Official Luthiers Forum!

What do most people use? Can't imagine using a guitar tuner!

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Pat

I use visual analyser from http://www.sillanumsoft.org/ set up using Gore's technical notes from http://www.goreguitars.com.au/attachments/Technical_note_on_collecting_spectrographic_data_R1.pdf. It is for the PC. There are others but I have been very happy using it.

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http://www.Harvestmoonguitars.com

Sometimes a tuner will pick these things up, but usually they're not clear enough (tightly enough defined in pitch) or sustained enough. It often seems to work better just to rely on hearing, along with some sort of pitch reference. With a bit of practice you can often pick out the 'main air', main top', and 'main back' pitches simply by tapping and listening. Keep trying until you can hum the pitch, and then match that up with a string at some fret. It obviously helps to know how these things work, so that you'll know where to tap and where to listen to get them to work right.

It's also helpful to damp out the ones you're not looking for. Partly this avoids confusion in trying to pick out one of many. An example of this is the 'masking' that occurs when you tap on the top at the bridge. This activates both the 'main air' and 'main top' modes. These are often appear around an octave apart, and the lower pitched 'air' mode masks out the sound of the 'top' resonance. By blocking the sound hole even in part, you reduce the power of the 'air' mode, and make it easier to pick the 'top' mode out.

Such selective damping also cuts down on coupling between the different modes that can displace the pitches. The 'air' and 'top' modes in question are a perfect example of this. They are so tightly coupled that they can be considered as the two halves of the 'bass reflex couple'. When the hole is open the two interact in such a way as to push their pitches further apart. A rigid guitar shaped box with a hole can have an 'air' (Helmholtz) resonance at around 125 Hz, and a backless box might have the 'top' resonance at about 180 Hz. One a real guitar, as has been mentioned, these couple in such a way as to end up more nearly an octave apart, at around 100 and 200 Hz.

Spectrum analysis software can help to see things in more detail; capturing some of the 'higher order' resonances that are very difficult to pick out otherwise. There are pitfalls in this. For one thing, some frequencies are radiated in a very directional way from the body of the guitar. The top 'cross dipole' resonant mode, for example, has one side of the top going 'up' while the other goes 'down', with a null ('node line') down the center line, Since it's usually at a pretty low pitch the sound output tends to be cancelled out along that center line, so if you put your microphone there you may not pick up what can be a strong resonance that can affect the timbre of the sound noticeably. It gets more complicated at higher frequencies.

There's also the question of what the 'real' pitches of these things are. We saw this in the case of the 'bass reflex couple', and, again, as you go up in pitch that sort of coupling becomes more prevalent. At some point, probably between 500-800 Hz or so on most guitars, you enter the 'Twilight Zone' of the 'resonance continuum'. Here there are so many resonances of the various parts of the guitar,and they are so strongly coupled, that it's impossible to separate them out. Just because you hear a sound coming off the top, say, it doesn't mean it's a 'top mode', although the top is certainly involved. Adding some mass, say, to the top at that point will almost certainly change the pitch or activity of the mode, but you might get an even bigger result by adding some mass to the back, or changing the sound hole in some way.

I could go on. I've spent 'way too much time in the past forty years trying to find resonances and figure out what they do. In the end, the simplest ways of finding them often work best, especially for the lower ones. So-
For the 'main air' resonance:
Lay the guitar down on your lap. Pinch the low E string between your thumb and finger down around the 2nd or 3d fret. Pluck it near the bridge. Move the pinch point up and down while you pluck as consistently as you can. The loudest pitch is the 'main air' resonance. The position of the pinch point will be a pretty good indicator, but a tuner will be more exact.

'Main top':
Hold the guitar up by pinching the end of the fingerboard between your thumb on top and your fingers inside the hole. Try to block the hole as much as possible WITHOUT actually touching the top. Tap in the center of the bridge, and listen just above the tapping point if you can.

'Main Back'
Hold the guitar as for the 'main top' mode but tap along the center line of the back. The lowest pitched tone you hear will generally be the 'main back' mode.

Top 'Cross Dipole'
Hold in the same way. Touch the top with a finger along he center line in the lower bout halfway between the bridge and the lower end to damp the 'main top' mode. Tap on the end of the bridge with your thumb. This is hard to hear unless it's a strong mode.

Top 'Long Dipole'
Ditto, except you switch the damping and tapping points. Again, may be hard to hear.

'Neck' mode
Hold the guitar up by pinching the neck up near the nut, with your finger damping the strings. Tap on the back of the headstock, and listen with your ear as close as you can get it to the front of the head without getting string ends stuck in your ear. You should hear a fairly clear low pitch; often C below the low E on steel strings. Sometimes, particularly on Classical guitars, this will be as high in pitch as the 'main air' mode, and if so the two will couple usefully.

Good luck.

Also use Visual Analyser. Just picked up a cheap $3 USB mic from a thrift stor. Just plug it into your computer, VA with the setup Mr Gore explains, your good to go. Oh yea, VA is free.

+1 on the VA / Gore stuff.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Thanks for all the details Alan! I will keep this as a reference and come back to it once I get set up.

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Pat

For the air-through-the-soundhole resonance I wrap the body in a towel to damp the wood resonances, then blow huge puffs of air into the hole as if trying to play a gigantic pop bottle. With a hard enough "P" consonant and a big enough burst of air the thing will make a quiet ghost of a low note. I identify the note using another guitar. Sometimes the note is a little lower than the lowest note of the guitar but can range up to about a fifth higher. I find the main wood resonance exactly as already described in this post. Having found these two notes I play chromatic scales in the lowest octave of the guitar looking for groups of notes that are louder than their neighbors or quieter. It helps to verify that the same pitches are bad on different strings. If the center of the too loud notes are strongly related to both resonances (unisons, octaves, fifths, fourths) I'll consider changing the resonances a little bit, not expecting miracles. All I've ever been able to achieve with this tinkering is to make an already great guitar a little bit more even and free singing. Before I enlarge or shrink the soundhole (smaller = lower resonance) or carving the braces (remove wood lowers resonance within reason) I compare the victim guitar with a known excellent guitar of about the same size, wood types, philosophy of weights of parts. You can't tinker a guitar into acoustic miracles. You can only get as good as other people have been able to get with a similar setup.

One of the best ways to lower the pitch of the 'main air' resonance is often to shave back braces. The back can work with the top pumping air through the sound hole, and the closer it is in pitch to the 'main top' resonance the more effective it is. The 'main air' resonance is lower in pitch than the true Helmholtz mode of a rigid guitar shaped box because of the added effective mass of the moving top, ad adding some more by getting the back into the back drops it even further. Normally backs are quite stiff, and have a higher 'main' resonance than the top , so shaving back braces will strengthen the couple, and drop the 'air' pitch.

It's usually easy to check the effect of lowering a resonance of the top or back that you'd get from shaving braces. A bit of mass added to the plate will have the same effects as shaving a brace or tinning the plate in that area. The good thing about using a wad of poster adhesive to check this is that it's a lot easier to undo. Even if you number the shavings it's hard to get them back in the right places....

Alan:

I appreciate so much the investment you make trying to help some of us understand this stuff. Speaking only for myself, I still don't quite grasp it, but I'll keep trying.