Official Luthiers Forum!

Hi to All,

I'm new to this forum and to guitar building, and just completed my 2nd LMI kit. So I hope you don't mind two really basic questions about planning the depth of an acoustic soundbox in the initial design phase of a build.

One of the things I don't understand is why the soundbox is always deeper at the tail end than the heel end of the guitar. I'm sure there's a really good basic reason, but I'll be darned if I can figure it out. The OM I just finished had a spec of 4.13" depth at the tail and 3.34" depth at the heel, which I followed. But I was wondering what would have happened had I made the tail and the heel the same depth.

The other thing I'm wondering about is what to do at the heel end if I wanted to make the tail end deeper (say on an OM). I've seen where some builders say they like to make their OM tail depths at 4.38" or 4.5" (I guess to get more volume?), but they don't say what they do at the heel end. Is there some kind of formula to use to figure it out?

Thanks for any help you can pass along.

John

I have done quite a bit of experimenting in body depths mainly with dreads and a few 12 fret Gibson style guitars. My dreads are quite a bit shallower than Martin specs and most hand builders. As a general rule If I go 3/32" shallower at the headblock I do the same at the tailblock. The decision, however, is yours and is what makes building your own so great.

Just a side note: my guitars, IMHO, have noticeably more volume and "liveliness" than most new Martins I've played. Leads me to believe the volume of the box isn't that critical unless you really take it to the extreme one way or the other...and I like the comfort of a shallower depth when playing. I'm currently building an OM that will be approximately 5mm deeper than a standard Martin OM.

Do your own thing. Do whatever you think is best and it will turn out fine I'm sure. Best of luck.

_________________
"We might not be big but we sure are slow"

I think that the main reason for the difference is to make the guitar more comfortable to play. I like to have a steeper slope than most do. Mine are between 3/4" - 1" difference between the tail end and the neck end. There is an effect on the sound, I believe, but I can't put may finger on what it is. The difference is subtle and I might just be fooling myself.

This is one of the places where builders express their individual design ideas so there is no formula. My sense of design, however, would make me keep the slop the same when I increased the depth of the instrument. If it used to be 4.13" to 3.34" and I wanted to add half an inch, I'd add it to both ends. 4.63" to 3.84"

But it is important to say that you don't have to do that. If you want to see what it would look like, make a body out of poster board and tape it together. I've don't this and it can be quite enlightening.

A deeper body helps to 'spread out' the pitch of the 'main air' resonance, and weakens it a bit, all else equal. The low end can be a little more 'even'. The deeper body _ought_ to make the 'main air' resonant pitch a bit lower, as well, but because of the way the top and the air work together it often doesn't. In fact, making the box deeper can drop the pitch of the 'main top' resonant mode, which is around an octave higher than the 'main air' pitch, even if you didn't do a thing to the top.

Some folks feel that more taper gives a more even sound. There is a resonant mode of the air in the box where it's 'bouncing' back and forth between the top and the back, up around 1000-1200 Hz. If the top and back were parallel that mode would be very strong, and tightly defined in pitch. Tapering the box spreads it out, and maybe some other air resonat modes as well.

I suspect you'd hve to make some pretty big changes in the taper before you'd notice a lot of difference in the timbre. Relatively small changes in depth are noticable, though, as I've found from cutting down a couple of guitars.

it would seem to me ( totaly amateur opinion here ) that making the body wider at the lower bout and thinner at the sound hole area and forward,would also help to force the air out of the body through the sound hole as the motion of the top ( and back ) pumps the air forward into a more "cramped" or diminished cavity area . Jody

Mine have a 1" range heel to tail. FWIW my latest box, a deep body OM, is 3.5" at the heel and 4.5" at the tail.

I too feel there is something to do with thinner at the top and deeper at the bottom but not sure why. I just know I like the results since going that route.

_________________
"We might not be big but we sure are slow"

Two related observations:

First, I'm not a classical player or builder, but, if I'm not mistaken, classical guitar bodies are generally not tapered in depth.

Second, on my first guitar, I tapered the body the opposite way, thinking it would be ergonomically better. I don't think it is any more comfortable to hold, but I don't think it's any less comfortable either. Most people who pick up and play the guitar don't even notice until I point it out to them. There is also nothing distinctive about the sound that I would attribute to the opposite taper - that is to say, it's a very good sounding guitar, and that's about all I can say about that. It also has an "offset" main soundhole and a side soundport, a floating fretboard tongue, an X-braced back... at the end of the day, it's just a very good, nicely balanced sounding guitar.

_________________
Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Mine is certainly tapered. 5/8" from tail to heel. Most plans I have ever seen are tapered.

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars