Acoustic Guitar Construction

Keeping all other measurements equal, what would be the effects of changing the waist as in the following pix?
Or, if the perceived change in sound is nil or very minimal, at what point in changing the waist width would the change be noticeable - that is, non-guitarlike?

Narrowing the waist (LB width and over all length being the same) reduces the size of the LB diaphragm reducing bass output and accentuating treble. ---- Bass drum/snare drum, violin/cello, woofer/tweeter etc. At least that is the way I understand it.

Whoa.....way too technical for this ol'boy

Somogyi explains this in one of his book. He says the width of the waist determines the width of the main vibrating part of the top. That is why dreads are loud in general. (he says)
To me that sounds reasonable.
Herman

I agree with Ken. Perhaps I can make it a little less technical.

1. The narrower the waist the smaller that area beneath the sound hole, the "money area" that does most of the air moving in long amplitude (bass note favorable) mode. This applies whether X-braced or ladder braced.

2. In X-braced tops, the narrow waist constricts the triangulation in the region just below the sound hole, creating smaller areas of easy movement. Small areas respond more vigorously to treble energy. There are adjustments to the angle that can be made, but all things being equal, there are more options with the looser waist because there is more room to move the legs of the X around when creating space between the brace and the side.

3. When I compare my Guild F-50 Jumbo (17 inch lower bout, narrow waisted) to a 16 inch David Russell Young dread, I find the volume to be about the same. Both are somewhat over built, so that is equal. (The F-50 has a very heavy laminated back.) The difference appears to be the extra interior volume removed by the narrow waist offsets some of the extra volume acquired in the larger lower bout and the extra tightness induced by the closeness of the upper legs of the X to the sides of the F-50. The F-50s heavy, maple back tends to dampen while the DRY rosewood enhances sustain. Sustain enhances the perception of loudness and bass. The dampened sound comes off, on the other hand, as a little more crisp, remotely similar to that of a Maccaferri gypsy instrument.

My F-50R, with conventional rosewood back and sides, as well as being lighter built, sounds more like the DRY. (This F-50R is a real peach of a git.) The pinching at the waist seems to more or less equalize the effect of the wider lower bout but the lighter build allows the bass to benefit more from the larger interior volume.

The playability advantage of constricted waists is they stabilize the instrument when you play it in your lap, especially if you are not built like an NFL linebacker. I think it would be very difficult to manage a 17 inch dread on an average guy's lap. When I talked with the local Guitar Center manager, he said guys complain about the slip and slide effect of dreads, even though that's what they mostly buy. Girls, he said, gravitate to OM type waists. They want to be in control of the instrument they are playing - a wise attitude, I think.

The classical Spanish tradition appears to have determined pretty much the ideal configuration for the kind of music they prefer. Steel string configuration is a much less defined field of play, just as the range of music played on our instruments is much less restricted, as is the way we play it.

The key to it as far as I can tell is found in the geometry of triangles and the relative size of the areas in the top that are free to move. Then comes the indefinables. Why are some 14.5 inch guitars just as loud as 17 inchers? I can't figure it out.

Maybe someone can tell me if there is a correlation between waist narrowness and 'quick response'?

I'll bite Dave. It is speculation, though. The narrower waist "tightens" the top, requiring it to dump its energy faster. Plus the somewhat lightened top can start up easier than the heavier one. It also may be influenced by the configuration the narrower space forces upon the upper legs of the brace.

But I have to imagine the difference would be small and easily over come by other factors. A lot of what goes on seems like mystery more than known science to me. Factories typically build to thickness specifications and at a givven thickness, the actual stiffness and weight characteristics of otherwise identical pieces of wood can vary widely.

Not much help from me I guess. I've always been better at living with mystery than faith.