Body depth and bass projection

[vindibona1]

Originally Posted by Brucebubs

Perhaps you should ask yourself why do so many builders offer a 'deep body' option on their OM guitars?

What made Chris Martin himself add an extra 3/4" in depth to the existing 0000/M body and create the Martin J-14 fret?

Great comment. I want me a Bourgeois JOM!

[zombywoof]

I would think the carve of the bracing has as much if not more to do with low end oomph than the body size or depth. The guitar I own with the deepest bottom is a 1942 Gibson J-50. While it may not be characteristic of a Gibson, the guitar has been described as having a low end that would make a pre-War Martin D-28 Herringbone run for cover. It has far more rumble than say our 1960 Gibson J-200. So it is not body size coming into play here.

[Alan Carruth]

I agree with Wade.

I've measured the response of similar guitars with deeper and shallower boxes, and even looked at one before and after cutting the sides down. The maximum amplitude of the lowest 'air' resonance is not as powerful in the deeper bodies, but it may be a bit more 'spread out' in frequency. It's possible (but hard to say for certain) that the overall available horsepower in the resonance is a bit greater with the deeper box, and it's likely to reinforce more notes, but I think it likely that it won't project as well over a distance.

Keep in mind that the lowest frequency sounds that a guitar makes pretty much radiate equally in all directions. The player hears them as well as anybody. As you go higher in pitch the sound tends more to go out of the hole and off the top, toward the audience. The player tends to hear that stuff more through reflections from the room. A guitar with lots of power in the low range does tend to sound louder to the player than it does further out.

I'll put up a short variation of Silly Moustache's explanation that I think hits it a little closer. When you pluck the strings they drive the top like a loudspeaker cone. This causes the pressure in the box to change in step with the top motion. For a given top and string set and so on the deeper the box the less the pressure change. Less pressure change pumps less air through the soundhole.

You might think that changing the box depth will change the pitch of the lowest 'air' resonance, which works in the same way as blowing across the mouth of a wine bottle. It doesn't, though, or, at least, not much. That's because a guitar body is not as rigid as a wine bottle. The air pressure changes inside the box push on the top and back, in particular, and cause them to move. In effect this adds to the mass of the moving air, and drops the pitch of the resonance. The shallower the box the more the top and back move, and the more they drop the air pitch. This almost exactly cancels out the rise in pitch that you'd expect from making the box shallower.

This is not to say that the sound doesn't change, though. To me a shallower box sounds a bit more 'open' or 'forward'. That tends to help projection, in my mind, so we're back to Wade's answer.

[Aecon813]

Originally Posted by Wade Hampton

Well, it's true that adding extra body depth to an OM will give you a more pronounced bass response. However, it actually has a bit of a negative impact on the acoustic projection of the instrument.

Projection and loudness are not the same thing. What sounds louder to the person playing the guitar and perhaps those sitting within six feet won't necessarily cut through when there are other instruments being played at the same time.

I'm one of the rare players who has for years favored 14 fret Triple O's and OM's as rhythm guitars (which is what the OM model was originally designed for back when it first came out.) When I first heard of deep-bodied OM's, I was very interested, only to discover once I'd gotten my hands on a few of them that they simply don't project as well as standard depth OM's.

So it's a trade-off. When you change something like body depth it affects more than just the tone.

Or, as my father used to say, "there ain't no free lunch." If you gain in one area, another aspect might shift at the same time.

Hope that makes sense.


Wade Hampton Miller

So I'm a rhythm player/Strummer, who struggles with the discomfort of Dreadnoughts (I'm of smaller stature). I have been searching for a deep body OM because I felt it could give me a Dread-like sound, buy in a more comfortable body. Are you suggesting that a standard OM would be a better fit?

[CopyCat]

Originally Posted by Alan Carruth

I agree with Wade.

I've measured the response of similar guitars with deeper and shallower boxes, and even looked at one before and after cutting the sides down. The maximum amplitude of the lowest 'air' resonance is not as powerful in the deeper bodies, but it may be a bit more 'spread out' in frequency. It's possible (but hard to say for certain) that the overall available horsepower in the resonance is a bit greater with the deeper box, and it's likely to reinforce more notes, but I think it likely that it won't project as well over a distance.

Keep in mind that the lowest frequency sounds that a guitar makes pretty much radiate equally in all directions. The player hears them as well as anybody. As you go higher in pitch the sound tends more to go out of the hole and off the top, toward the audience. The player tends to hear that stuff more through reflections from the room. A guitar with lots of power in the low range does tend to sound louder to the player than it does further out.

I'll put up a short variation of Silly Moustache's explanation that I think hits it a little closer. When you pluck the strings they drive the top like a loudspeaker cone. This causes the pressure in the box to change in step with the top motion. For a given top and string set and so on the deeper the box the less the pressure change. Less pressure change pumps less air through the soundhole.

You might think that changing the box depth will change the pitch of the lowest 'air' resonance, which works in the same way as blowing across the mouth of a wine bottle. It doesn't, though, or, at least, not much. That's because a guitar body is not as rigid as a wine bottle. The air pressure changes inside the box push on the top and back, in particular, and cause them to move. In effect this adds to the mass of the moving air, and drops the pitch of the resonance. The shallower the box the more the top and back move, and the more they drop the air pitch. This almost exactly cancels out the rise in pitch that you'd expect from making the box shallower.

This is not to say that the sound doesn't change, though. To me a shallower box sounds a bit more 'open' or 'forward'. That tends to help projection, in my mind, so we're back to Wade's answer.

Thank you for this excellent explanation. It really helps to visualize what’s going on with these lovely wooden boxes.

[Dogma]

As the tireless promotor of the Luthier on Luthier podcast, may I suggest a listen to episode #45 which includes interesting discussion of this subject by host Michael Bashkin and his guest Michael Greenfield.

[AcousticDreams]

Originally Posted by Dogma

As the tireless promotor of the Luthier on Luthier podcast, may I suggest a listen to episode #45 which includes interesting discussion of this subject by host Michael Bashkin and his guest Michael Greenfield.

I tried googling Luthier Podcast #45 and could not find this. Could you post this link?

[nightchef]

I wonder if an analogy to drums is useful here. With drums, it’s the diameter of the head (analogous to lower bout width) that determines the range of pitches it can be tuned to, i.e. how low it can go. But it’s the depth of the shell (analogous to body depth) that determines the length and power of the drum’s resonance. With that in mind, you’d expect a guitar with a wide lower bout but a shallow body to have a deep but dry, quick low end, and a guitar with a narrow lower bout but a deep body to have a rich, resonant, but not particularly deep low end.

[Dogma]

Originally Posted by Knives&Guitars

I tried googling Luthier Podcast #45 and could not find this. Could you post this link?

Why certainly:
https://luthieronluthier.libsyn.com/...ael-greenfield

[Mr. Jelly]

My experience with Gibson L-1 and L-00 guitars, with the L-1 being a deeper body, I found that the deeper body affords more depth and a little robustness of sound. Bass response opinions depend on ones outlook as to what that means. A treble more immediate response from the bass strings is far different than a big round bass presence.

[KevWind]

I am not a Luthier and do not play one
but I do play A Breedlove Phoenix (Ziricote back and sides Sinker Redwood top)

which compared to my Taylor 810 ce Dread, is smaller and shallower overall and yet has a bit more noticeable (if different sounding ) bass response