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Hesh asked me to enlarge on what I posted in the other thread, so here comes. I'll note that this in my opinion, based on my own experience, reading, and research.
"1) Top density and thickness for volume/loudness."
Plucked strings don't have a lot of horsepower in them; that's why the fiddlers can usually drown us out. If you don't have much horsepower in the engine, the only hope you have for acceleration (= high end) and top speed (= sound power) is in keeping the car light.
All of the sound has to go through the top, so that's the most important part to keep light. The limit on how light it can be is the torque on the bridge; if the top's too thin or too lightly braced to hold that for any length of time, you're fried.
Most of the weight of the top is in the top plate itself. That might weigh 110-150 grams, depending. ALL of the bracing together is typically 30-50 grams, and the bridge will be almost as heavy as the bracing; something like 20-35 grams. The place to look for weight savings is in the top itself first, and then the bracing.
I simplify things by only considering the stiffness of the top wood along the grain. Since the top will 'cold creep' over time it seems to me that the crosswise stiffness, as much as it effects the tone, is not as much use in countering the bridge torque. I _know_ that's not strictly true, but it's a useful simplification and probably won't get me into structural trouble.
Assuming you're talking about a single design, the stiffness of the top plate will depend on the Young's modulus of the wood, and the thickness. Basically, as has been discussed in another thread, all else equal, the stiffness is proportional to the cube of the thickness multiplied by yhr Young's modulus.
Now, it turns out that the Young's modulus (E) along the grain of all of the top woods (seven varieties) I've tested scales with the density. If the wood weighs 300 kg/meter^3 the E value along the grain will be close to 6000 megaPascals, and if it's at 500 kg/m^3, the E value will be close to 17000 mPa. ALL of the top woods fall near the same line, regardless of species, and the scatter is something like 10% plus or minus, for the most part.
The important thing to see is that E scales more or less linearly, while stiffness goes as the cube of thickness. Making a given piece of wood twice as thick makes it eight times as stiff. If you make two tops of different density wood, but make them to the same thickness, you'll find that the one with the lower density will be a bit lighter in weight. It will be thicker, of course, but not enough thicker to add so much weight as to make it heavier. Lighter=easier to push. There may be other benefits as well, but they are harder to demonstrate and more subtle.
This is where I need to say something about bridge weight. Obviously, the bridge has to move if the top is to move, and a heavy bridge will slow things down, no matter how light the top is. There goes your treble response. We could get into a long discussion on 'impedance' here, but I'll try to resist in the interest of covering some other ground. Suffice to say that the higher impedance of a heavy bridge has a lot to do with the added sustain.
"2) Shallow boxes and volume."
So you've got a top=bridge system of a certain weight and stiffness, and you stick it on a box. Given a certain amount of input energy from the strings the top is likely to vibrate with a given amplitude. As it moves 'in' to the box, the pressure inside goes up, and some air is pumped out of the soundhole, and the opposite happens when the top is moving 'out'. The shallower the box the greater the pressure change for a given amount of top motion, and the more air is likely to be pumped in and out at the 'air' resonance pitch.
At this point, we have to talk a little about 'loudness (or volume)' and 'power' and 'projection'. I tend to use 'power' to mean the actual horsepower output, regardless of what the thing sounds like. 'Volume', for me, is how loud the thing sounds close up, and 'projection' is loudness at a distance. 'Power' can be measured, although it's not as easy as you'd like it to be. The other two are subjective, and have very little relationship to the power in the signal. In fact, I've got one acoustics text ('Fundamentals of Musical Acoustics' by Benade) that spends a whole chapter on the different ways of calculating 'loudness' from power and spectral measurements, and ends up concluding that none of them works well in all cases.
One of the many things that seems to correlate with close up 'loudness' is the strength of the lower frequency part of the spectrum. This is the 'bass reflex' range of the guitar, where the 'main top' and 'main air' resonances dominate the output. Sounds below the 'main top' pitch (often around the open G string) tend to radiate in all directions, while higher pitched stuff is more and more directional. A guitar with a strong bass reflex range will usually sound loud close up, and to the player, and this is particularly true if the attack is fast. A shallow box helps with that fast attack, but doesn't (surprisingly) alter the 'main air' pitch much.
'Projection' can benefit from a strong bass reflex, but not a lot. If there is twice as much power in the bass reflex range (3dB stronger: a huge change) the sound won't be much 'louder' close up (because of the way your ears work), but, owing to the inverse square law, it will be audible about 1.4 times as far away. However, good projection seems to have a lot to do with the high frequency content of the signal, between 2000-4000 Hz, because that's where your ears are most sensitive. Sound in that range is coming off the top and out of the hole (mostly) and heading away from the player, often in 'beams' in particular directions. I have seen and played guitars that were barely audible to the player, and absolute cannons thirty feet out in front, becuase of this.
"3) Shifting the sound hole forward for greater volume."
What I said, iirc, was: "making a larger soundhole closer to the neck end". The larger soundhole obviously 'breaths' easier, and tends to give more volume within reason. There does seem to be an 'optimum' size, but I suspect it's a little bigger than the usual. Making the hole larger raises the 'Helmholtz' air resonant pitch, and thus probably the 'main air' part of the bass reflex. You may not want that, as it tends to reduce the 'fullness' of the bass notes (that's why the 'standard' soundhole is not larger). Shifting the hole up toward the neck helps to drop the 'Helmholtz' mode (and loses you some useful frets: it's all a compromise). See Allen's article, 'The Basics of Air Resonance' in American Lutherie #1 or the first Big Red Book. Again, there may be other, more subtle effects, that could add to the 'loudness' as well, but the research remains to be done.
"And anything else related."
This post is long enough already.....
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