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I've read one of Roger Siminoff's books where he discusses tuning the sound chamber. As I understand it, sound chambers with the same dimensions all have the same natural frequency. So all dreadnaughts, or all OMs, or all 000s built with the same body shape and depth would have the same natural frequency. So it seems all dreadnaught's built with a specific body shape, depth and sound hole size would have a sound chamber with a similar natural frequency.

If I understood Siminoff correctly, you can change the frequency the sound chamber produces by altering the size/location of the soundhole. So if we built the same dreadnaught guitar described above and made the sound hole smaller, the frequency of the sound chamber would lower......maybe down to an E for example.

Here is where I need a little feedback. If I understand correctly.....the natural frequency of the sound chamber does not change when the sounhole size is changed. The note it produces changes, but the natural frequency of the sound chamber does not.....correct? My understanding of Siminoff is that if you match/tune the note the sound hole produces to the natural frequency of the sound chamber......then the guitar becomes much louder. Correct?

Assuming my last statement is true, is the soundhole size on most guitar designs correctly sized to match the natural frequency of the sound chamber? For example, is the diameter of the sound hole on a Matin OM "tuned" to match the natural frequency of the OM sound chamber? The Norman Blake model has a slightly smaller sound hole.......so is it not tuned or is the normal OM not tuned? How much did changing the size of the sound hole affect the sound of an NB compared to a standard OM? Another example, the OLF plans for an OM show a deeper body than a standard Martin OM (best I understand it anyhow) so was any thought given to changing the sound hole size?

I guess you can see where I'm going with this........how much difference is made by changing the size of the sound hole? Has anyone tuned the sound hole size to match the natural frequency of the sound chamber? How much difference did it make? (Siminoff claims a large increase in volume). Was the "tuned" sound hole size much different than the typical sizes in use?

Appreciate your thoughts and experiences.

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Formerly known as Adaboy.......

Daryl, you've got quite a few thoughts there, but I'll just respond with my opinion that your whole premise of a body shape/size having a natural frequency is invalid. The guitar is a complicated system and the main resonant frequency is affected by many many things. A guitar built to a certain design will have a main resonant frequency that is predictable, but change any aspect of the design (body shape, soundhole size location, plate thickness, bracing layout, etc) and you'll get a new resonant frequency.

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David White, Toronto

"All my favourite singers can't sing."

What Siminoff says would be true of thin walled _rigid_ boxes with soundholes of the same size in the same place. This is what I call the 'real Helmholtz resonance'. The thing is that any guitar you see departs very far from this ideal situation, and so does the pitch of the air resonance.

What happens on real guitars is that the movement of the top, and even the back and sides, produces what's called a 'bass reflex couple'. The air pressure changes caused by flow in and out of the hole push on the top (at least), and the top, of course, pushes air in and out of the soundhole. The top and the air flow have to work together, coming to a sort of compromise in terms of frequency.

The 'main air' resonant pitch you see on a real guitar will be lower than the 'real Helmholtz' resonant pitch, and the 'main top' mode frequency will be higher than it would be otherwise. On one guitar I made closing off the soundhole dropped the 'main top' resonant pitch by about 20 Hz, from about 190 to 170; something of a measure of the influence of air flow through the hole. It works about as strongly with the Helmholtz resonance, which is pushed upward by the interaction.

All of this is pretty well understood, and effects mostly the low-range sound. There are lots of other air resonant frequencies in the box, most of which are pretty well set by the size and shape, but with some influence from motion of the walls as well. As you go up in frequency, things get more and more complicated.

A good article to look up is 'Fundamentals of Air Resonance' by William Allen. It was in American Lutherie #1, and is reprinted in the first of the 'Big Red Books' from the GAL.

Thanks for the replies Al and David. And thanks for describing the coupling affect. Are the "Big Red Books" available from GAL? Can you find them online?

Al, it's interesting that you mention a guitar with a resonant frequency of 190 Hz. I read another poster stating that the guitars he had built had a resonant frequencies between G and F# which is close to 190 Hz. Was this on an OM sized guitar?

What are comon frequency of the air chamber on specific sized guitars? Would it not be lower on a larger guitar than a smaller guitar even with the coupling affect? Has anyone played with the size of the sound hole to shape the tone on a guitar? If so, does a small change have a large or small effect?

I'm curious about this topic so I appreciate you input.

