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This is something I've been pondering, and I hope my thinking is heading in the right direction please correct me. Initially I though the soundboard moved up and down and pumped the sound out via the sound hole, then I started thinking about f holes etc and am now wondering does the soundboard move up and down and the sound radiate from the surface of it and the sound hole more like a pressure equaliser for the system. Then i started wondering about the sound hole being smaller promoting more bass and thought does the diameter of the sound hole help with coupling the top and back, and tuning the main frequency of the box?

All pondering which I'd like to firm up and premote discussion really

thanks

John

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Formerly JJH

I learn more from my mistakes than my successes

Ervin Somogyi has some good articles on the subject.

"The importance of longitudinal bracing is central to the success of the guitar because the guitar is basically an air pump, and in lutherie we need to concern ourselves with how efficiently the guitar can pump air. All other considerations, such as choice of woods and how pretty they are, must be subordinated to this if you want to make a successful guitar."

http://www.esomogyi.com/principles.html

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Jeremy Douglas

John, this is a great question, and also a huge question. I suspect you aren't getting more responses because those who have pretty good grip on it are thinking, "Sheesh, where do I start?"

I would suggest one good place to start would be some books on the subject of musical acoustics. Arthur Benade's Fundamentals of Musical Acoustics is a good one. Another good source would be every article Alan Carruth has written for American Lutherie, as well as his many contributions on that subject right here on the OLF. Do a "search for author" on him here and start reading. There have been quite a few good discussions on the subject here.

It's a fascinating subject. Have fun delving into it!

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Todd Rose
Ithaca, NY

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

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

Glad it wasn't a dumb question and everyone was thinking doesn't everyone know that

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Formerly JJH

I learn more from my mistakes than my successes

i would imagine (key word, imagine) that the movement of the sound board changes the pressure in side the box. this pressure then escapes the box via the sound hole. this then creates slight changes in pressure in the surround areas and this dominoes ie, it creates a sound wave. the air wants to remain at a constant pressure (think about things getting sucked into vacuums) so surround molecules will move into the space left by the molecules that went to fill in the pace made by the previous ones...

so the size of the sound hole affects how quickly the box and the nearby air can get to an equilibrium pressure. i suppose this is why a slammer sound hole would lower the note of the box, because lower notes have a lower frequency, so the pressure changes needed are slower?

and lowering the volume (asin units cubes) of the box would mean there is less air to pressurise/de-pressurise, so the time taken to equilibrium is less so the period decreases, so the frequency can increase, ie the note will be higher?

got no idea what any of this has to do with the tone or volume (as in musical) of the instrument though

so are my assumptions correct? or did i make an "ass out of you and me" (well, just me really lol)

Thanks

Jonny

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"Anything that happens, happens. Anything that in happening causes something else to happen, causes something else to happen. Anything that in happening happens again, happens again. Though not necessarily in that order." Douglas Adams

I've been asking myself that for the past thirty years or so. The more I look into it, the more complicated the answer gets in some ways.

A good start is the 'bass reflex' action of the box: that accounts for most of the actual power, according to some researchers, and has the most to do with the bass tone. There's loads of literature on this in the speaker design world.

Basically the top moves in and out, changes the pressure of the air in the box, and radiates some sound out in front of the guitar. Air moving in and out of the soundhole also makes sound in the room as it pushes on the top from the inside, so the two things work together.

The 'Helmholtz' air resonant pitch is set up by the size of the box and the size and location of the soundhole: smaller box or larger hole gives a higher Helmholtz pitch. Note, though, that you only get a real Helmholtz resonance with a rigid box, as soon as the walls flex things change.

The top has it's own resonant pitch, determined by the stiffness and mass of the lower bout (mostly). You don't see a 'pure' main top resonant mode on a playable guitar because of the interaction with the air. If you cut out most of the back you would, though.

On an assembled guitar the 'Helmholtz' and 'main top' resonances cooperate to give two peaks in the output spectrum. The lower peak, usually around G on the low E string, is often called the 'main air' resonance, because a lot of the energy in the system is tied up in the air moving in and out of the soundhole. It's at a lower pitch than the pure 'Helmholtz' resonance, and the difference is a measure of how much it's being effected by the flexibility of the walls of the box. The higher pitched peak is called the 'main top' resonance, again, because that's where a lot of the energy is coming from. It's at a higher pitch than the 'main top' resonance of an open-backed box, but, unlike the air resonance situation, we don't have seperate names for the two conditions. I sometimes talk about the 'real' isolated main top resonance as against the one you see on assembled guitars.

At the 'main air' pitch the air is moving out of the soundhole as the top is moving inward. Some of the air just sort of sloshes around to fill in the volume that the top vacates, so there is not as much power output as there might be. Still, despite the 'phase cancellation', this can be a pretty strong peak. At the 'main top' pitch the top is moving outward as the air is coming out of the hole: everything is pushing in the same direction, and this is normally the tallest peak in the spectrum. This peak often comes in around the open G string pitch.

If that was all that was happening the timbre of every note above that open G string would be pretty much the same, and the sound of the guitar would probably be boring. Fortuneately, there are a lot of other resonant modes of the various parts of the guitar too, and each one of them colors the sound in some way. Very few of them are anywhere near as efficient as the 'bass reflex action' though; generally they, too, involve a certain amount of phase cancellation.

As you go up in frequency there get to be mor and more resonancesof different parts of the box and the air inside. When you get up around the pitch of the 12th fret E on the high string it's often difficult to isolate things and say that a particular peak in the spectrum is caused by the way a certain part vibrates. Everything is effecting everthing else. Even at lower pitches the interactions (often called 'coupling') of the various parts can cause things to work in baffling ways. For example, cutting down the height of the sides ought to raise the the Helmholtz pitch, and thus make the 'main air' mode of the completed gutiar come in at ahigher frequency. Butit can happen that the 'main air' pitch remains the same, while the 'main top' pitch is the one that goes up: and you didn't do anything to the top! It's this sort of thing that keeps it interesting.

Another reference if you're interested is Jansson's 'Acoustics for Violin and Guitar Makers', which you can download free from:
http://www.speech.kth.se/music/acviguit4/
Get part1.pdf through part9.pdf
There's also a neat sound recording and analysis program, 'Wavesurfer', at the same price from the same folks. He gives details in part9.

John, I'm really glad you posed this question, and I'll be following the answers carefully. I'm continually amazed by the number of people who think all of the sound comes out through the sound hole. Well, certainly, some of it does, but it's coming directly off the top, sides, back, and in fact every other part of the instrument, too. The myriad ways these parts interrelate to give shape, color, tone, projection, sustain, etc. etc. etc. is really quite fascinating, and it's that limitless potential for experimentation that challenges all builders.
Patrick