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I used an air compressor to blow air over the sound hole of my Torres type CG and got a reading of D. Is this an accurate method of getting the body resonance?

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A higher purpose for wood.
Rich Smith
Issaquah, WA

Alan Carruth where art thou?

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A higher purpose for wood.
Rich Smith
Issaquah, WA

Since no-one else has replied, I'll post my humble opinion...

The Helmholtz air mode should be the most distinct of the body modes, so I guess whatever you hear when you blow across the soundhole will most likely be the Helmholtz. But have you tried humming in the soundhole? I think that's a very easy way of finding air modes. There will be a very noticeable rise in volume when you hit the pitch of the Helmholtz mode.

D doesn't sound too good, I think. Supposedly most guitars are from G sharp to F sharp, and some wonder guitars might be as low as E I heard, but a low D?

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Build log

Sondre - good suggestion. When I hum in the sound hole it really responds to G# which pleases me! Thanks

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A higher purpose for wood.
Rich Smith
Issaquah, WA

Busy trying to finish up stuff for Montreal....

Which D?

There are several air resonances that do radiate from the soundhole, and any of them could be driven by the blast from the compressor. The low frequency stuff seems to work better with high volume/low pressure.

I usually use my signal generator and speaker. I set the box up on foam blocks, and put my dB meter on the top where the fingerboard extension will be, facing the hole. Then I jack up the siggen a little, and start looking for the frequency that makes the needle on the meter jump.

any time you're looking for that air mode, make sure you don't get the driver too close to the hole, as that will drop the pitch.

Thanks for the tip Alan!

_________________
A higher purpose for wood.
Rich Smith
Issaquah, WA

just out of curiosity , would you rather have a dread at D#2 or E 2? if you could thanks jody

Jody:
Almost every Dread I've ever tested has had the 'main air' (Helmholtz-type) mode at around G=97.9Hz. I've never seen one that had that mode anywhere near any D. How are you testing it?

Thanks Alan, I am using the strobo soft, tapping the top softly , holding the sound hole about 12-18 inches from the micro phone , and getting a fairly consistant reading at just about 78.8. That is after opening the sound hole to about 4 1/4". it was just above D2 when I started. now I am not sure if I should try to bring it back to the D# 2 proper ,or " go for it " and try opening it up to E 2. Jody

Jody, wouldn't tapping the top give you the resonant frequency of the soundboard instead of the main air resonance? I ask as I'm trying to learn myself.

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Formerly known as Adaboy.......

no, on the assembled body, you need to block the sound hole to get the resonance of the top. Jody

79 Hz is much lower than I think I've ever seen the 'main air' resonance on a normal guitar. The only thing I can think of that might be that low is the so-called 'neck mode'. How are you holding the guitar?

The only way to hear the 'real Helmholtz' resonance would be to encase the whole guitar is something like sand, with only the soundhole open. As soon as you get any motion in the walls you're seeing some sort of 'bass reflex' behavior, and the Helmholtz-type resonant pitch is shifted down by some amount. According the Rossing a Dread body in sand would have a 'Helmholtz' resonance up near 125 Hz (B or so), iirc. It would take pretty compliant walls to drop it to D.

Alan , I am holding the guitar by the neck , near the middle,to damp the stringsl.with the strings off the guitar , holding the guitar by the neck , I still get the same resonance, within a few hertz. jody

Alan ,to be more precise, I grasp the neck, near the center , with the centerline of the back of the neck crossing my palm with my four fingers perpendicular, I then wrap my four fingers around the treble side of the neck , and my thumb around the bass side , allowing me to hold the guitar firmly , and damp the strings , leaving my right hand free to tap the top. Thanks Jody

You could be hearing the so-called 'neck mode', which is actually the lowest 'bar-type' mode of the whole body. The nodes for that are actually normally closer the first fret, and across the wide part of the lower bout, but if it's active enough you might still hear it even holding the neck in the middle. The node line at the nut also moves down the neck a bit when the machines aren't on, and the pitch of the mode is higher without them as well, since they add a lot of mass in theplace that has the most motion. This mode does usually come in lower than the 'main air' resonant mode, but it's not often paid much attention, since it can't put out much sound by itself.

You can check this a couple of different ways. The obvious one is to hold the guitar up in the same way, but with your hand nearer to the nut. If what you're hearing now is the 'neck mode', you should hear the same pitch, but it will most likely be clearer.

Another check is to block off the soundhole by taping a piece of cardboard over it: that will eliminate the 'main air' mode, so if you hear the 79 Hz pitch with the hole blocked, you know it's not the air mode.

