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I’ve noticed that many builders use deeper bodies to modify the internal air volume. What are they trying to achieve with this? Is there any general effects on sound that’s related to bigger or smaller volume?
Is this something you take into consideration when planning a new build? If so, what would be some numbers to strive for for different body sizes, and how do you calculate it?
As usual it’s problematic to try to isolate and pinpoint the effects on sound out of one single design factor. But assuming all other things being equal what do you guys think?

//Oskar

Up to a certain limit it tends to enhance bass response and fundamentals a bit, at the cost of some projection and perhaps overtones. I can be compensated somewhat by enlarging the soundhole.
I am not aware of a way to calculate the depth, or of any magic numbers. Look at Martin's dimensions for their different models, they have a long history and settled on the numbers that presumably work the best for their building style.
Minimum depth I've used is 3 7/8" at the tail. For 0 and 00 sizes I would not exceed 4 3/8" -I mostly use 4 1/4"-, 4 1/2" for 000 -I mostly use 4 3/8"- and 5" for D and over. I've built some large(r) guitars measuring 4" at the tail with excellent results.
It's always easier to get more bass response out of a larger body size. Conversely it's easier to make a small body sound "balanced" and project well. All IMHO, of course…

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Very interesting, Laurent. Thank you.

I've always wondered about body depth and body taper. Martin OM's are 4.25-3.25". Goodall Grand Concerts are 4.25-3.75". What's that 1/2" at the neck block likely giving them?

I agree with pretty much everything Laurent said. An added benefit of working with a shallower body is that you have much more wood to choose from. It is a lot easier to find 4" sides than even 4 1/2 sometimes. I really like the larger bodied guitars I have made that were 4" deep at the tail. They still sounded full but projected very well and very cleanly.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

I agree with Burton. The wider guitars that I have built have had very good bass but also projected well. That is why one of my favorite body styles is the Martin M (0000) . It is a deep as an OM but as wide as a jumbo. It is so easy to make these guitars sound good.

It's not just about air volume, IMO. You need to look at the relationships between air volume, top/back area, and soundhole size and location. All of them work together to shape aspects of the guitar's timbre.

Helmholtz showed how to use the relationship beween the volume of air in a bottle and the size of the neck to figure out the pitch of the air resonance that's named after him. For this to work right, the 'bottle' should be a pretty simple shape, and have _rigid_ walls. The guitar satisfies neither of these conditions very well. Nonetheless, in general, for a given hole size, a large body volume will tend to have a lower 'main air' pitch. For a given box, the larger the hole is the higher the 'main air' pitch will tend to be.

Allen, in his excellent article on air resonances in the first 'Big Red Book' shows that, all else equal, moving the hole from the middle of the box to the upper end will tend to drop the 'main air' pitch by a fairly large amount. This obviously effects the bass end of the range, but also has effects higher up as the hole 'hears' a different set of interior resonances.

You could, if you wanted, make a really shallow Dread and a deep '0' sized box, and, by manipulating the hole size and location, get them both to have the same 'main air' pitch. They'd sound different, though. Basically, for a given amount of top motion there would be a much larger change in inside air pressure on the shallow Dread, simply because the top accounts for a lot more of the surface area of the box than it does on the '0'. This would help the Dread get back some of the 'punch' it would lose from the tiny soundhole you'd need to put in it to keep the 'main air' resonance down. Making a given size box deeper tends to 'smooth out' the bass response somewhat, and possibly make it weaker, unless, of course, you make the hole larger.

Surprisingly enough, changing the depth of the box on an existing guitar does not alter the _pitch_ of the 'main air' mode much. This is due to the fact that it's not a simple 'Helmholtz' mode, but rather the product of coupling between the top and air, with some contribution from the back. If it were a matter of a pure Helmholtz mode, the deeper box would have a lower pitch, but since the pressure changes less for a given amount of top motion the coupling between the top and the air is weaker, and the coupling itself has the effect of dropping the 'main air' pitch relative to that of the 'Helmholtz' resonance. The two effects nearly cancel each other out, so the 'main air' pitch stays about the same as you cut the box depth down. That was the resukt in an experiment that Fred Dickens did back in the late 70s, if I have the date right.

as of listening to my larger guitars compared to other not so deep guitars I would say the deeper guitars (4.75"end block) sounds way more airy ! and also like already been mentioned, if they are deeper its not as quick responce on the wish in my opinion leads to a warmer softer tone with not as much attack.but ofcause it has to do with so many factors. bracing, top thickness etc etc. but in general that is my feeling towards it. I personally do not agree that it will lead to less overtones, atleast not on my guitars ! but I have mostly used german and maple bridge plates on the large ones so far.

Lars

I love these type discussions. Thanks for all who've shared their thoughts.

While we are on the subject, what if we increase the air volume by lengthening or widening the body to increase the air volume instead of changing the body depth........how does this change the sound?

As far widening the body (the lower bout), it would not only increae the air volume (lowering the main air resonance), it also seems it would have an effect similar to lowering the cross-grain stiffness of the soundboard (whatever affect that has.....feel free to share your thoughts).

I've read (somewhere, not sure) that lengthening the body increases bass and is one of the reasons a dreadnaught has more bass than a 000 sized body (is this the primary reason?). Like the other concepts mentioned (increasing depth or increasing width) this also increases the air volume of the body (so would likely lower the main air resonance). If the soundboard and bracing thichnesses/shape were kept the same, seems this would have the affect of decreasing stiffness along the grain which might lower the resonant frequency of the top (similar to thining the top except that would also decrease cross-grain stiffness).

So a couple of points/questions. First, any sound difference between adding length above the bridge (12 fret body vs 14 fret body) vs adding the length below the bridge? I've no clue but would speculate more area below the bridge decreases the stiffness of that area which might lower the frequency and allow more movement of the antinode peaks so the top could move more air (at least in the main "speaker mode" where there is a closed ring in the lower bout).

Second, if additional length in the lower bout provides a little more bass and a little more volume, then couldn't one add bass and volume by "flattening" the bottom of the lower bout so more surface area of the top has the additional length? I'm not talking square off the lower bout or anything drasitic like that.........but it seems if you wanted to add a little bass to a 000 body (for example), you could make the bottom of the lower bout flatter and extend the flattened area out a little further to the sides before rounding over.
Just some hair-brain things I've wondered about.

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Formerly known as Adaboy.......

Adding length to the upper bout probably won't change the sound much. The upper bout is almost all structural and contributes little to the sound. The extra air volume that you get won't make that much of a difference either. Consider guitars that have cutaways. The have significantly less air volume than their non cutaway brethren yet they don't sound very different. It has always amazed me that making such a huge change like adding a cutaway makes so little difference in sound.

Adding length to the lower bout, loosens up the area between the bridge and the tail, which is the area of the top that is most active in bass production. Making the lower bout flatter at the tail might add some additional looseness, but not as much as lengthening, which loosens up the area between the bridge and the tail, specifically. At least that's how I see it.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Mike is right. The upper bout is not super active, even guitars I have built with a floating fretboard didn't impress me as having a radically different tone. When the fretboard is glued to the top, that big chunk of ebony, or any other hardwood, will seriously impede vibrations. If anything happens, it would be in the upper partials in any case.
Dreadnaughts have more internal air volume because the waist is so flat, compared to, say, a similarly sized 0000 (wider in the lower bout in fact).
I have moved my soundhole one fret north on my last dozen builds and I can hear a difference. Having more wood in front of the bridge seems to give me more bass response and a wider dynamic range. It is subtle but it is there. It allowed me to use a larger soundhole as well, which I really like.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/