PS Now that I think about it, I guess I could test this on a finished guitar. I could build an insert that fits the sound hole with a smaller diameter hole and see how it changes the sound of the guitar. The only variable would be the increase in weight of the soundboard. Maybe I could use foam or other lightweight material that wouldn't leak air. Maybe I should buy the Peterson Strobosoft tuning software beforehand to measure any changes......but it wouldn't measure volume as far as I know.

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Formerly known as Adaboy.......

You can order directly from GAL. Better yet, join the GAL, get a discount on the 'Big Red Book', and get the quarterly magazine American Lutherie. GAL can be a bit slow if you're used to Amazon and so on but they are good folks to deal with; be a bit patient.

http://www.luth.org/books/book.htm

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Steve Smith
"Music is what feelings sound like"

Darryl, I too have been thinking about how the size of the sound hole affects tone on an acoustic guitar. I also think that the top plate and braceing would have to support the hole size as well.And body depth might also come into play.The sound hole size would surely affect the amount of volume being pumped in and out of the box from the plate and affect the transfer rate of that air and mabey some tone.

Strobo soft 2 is a good package and will give a value to what you hear, all that remains is learning what are the values to strive for. I have read lots of conflicting things about tap tuning guitar parts and the completed box. I have had great fun with strobosoft on my laptop but realy don't know if it has made much difference in the tone or volume, as I have yet to finish 1st guitar I used it to evaluate.

Fred

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Fred Tellier
http://www.fetellierguitars.com
Facebook page http://www.facebook.com/pages/FE-Tellier-Guitars/163451547003866

Changing the sound hole size is indeed an easy way to get a good feel for this. I use the sort of card faced foam board you get at office supply stores for making displays. It's light and reasonably rigid, and if you cut the plug carefully it will friction fit without marking the top or finish. Use a layer of card stock cut a bit large to keep it from slipping into the hole: these things are harder to get out than picks!

Most guitars seem to end up with the 'main air' resonance between about F# and A, with G or G# being the most common. Larger boxes tend to have larger soundholes: it helps keep the volume up and also maintains the pitch of the air mode.

An easy way to find the 'main air' pitch is to use the strings. Lay the guitar down in your lap, and pinch the low E string between your thumb and finger near the nut. When you pluck it you should get a sort of tuned 'thunk'. Slide the pinch point up and down as you keep plucking, and find the loudest sound. That's the 'main air' pitch. One high scool student I told about that trick won a science fair prize by going through about four big NYC music stores, and noting down the air pitches of all the guitars one Saturday.

Does anyone think the "main air" pitch is a large contributor to the great bass "cut" you hear on some of the old Martins.......the sound bluegrass folks really like? Within reason, wouldn't lower be better for this sound/tone?

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Formerly known as Adaboy.......

Daryl Young asked:
"Does anyone think the "main air" pitch is a large contributor to the great bass "cut" you hear on some of the old Martins.......the sound bluegrass folks really like? "

I'm sure it is.

If you look at the spectrum chart of a guitar there will almost always be two main peaks in the low range; associated with the 'main air' and 'main top' resonant modes. Usually the low 'main air' peak is lower than the 'main top', often much lower, in part because there is some phase cancellation between the air and top movement at the low pitch. Air is flowing out of the soundhole at the same time as the top is moving inward, and some of the air just goes and fills in the space the top is vacating. The relative phase of the air flow changes for the 'main top' peak, and that's one reason it tends to be taller. The guitars with the tall, broad 'main air' peaks, sometimes nearly as big as the 'main top' ones, are those Dreads that the Bluegrass guys all love. There's more to it than that, I think, but moving lots of air at a low pitch can't hurt for that application.

I do believe it's possible to get that 'main air' mode too low in pitch, though. Notes near the peak of that mode tend to be louder, and have more fundamental, of course. If you put the 'main air' mode down around the low open E, then that note will be strong, but as you go up from there they will tend to get weaker. There's usually a pretty big dip in the output between the 'main top' and 'main air' pitches, a gap of early an octave usually, where the overtones have to do most of the heavy lifting, and making the 'main air' pitch too low leaves you with a big gap. I suppose you could make it up by making the top looser, but that has it's own costs, especially in the trebles. Siting the 'main air' pitch at around G means that all the notes between low E and A get some help in most cases. Once again, the old boys knew what they were up to, even if they didn't know it.