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This is a question about my first build. I planned to make the body depth of this OM 4 3/8" at the tail and 3 3/8" at the heel. After a two week interruption I forgot that I was going to taper down the heel side a little more. At this point, the kerfing is on, side braces are on and the mortices for the bracing are cut. The body is ready to assemble and what I will have with top and back on is 4 3/8" at the tail and 3 9/16" at the heel.

What will a deeper body depth at the neck do for sound? Voicing is complete so I hesitate to mess with the top at this point. Maybe it's just fine. Thoughts?

It is fine . 5mm extra depth at the tail would make for an insignificant sound change, and this should be your least important issue right now.

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Build log

Thanks Alexandru. Glad to know that among the multitude of issues I am encountering this shouldn't be one of them.

Is there a desired drop in depth from tail to heel for optimum tone? Resonance? I notice most depth changes are around 1". I guess this has got me wondering how this relationship would affect sound production. If one were to make the two depths the same how would that differ from a body that had a drop of 1 1/2" from tail to heel?

I'm trying to recall where I heard/read it, but I think the taper is primarily a player comfort consideration, and not so much about tone/sound.

Am I off base here?

Edit:
Come to think about it, I think it was something I was reading about Linda Manzer's wedge, so maybe that wouldn't apply here...

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Jim Hansen

We had a couple threads on the subject here recently, and I most people guess it is for comfort. Pretty hard to imagine a sonic reason to it. As for the air volume change made by 1/5inch" at the heel, it is really not much at all. You would probably need to add or take some more than that, and over the entire surface, to really make a change. Just a guess.

Alan has posted about another builder who made an extra deep guitar then kept rebuilding it shallower but I don't recall the results.

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Build log

I recently asked some questions about this: this is what I have found.

Typically across many makers and many models, the typical taper is 20 - 22% reduction in depth towards the neck heel.

Though as many have said, this appears to be more based on comfortability in play and aesthetics, there is some relationship to the total box volume and guitars ability to move that volume. Efficient guitars will move more. That is why some smaller volume guitars will sound "louder" than larger bodied guitars in that they can more efficiently exchange their air volumes. Thinking this through - there should be a close match of the body volume of the guitar to the prescribed mechanical input - or the excitation of the top. If the excitation force is too small in comparison to the body, the resulting motion may be too small (i.e.) a weak response - insensitivity. If it is too large for the body, you could 'overdrive' the guitar and invoke awkward resonances - saturating if you will. All that being said, though I am sure it makes [i]some[i] difference, I do not believe it will make a significant change - considering the fact that when building to rough standard specs, you are already 95% of the way there.

2 Cents

Stephen

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www.ziegenfussguitars.com

Thanks for the responses. It seems I'll be ok as is. I'm at about 19% change in depth.

It seems logical that too much of a change from the standard would affect response. If this were the case and more air needed to be driven through a larger box, other factors would most likely come into play (aggressiveness of the player, bracing pattern, thickness of top) and so on. It seems fairly complicated.

I wonder if there are other considerations that one could use to compensate for larger or smaller air volume in order to acheive a certain type of sound. It doesn't seem that I'm in this category on mine, but I'm curious for the sake of understanding the mechanics of sound generation

Alexandru Marian wrote:
"Alan has posted about another builder who made an extra deep guitar then kept rebuilding it shallower but I don't recall the results."

Fred Dickens built a classical that was, iirc, about 6" deep, and then cut the sides down by an inch or so at a time until he got down the 3" or so. He found that the air resonant pitch only changed by 7%. The reasons are complicated, but, at any rate, he was not looking at taper.

Zigenfuss wrote:
"Though as many have said, this appears to be more based on comfortability in play and aesthetics, there is some relationship to the total box volume and guitars ability to move that volume. Efficient guitars will move more. That is why some smaller volume guitars will sound "louder" than larger bodied guitars in that they can more efficiently exchange their air volumes. "

I tend to think of it in terms of the relationship of top area to body volume. For a given size top and amount of top motion, you'll get more of a pressure change in a shallow boax than a deep one. This will tend to pump more air in and out of the hole. The 'main air' resonance peak becomes taller, and looks 'peakier' in a spectrum plot, which tends to make the guitar sound a bit 'sharper', or 'clearer', and probably 'louder'.

One reason small guitar can be louder than big ones is that as you increase the span of the top you have to beef it up to retain enough stiffness to resist bridge torque. Weight tends to go up faster than top area, as it turns out, so with more mass for the string to move, and not that much more top to move air, the guitar is not as loud. Smaller guitars tend also to be more 'treble balanced', and your ears are generally more sensitive to high frequencies, so that even if the actual power output is the same, the small guitar would tend to sound louder.

It is very interesting trying to quantitatively identify changes in parameters with qualitative phrases like "louder".

Alan,
if we I am comparing two identical top plate areas with the same excitation forces: the only difference being the volume of the boxes that they are driving, wouldn't the pressure response technically be the same (referenced from the plate)? In its fundamentals, we are just looking at a force over an area. I feel like I understand what I am asking here, but it may not be coming across that way...There has to be some additional affect going on - i.e. the compressibility of the fluid - and wall effects. Smaller total volumes translate into greater particle excitation overall compared to larger volumes for a given input.

Stephen

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www.ziegenfussguitars.com

Thanks Alexandru, Jim and Zeigenfuss. And thanks Alan for such a clear explaination. This is all very helpful. I am feeling new neural pathways being forged here!! New insights into sound production are exactlly what I am wanting. Thanks again.

Ziegenfuss wrote:
"It is very interesting trying to quantitatively identify changes in parameters with qualitative phrases like "louder". "

Indeed, and I plead guilty to sloppy use of language. What I should have said was that smaller guitars can be more _powerful_ than larger ones. The fact that they often sound 'louder' is sort of fortuitous.

"If I am comparing two identical top plate areas with the same excitation forces: the only difference being the volume of the boxes that they are driving, wouldn't the pressure response technically be the same (referenced from the plate)? "

If the plate was acting in isolation, yes, but it's not. The low range response of the guitar comes from a 'bass reflex couple' between the air in the box and the top plate. Top plate excursion drives pressure changes inside the box which move air in and out through the soundhole. The frequencies of the resulting spectral peaks depend on the pitches of the 'Helmholtz' resonant mode, and the 'main top' mode, and by how strongly they are coupled. Generally speaking, the 'Hemlholtz' mode on a guitar will be a bit lower in pitch than the 'main top' mode, if you could look at both of them in isolation. When the guitar is together, they are coupled by the air pressure changes in the box, and this shifts the pitches. The 'air' pitch is shifted downward, and the 'top' pitch upward. The stronger the coupling, the further apart the final pitches will be.

This is why making the box deeper usually does not make the 'main air' resonant pitch lower. The 'Helmholtz' pitch of the deeper box is lower to begin with, but since there is less pressure change for a given amount of top motion, the coupling is not as great, so there is less of a pitch shift. Of course, this also means that the 'main top' pitch is shifted upward less, too, so that you'd expect the see a lower 'main top' pitch on a deeper box, all else equal. In practice you might not see much change, since the top, being heavier, shifts pitch less than the air. However, I did see that happen once when I cut a guitar down.

In terms of power output, it's hard to see why box depth should effect that over the broad band. The frequencies of the peaks change, and it's likely that the relative, or even absolute, heights of the peaks changes, too. What may or may not change is the area under the curve, the 'total available horsepower', so to speak. It's hard to see why it would, in general, but I would not be surprised if one setup turned out to be more efficient than another, and thus produced more power overall.