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I have a question that I feel confident that hasnt been asked before.... dont tell me you havent done this either... I know everybody must do it and I'm not completely crazy. well, maybe a little, but not completely

Before the neck is even on a guitar, I have been humming into the sound hole and tracking where the resonance point is. Seems like all of my dreads resonate at about the same point, but a few are off by as much as 3 half steps. What I mean by this, is that I start by humming as low as I can and then at some point the guitar comes alive with vibration when I raise my voice to a certain pitch. If I keep raising my voice, several other points of vibration come alive as well. I call these the "harmonic points". Most of the time, they are pretty consistant with the first vibration point.

Ok, here is the deal.... I dont know how I manage to do it, but the main vibration point for my dreads is 98% of the time a perfect "G" note. (I have actually checked this with a tuner). I also know what to do to alter this vibration point. (several things that I can do).

Have any of you paid attention to this vibration point?? if so, what note does your guitars come alive on?  Do you feel that this may  affect the final tone of your guitars?

_________________
Reguards,

Ken H

That's the Helmholtz frequency of your box. It's determined not by tuning of
the plates, but rather the ratio of the volume of the box to the area of the
soundhole (in it's simplest form anyway). If you're getting results that
consistent it means your building is very consistent dimensionally. Blow
across the soundhole at the right angle and you'll get this same pitch.

Most dreads will end up around G. I'm convinced this underlies a
conspiracy by bluegrass players that led to A being brought up to 440 early
in the last century. This way when the guitarist gets tired they can just angle
their guitar to the wind to hang on a G, then let the mandolin player do all
the work.

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Eschew obfuscation, espouse elucidation.

David is correct. I believe this is what Alan Carruth calls the 'main air' resonance as well.

The resonance is somewhat directly proportional to the size of the soundhole and somewhat indirectly proportional to the volume of the box. In other words, this frequency will increase with a larger soundhole and decrease with a larger volume box.

Actually, one of my friends came over the other night and we had a humming contest. I'm glad nobody saw us doing this, but we were both amazed at the vibration of the Ash guitar.

_________________
Reguards,

Ken H

Ken,
Here’s the other thing about that note that vibrates your box.
If you play that G note, you may notice it does not sustain as well as others. You would think it would be the dominant tone the guitar would produce.
I have heard it called a wolf tone. It has a quick bark but then fades somewhat compared to say an A note.


Wade

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Wade
Nashua, NH
http://www.wadefx.com

It's the 'main air' resonance alright, but that's not exactly the same thing as the 'Helmholtz' resonance.

A real Helmholtz resonator is a spherical bottle with rigid walls and an opening. Suppose a slug of air is moving into the opening, compressing the air inside. That slows down the flow until it stops, but by then, because of the iniertia the air had to begin with, the pressure inside is higher than the pressure outside, so the air starts moving out again. When the pressure on both sides is equal the air in the opening is moving as fast as it can, and has enough inertia to pull some more air out, so the inside ends up being at a lower pressure than the outside. More air gets sucked in, and off we go for another round. The important thing about this is that all of the forces involved, the inertia of the moving air and the pressure inside the bottle, come from the air itself, the bottle doesn't contribute anything except to confine the air.

Some years ago Tom Rossing did an experiment where he made a Martin D-28 (iirc) into a 'real' Helmholtz resonator, by burying it in sand to kill all the wall vibrations. The real Helmholts air mode was up around 125 Hz, I believe, which is a bit higher than A#.

What happens on the guitar is that the air moving in and out of the soundhole pushes on the walls. It takes energy to move them, and that energy comes out of the air flow, slowing it down. Thus the 'main air' resonant frequency on real guitars is lower than the true 'Helmholtz' frequency. How much lower depends on the mass and stiffness of the top, and, to a lesser extent, the back, how much air is being moved, the soundhole size, the way the stock market is moving, the phase of the moon, and the wholesale price of peanuts in Burundi. All the usual variables.....

