Acoustic guitar geometry?

[lowrider]

Is there a rule, or guideline, as to how the length and width and depth of the soundbox relate to one another? Too deep, you get this. Too shallow you get that. Get it right and its, well, just right!

I ask because of the Martin reduced depth models. I was interested in the d-16e. I thought that with the dread body it would still put out a good sound even with the 000 depth. It sounded nice but it just had no oomph, if you know what I mean. I also tried GPC-16e reduced depth and felt that it gave me less than an 000 would.

I remember being disappointed by the 0000, M-36 and feeling that my OM-21 Special would take it's lunch money. So I ordered a Custom Shop J-17 and it has tone to kick out the windows.

I've seen where people order custom 00 deep body guitars and wondered just how deep you can go.

So how do you figure the measurements? And what is just right?

[Flat5]

I’m afraid there’s not, in the sense that there’s no simple answer.

What is there are “standard models”, with recurring shapes and sizes.
The overall voicing process is the one that determines how your guitar will sound. That’s where the luthier tweaks bracing and thickness and overall structural configuration to get the desired result.
I’ve played small footprint guitars which would blow you away and ginormous jumbos with muted dull voice.

Especially with industrial made instruments, your best bet is to go out there, close your eyes, and try as many as you can. Once you find the one, you’ll know. And, most importantly, if it’s not the one, regardless of the make, size, shape, appointments and price tag, you’ll know.

Or go to a respected experienced luthier, with several dozens of boxes under his/her belt, and spell out loud what you want.
They’ll beat any Martin Custom Shop any day, in terms of end results.

[Tim McKnight]

Wellllllll ... yes and no. Altering the depth of the guitar changes the Helmholtz frequency (google that if you want your head to spin) and it also alters the response time in milliseconds. Here is an easy experiment, blow across the mouth of a full soda bottle and listen to the tone. Pour out a few ounces of liquid and repeat until the bottle is empty. As more air space is created the frequency lowers and also the response time slows, if you are able to hear it?

As a general rule of thumb, shallower bodies (shorter air spring) will have better projection and faster response time because the sound waves have less distance to travel to reflect off the back and those waves can travel at a faster reflective speed. In general, they are often brighter overall and lack a boomy bass response. A shallower body tends to throw the sound down the hallway and out the window, so to speak.

A deeper body has a slower response time (longer air spring) because the sound waves have a further distance to travel and tend to have less projection and may fill a small room and envelope a player it sound. They can also be slighter lower frequency due to additional air space.

Alter the thickness, graduation of plates, size of the sound hole(s), bracing flexibility and final voicing of the top and back and the luthier can control and shape the sound based on their experience, skill and desires of the customer. However, size does matter and there is no way to plainly ignore the physics involved considering the given column of air inside the sound box one has to work with.

Cut a side sound port in the sound box and that adds another dimension to consider.

Hope that makes sense?

[lowrider]

Thanks, that makes good sense. But to take a step further, does making a guitar wider with the same depth as in 000 to 0000 upset the balance or the ratio so that the sound just doesn't come out?

Obviously, I don't know what I'm talking about, just what I'm hearing.

[ChuckS]

Originally Posted by Tim McKnight

As a general rule of thumb, shallower bodies (shorter air spring) will have better projection and faster response time because the sound waves have less distance to travel to reflect off the back and those waves can travel at a faster reflective speed. In general, they are often brighter overall and lack a boomy bass response. A shallower body tends to throw the sound down the hallway and out the window, so to speak.

A deeper body has a slower response time (longer air spring) because the sound waves have a further distance to travel and tend to have less projection and may fill a small room and envelope a player it sound. They can also be slighter lower frequency due to additional air space.

…[snip]...

My experience is totally in line with Tim's great explanation. For me, it effects projecting vs wrapping/filling.

Also, I feel deeper bodies tend to have notes blend more (over time) where shallower bodies allow the next played note (i.e. next bass note) to take over how the box will be resonating. (maybe it has something to do with the top / back coupling at various body depths).

[printer2]

You have a bigger top, more surface area. Also bracing might have to change. A guitar is a collection of things, some matter more some less. Sometimes changing one thing, needs you to make other changes. Change two things, is it the result of the one, the other or the combination of the two.

[Flat5]

The main thing that happens by increasing the body depth is that you change the volume of air inside the box.
In so doing you’re changing its resonance peak.
Technically, the resonance frequency is inversely proportional to the square root of volume.
In theory you can have the same effect by reducing the sound hole: the resonance frequency is also directly proportional to the square root of the sound hole area.
Or, leaving all else untouched, you can increase body depth and compensate with a sound port, and air cavity resonance will remain unvaried.