Alan, these are the resonances i can measure,with an open sound hole lightly tapping the top several times ,i get a measurement rough average of 78.81 , the back gives me , 145.5. with the sound hole blocked, tapping the same spot I get, of the top 140.75-141.67, the back 141.57-142.22.
to measure the "neck mode" ( or what I think is the neck mode) I hold the guitar at the nut , letting it settle to be cradled or lightly pinched and hanging between the crook of my thumb and index finger, I then wrap my fingers lightly around the neck with just enough pressure to damp the strings. I then rap the head stock solidly with a felt covered tapping hammer.doing that gives me a reading of 52.7- 53.60. thanks Jody

So it would seem that you do have a 'main ar' resonance around 79 Hz. The seemingly close agreement between the 'main top' and 'main back' modes would argue that they are coupling in very effectively in the 'bass reflex action', and that would tend to push the real Helmholtz resonant pitch down. I'm still surprised it's that far down, though. Rossing found top and back resonances on some Martins that he tested at around 165, so your plate modes are on the low side, which would also tend to drop the main air pitch. Thi sis all so much easier when I can actually have the guitar on the bench, and look a Chladni patterns!

Alan, I know you are a busy man, I would be more than happy to bring the guitar to you, so you can " disect" the modes , and qualify or dispute the anomaly. assuming I am correct in remembering you are located somewhere in new england. Jody

Jody:
I'm in Newport, NH; about 1/3 of the way up the western side of the state, and near Mt. Sunapee and Hanover. I'd be happy to entertain a visit AFTER the Montreal show! Give me a couple of days notice so I can move and leave no forwarding address......

I use adobe audition(cooledit) to analyze the taptone clip. Place the mic near the soundhole or soundboard to get the main air resonance.

I've read that Julian Bream actually encourages makers to make the helmholtz resonance lower than the 6th string. The only Liam Romanillos guitar in Taiwan is said to have the main air at E2. The owner, who has joined the Romanillos class for two or three times, has finished a Romanillos copy several years ago with the main top at E and main air at D or D#, IIRC. That guitar has a tall 18-hole ebony bridge and sounds quite muted.

Sure Alan, I can wait, it is probably at least a 5 to six hour drive for me ,so I will do some sight seeing along the way( or along the way back ) and stop overnight somewhere along the way . if I had your zip code I could do a mapquest.It would be nice to have someone that knows what they are doing evaluate my work, even if it is brutal honesty ! !
CW right now my main air is just above D# 2 that is after raising it from just above D2. I tried closing it down to D proper , but that definatly muted ( or muffled) , and inhibited the sound, at least for the player. It seemed to open up a bit after raising the pitch .it has a nice smooth sound now, for a mahogany dread the bass is fairly strong, the trebles are cutting, and I am getting ( what I think ) is a nice chorus, from the chords.I am just not sure if I should try going to the E2 proper. Jody

There's a problem with having the 'main air' mode too low, IMO. If you look at the spectrum plot the 'main air' peak will be the lowest frequency one, and there's usually a pretty big 'flat spot' between that and the 'main top' peak up around the open G pitch (usually) or higher. This means that the notes between these peaks are going to be somewhat low in energy in the fundamental, and can lack 'fullness'.

If you have the 'main air' peak down around E=82.4, or lower, that gives that lowest note a nice full sound, but you start to lose that solid timbre as soon as you play anything higher pitched, and don't really get it back for a while. If the 'main air' peak is around G or so the low E might lack fundamental, but you're gaining a you go up from there, and don't really start to lose it until you hit A or B, given the breadth of that peak on a normal box. This cuts down on the gap.

Another way to look at it is simply to ask why you would want a resonant peak at such a low pitch that you don't even get to use half of it?

The best way to extend the reach of that peak, both up and down in pitch, is to 'couple' it with another mode. The other low-pitched mode on the guitar is the 'neck' mode that I spoke of earlier. If you can get that one up as high as the 'main air' pitch it will couple, and you'll end up with a broad double peak in the low range. The height of the resonant peak in the output will be lower, but the extra width in pitch means that you actually gain in 'available horsepower'. The lower peak height also translates into less liklihood of a a 'wolf' on the 'main air' pitch. Guitars with this sort of matching often have a 'rich' or 'dark' low end sound. It's hard to do, but worth the try.

so Alan , if I z poxy the back , and egg wash the top, is it going to mess with your readings? > thanks Jody

Jody:
We'll work with it as is.