That's not all, of course. Rossing also checked out what the top of the guitar would do if the air volume was not enclosed, to eliminate the pressure changes pushing on the top. The 'main top' resonant mode, the one that looks like a loudspeaker, was at around 170 Hz, iirc. When the box is closed the air pressure at that frequency will be rising as the top moves 'in' and falling when the top moves 'out' of the box, so the pressure works against the tops motion. That's like making the top stiffer, so the pitch of the 'main top' resonant mode rises. Again, there are a lot of things that effect how much the pitch rises, but it often ends up around G=196 Hz.

Those resonances can suck all the energy right out of a string, and dump it into the air in a hurry. The sound is loud, but the sustain is lost. You don't usually notice the extra power, but the loss of sustain jumps right out.

I know this all sounds a bit esoteric, but it's important to remember that the 'main air' and 'main top' resonances that you hear on a completed box are not what those parts would do if they had their druthers. There are all sorts of ways to fine tune things. For example, making the box shallower _should_ raise the 'main air' pitch, but it often doesn't. Instead what can happen is that the top resonance rises, and you didn't even change the top! It has to do with the strength of the coupling between the top and air, and what happens to the 'real' Helmholtz mode.

One last thing, if you have your face much closer to the hole than about two hole diameters, you're blocking the flow enough to alter the pitch.

I'm am SO glad to hear this. I had no idea what the "main air" resonance was...and was reluctant to ask. I'm guilty of humming myself.

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Eagles may soar, but weasels don't get sucked into jet engines.

[QUOTE=Bill Greene] I'm guilty of humming myself.[/QUOTE]

I knew it!!

 

Thanks for the explanation Alan!  At this point I dont really know what effect that this tone has on the guitars. I guess I should start writing down what note the guitar resonates at and then paying attention to the final sound of the instrument to see if I can place some kind of theory on the tones.

I'm not really blowing into the guitar, but with my face more or less very near the hole I am humming (mouth closed). I can hear the tone get louder and then feel the higher amount of vibration in the whole guitar when it reaches a certain tone. This will usually respond accordingly when the tone is one octave higher, but the vibration and volume are both less than the lower tone. I also seem to get some variables with other tones, but these are also lesser in volume and vibration.

 

_________________
Reguards,

Ken H

Ken Hodges wrote:
" At this point I dont really know what effect that this tone has on the guitars. "

That's alright; neither does anybody else. Or, rather, there's a certain amount of dissagreement. Partly it's simply that it's so hard to change _only_ the 'main air' resonant pitch, so you can't do well controlled experiments.

At least, you can't do that on real guitars. One study, by Howard Wright in 1996, used a computer model of a guitar, which allowed him to change anything he wanted. By virtually playing the virtual guitar, and virtually recording the sound, he could play back pairs of sounds with only one thing changed, and ask a lot of people if they could hear any difference. The conclusion was that the pitch of the 'main air', or any other resonace, really only effected the timbre of the guitar around that paricular note. This is reasonable, but, I'm not fully in agreement with it. For one thing, the model was pretty limited, and, for another, he didn't check a couple of the most interesting cases of different pitch relationships within the guitar structure.

The 'bass reflex couple' between the 'main air' and 'main top' resonant modes does seem to be the thing that produces most of the power output of the guitar. These are the most effective sound producers, so it stands to reason that you'd want them to be pretty strong. The amount they are 'spread out' also seems to make a difference; when the 'main top' resonant peaks is very narrow the tone seems to get 'sharp', 'harsh' or 'cutting'. A broad 'main air' peak tends to give a 'warm', 'full' or 'dark' bass tone. Generally speaking, a smaller hole or a deeper body will tend to give a more 'spread out' main air peak, as well as possibly dropping the pitch.

As far as I'm concerned, having the 'main air' pitch end up too high or too low is a bad idea. When it's much above A=110 the tone starts getting too 'brash' for my taste, and much below F at about 88 Hz, it tends to be 'tubby'. you have to be careful about this stuff, though, there can be other things going on down that low that can make the 'air mode' active at a low pitch, and still leave it sounding good. This stuff gets real complicated!     