But that’s all an oversimplification: the Helmholtz frequency is also driving where the resonance of the top and the back will be placed: if you just deepen the body and don’t adjust the top and the back accordingly, you’ll mess the response up across the sound spectrum.

Projection, on the other hand, is driven mainly by the particular mix of vibration modes of the top: long dipole gives projection, while cross dipole gives best response near the instrument.

[Alan Carruth]

Tim McKnight wrote:
"Altering the depth of the guitar changes the Helmholtz frequency (google that if you want your head to spin) and it also alters the response time in milliseconds."

Well, yes, but...

Several decades back Fred Dickens made a guitar that had 6" deep sides. After he'd played it a while he cut them down to 5", and put the back back on. He kept doing that until the sides were too shallow to allow it any further. In the whole process the Helmholtz type 'main air' resonant pitch rose by...........7%. Not much more than a semitone.

Of course, if the box had been rigid the Helmholtz resonance would have risen by something like an octave, if I remember what he told me about the experiment. It didn't because the 'main air' resonance has been displaced downward due to the coupling of the 'air' and 'top' resonances. The stronger the coupling the more the Helmholtz pitch drops as it becomes the 'main air' mode.

When the box is shallower a given motion of the top creates a larger pressure change in the box and pushes more air out of the hole. Similarly, a given flow through the hole produces a larger force on the top. The couple between the air and top gets stronger. That tends to shift the Helmholtz mode downward more in pitch. As it turns out, the rise in the Helmholtz pitch is almost exactly balanced by the increased coupling strength, so the end result is that the 'main air' pitch doesn't change much as the depth of the box varies, all else equal.

"As a general rule of thumb, shallower bodies (shorter air spring) will have better projection and faster response time because the sound waves have less distance to travel to reflect off the back and those waves can travel at a faster reflective speed."

Similarly, you could just as well attribute the 'brighter' sound of the shallower box to a faster response owing to the quicker pressure rise in the smaller volume. It really has nothing to do with the time it takes for the wave to reflect. Most guitars have an internal 'air' resonance up around 1500 Hz or so, which is the pitch of the standing wave between the more or less parallel top and back plates: the reflected wave comes back in 1/1500th of a second or so, not enough delay to notice.

I never got to play that guitar, and Fred didn't say much about any changes in the timbre of it. It's too bad. It would be a nice experiment to repeat in one's copious free time....

[Howard Emerson]

Originally Posted by Alan Carruth

Tim McKnight wrote:
"Altering the depth of the guitar changes the Helmholtz frequency (google that if you want your head to spin) and it also alters the response time in milliseconds."

Well, yes, but...

Several decades back Fred Dickens made a guitar that had 6" deep sides. After he'd played it a while he cut them down to 5", and put the back back on. He kept doing that until the sides were too shallow to allow it any further. In the whole process the Helmholtz type 'main air' resonant pitch rose by...........7%. Not much more than a semitone.

Of course, if the box had been rigid the Helmholtz resonance would have risen by something like an octave, if I remember what he told me about the experiment. It didn't because the 'main air' resonance has been displaced downward due to the coupling of the 'air' and 'top' resonances. The stronger the coupling the more the Helmholtz pitch drops as it becomes the 'main air' mode.

When the box is shallower a given motion of the top creates a larger pressure change in the box and pushes more air out of the hole. Similarly, a given flow through the hole produces a larger force on the top. The couple between the air and top gets stronger. That tends to shift the Helmholtz mode downward more in pitch. As it turns out, the rise in the Helmholtz pitch is almost exactly balanced by the increased coupling strength, so the end result is that the 'main air' pitch doesn't change much as the depth of the box varies, all else equal.

"As a general rule of thumb, shallower bodies (shorter air spring) will have better projection and faster response time because the sound waves have less distance to travel to reflect off the back and those waves can travel at a faster reflective speed."

Similarly, you could just as well attribute the 'brighter' sound of the shallower box to a faster response owing to the quicker pressure rise in the smaller volume. It really has nothing to do with the time it takes for the wave to reflect. Most guitars have an internal 'air' resonance up around 1500 Hz or so, which is the pitch of the standing wave between the more or less parallel top and back plates: the reflected wave comes back in 1/1500th of a second or so, not enough delay to notice.

I never got to play that guitar, and Fred didn't say much about any changes in the timbre of it. It's too bad. It would be a nice experiment to repeat in one's copious free time....

Hi Alan,
This pretty much explains why the Flammang L-40 that David Flammang built to my specs has so much to say, with little effort. It's only 3 & 7/8" at the tailblock, which is basically what a Martin 00-18 measures.

The whole 'deep body' thing is not what it's cracked up to be.

John Monteleone is currently building me a 12 fret maple/red spruce cutaway using the same depth at the tail block. I expect it will have even more to say being a 12 fretter.

Best regards and have a great holiday!

Howard Emerson