' type mode tends to mask the 'top' resonance that is usually about an octave higher. Blocking the soundhole as you tap on the bridge can help you hear the top resonance.

[Paul Micheletti]

I visited a long-time steel string builder who showed me his database of builds. I won't name him as I have not gotten permission from him to do so.

He measured the top resonance, the back resonance, and the air chamber resonance on every guitar he made. I have no idea what methods he used to do so, but they were logged by frequency in his database. Then he compared these resonances against which guitars sounded best. He found that the ones that sounded best had the top/back/air resonances far away from each other on the scale. If any of these resonances were within a half-step of another one, the interaction caused issues with the overall sound quality. So he varied his bracing goals to create this dispersion of resonances for each body size he created.

I have no idea whether that also holds up for classicals, but it was an interesting observation.

[Alexandru Marian]

On most guitars the air and top are exactly 1 octave, or a slightly bit more, apart. This doesn't seem to affect them adversely. Putting more distance is difficult. The tornavoz is the most effective design for this. Another version is the later Hauser I design, which imo relies on the perfectly flat top to get a lower air resonance than usual. I noticed that some later Romanillos guitars are 1 octave and a maybe a full step apart, and this is probably helped by the low stiffness harmonic and back braces. Compared to the Hauser, Romanillos' are stiffer, with the top being around G# rather than F#. One important aspect here is the smaller plantilla and wider linings.

[Jeff Highland]

I tune to target frequencies rather than scale notes for the Air, Top, and Back frequencies.
I use a free program Visual Analyser to measure these
I am using the principals set out in Trevor Gore's books and the seminar I attended in January.They work to avoid wolf notes and promote even response.
Basically
-Do not tune to exact scale notes tune midway between
-Do not stack resonances exactly an octave apart
-Position the back resonance about 4 semitones higher than the top
He also gives guidance on what target frequencies work well and sound "in Genre" based on analysis of "good " guitars from a range of makers

The books are well worth the money
I just retuned a 2 year old Spruce / Rosewood classical taking a whopping 17Hz off the Top resonance using Trevor's Methods.
It made a huge positive difference,

[Alexandru Marian]

4 semitones? Interesting. I thought Trevor likes a very active back. My backs are usually at about 3.5, sometimes full 4 semitones higher than top and I thought I am not working light, although my guitars are certainly prone to some "belly dampening".

[Jeff Highland]

The approach to the back is
-light panel, stiff bracing which can then be tuned by scalloping the centre of the lower bout brace to bring the pitch down to target

[Alexandru Marian]

Cool, sounds familiar

[Dofpic]

I double checked the body resonance and sure enough one guitar is E one is F one is F# and one is G. Which is better? Or can someone describe what this means for overall characteristics of each instrument?

Thanks Really curious your thoughts...

[Alexandru Marian]

It's definitively not about being better. These main resoannces are just a mark of a certain overall constructive balance and say NOTHING about the fine and more important details. Lower resonances indicate a softer build which typically produces a deeper bass, and that's is about all. The easiest way to check is to simply look carefully at the middle of the string while plucking the notes. For the air, check from open 6th string up to the 4th position, and for the soundboard, from the 7th to 12th on the 5th string. As you approach the resonance, the sustain decreases, and same for the vibration amplitude of the string. As the string quickly vibrates it creates a blurred, spindle(?) shape. When the sustain is good, the vibration takes longer and the shape decreases gradually. Near the resonance the response is rougher, short, the shape width is lower. Look carefully, can't miss it.

[Jeff Highland]

As Alexandru says, a softer build (top back and sides) will give a lower air resonance, the size of the soundhole is also a factor, smaller gives a lower resonance, and if you want even lower , then a tornavos will do it

[Ernie Kleinman]

Jeff, I/m still reading the gore/gilet book .What meter or tester do you use to measure the live or inactive back frequency with ??thanks.

[AdamX]

You use a computer with software. Lots of choices.