Just for fun, try this singing/humming thing with all sorts of "boxes": shower stalls, closets or other small rooms (empty ones work best, esp those with hard walls), various containers... even a deep sink or bathtub will give you resonant frequencies if you sing right over it. You will often find that the sound of your own voice no longer seems to be coming from you, but from all around you.

Speaking of the voice, resonant frequencies like this have a lot to do with how the human voice creates pitch (there's stuff going on with both the vocal "cords" and the shaping of the throat/mouth that determine vocal pitch) as well as different vowel sounds. Singing through a certain size tube, like a metal pipe, can make it very difficult to sing certain pitches, because the resonance(s) of the pipe don't agree with what your vocal "cords" are trying to do. Likewise, if you are having trouble singing in certain areas of your singing range, e.g. your voice is "cracking", it may be that you need to learn to do something different with the shaping/sizing of your throat/mouth.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Ken, what are the several things that you do to alter this pitch? I know that changing the size/location of the soundhole will affect this, but that's not so easy at this point when the box is built.

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David White, Toronto

"All my favourite singers can't sing."

I'm not Ken, but....

Since the 'main air' pitch is really a product of the cooperation between the top, back, and air, changing the top and back can alter it. One way to drop the 'main air' pitch is to shave the back braces, to make it more compliant. In theory, the 'main air' tone would be lowest for a given box geometry when the 'main top' and 'main back' tap tones are the same. The problem with that is you're likely to get one or more of several different sorts of 'wolf tone' whith that relationship. It works quite well to pitch the 'main back' tap tone a semitone above the 'main top' tone. Having the back tap tone lower than the top tap tone works well too, but is not as safe, since the pitch of the 'main top' tone tends to drop over time.

In theory, anything you do to increase the compliance of the box walls would do the same, but in practice the back gets you the most bang for the buck, since it's pretty big, and shaving back braces is not such a structural risk as shaving top braces. In some cases thinning the sides in the 'flat' area just below the waist can be useful.

Believe it or not, in some cases shaving the neck or swapping out machines can alter the 'main air' pitch. There is a 'neck mode' (which actually involves the whole guitar bending, but the neck most of all) that is generally lower in pitch than the 'main air' resonance. However, if the air mode is low, and the neck is stiff, the neck and air modes can work together. This will result in a double peak in the 'main air' part of the guitar's spectrum, with the dip in between being at the actual 'main air' pitch, more or less. This gives a really nice, rich bass, in my experience.

You can find this 'neck mode' pitch by holding the guitar up at a node point, usually around the nut or first fret location, and tapping on the back of the headstock. I listen at the top of the head. If this mode is close to the 'main air' pitch, you might try adding or subrtacting weight from the headstock to see what happens. Remember that swappping the metal buttons out for wood on a set of Schaller or Grover sealed gear units will be about the same as leaving off two machines.

As I say, usually this mode is low, particularly on 14-fret instruments with narrow necks. Classicals with cedro necks that are deeper at the body than the head end are usualy the best candidates for this sort of matching. I did have one archtop with a maple neck that had the mode match, and on one short scale classical I actually had to add weight on the head to get it to come in: that gutiar needed the extra bass, as it had really strong trebles.

BTW, there is a reasonably safe way to try out some of this stuff. You can add weight to things to drop the pitch, rather than shaving them down, and it's reversable. Get some poster adhesive; 'FunTac' and 'BluTac' are a couple of brands. You can stick a wad onto the back or the headstock to hear what would happen if you shaved something to drop the pitch. By moving the weight around you can find the most effective spot to remove wood. If you don't like what you hear with the weight on, there's no damage done. DON'T use modeling clay: that has mineral oil in it that can get into the wood.

Thanks Alan !  I couldnt have said it better myself.

One thing I might note is that the changes I have made that affect this tone were done before I closed the box. It is not something I do afterwards. I doubt that I would even do anything after I close the box just for this resonance unless I could prove that it would affect the final tone in a positive way.

_________________
Reguards,

Ken H