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two schools of thought for guitar backs ?
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Author:  Jody [ Thu Jul 03, 2008 8:54 pm ]
Post subject:  two schools of thought for guitar backs ?
one thing that became clear to me at the Gal convention , which I have never heard discussed, is there are ( basicly) two differing concepts for the backs of guitars , one we could call the " bass reflex speaker " school , where the back is rigid and the top seems to act as the speaker cone of a cabinet speaker , and the other we could call the "tennis racket " school , where the back couples with and directly moves with and influences the movement of the top.
I know great sounding guitars can be made with either concept. so i guess my question is for some the the more experienced builders. which style do you use or prefer and why ? do you see any advantages or drawbacks of one over the other ? do you stick to one school ? or use both? if so when would you use which one ?
Thanks Jody .
Author:  Andy Zimmerman [ Thu Jul 03, 2008 9:15 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Jody
I am a big believer in the 2nd approach....that the back contributes to sound and couples with the top.
My earlier guitars were overbuilt and the top was very stiff and the back contributed nothing. My more recent guitars are FAR superior.

Ervin S. taught me that most guitars are overbuilt...both top and back. Since I have made this change there has been a dramatic difference. In order to handle the thinner top and thinner "contributing" back I build VERY rigid sides. I use the "double side" technique where I bend 2 sets together and laminate them together. Save overall thickness side, but "2 ply". They are dramatically more rigid. I can almost build a guitar without the use of a mold.

To confirm this point, I took the top off a parlor of mine and made it lighter and also thinned the back. The guitar was much better. Also I just made some changes to one of my OM's. Thinned the top and back without changing the box. I will see if this improves the tone. Before, the guitar was overbuilt. No oomph. The back definitely didn't contribute. Tapping after thinning seemed to make a difference. I will have this one refinished and we shall see.
Author:  Mike Collins [ Fri Jul 04, 2008 6:09 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Take a loose back guitar and hold it up to your body and the volume drops off.
Because you stopped the back from vibrating.

This is why I build supple tops with stiff back & sides.
The player can get up against the guitar without loosing tone or volume.

Mike
Author:  Dave White [ Fri Jul 04, 2008 6:43 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Mike Collins wrote:
Take a loose back guitar and hold it up to your body and the volume drops off.
Because you stopped the back from vibrating.

This is why I build supple tops with stiff back & sides.
The player can get up against the guitar without loosing tone or volume.

Mike


Mike,

I'm a bit puzzled by this. When you hold the "responsive" back tight against your body, isn't this the same as stiffening the back and making it reflective? Or are there different sorts of "stiffness" - that's not a euphamism by the way :oops:

I'm in the second camp by the way, even when I have my "belly clamp" on the guitar the back is still active and working. Apart from any physics, the "psycoacoustics" of feeling this response makes me more aware of the instrument and improves the way I interact and play it. Also there are other ways to get projection if that is what you are after with a stiff back.
Author:  David R White [ Fri Jul 04, 2008 11:13 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Quote:
When you hold the "responsive" back tight against your body, isn't this the same as stiffening the back and making it reflective?


Personally I doubt that this tells you a lot. Building a flexible back and dampening it is not the same as building a stiff back.

I am in the flexible back camp.
Author:  James Orr [ Fri Jul 04, 2008 11:20 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Andy and Dave, would you guys mind mentioning how you go about making flexible backs?

The back of my current guitar is .90 with 1/4" x .650" lutz braces throughout. Does that sound like the right direction?
Author:  Dave White [ Fri Jul 04, 2008 12:35 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
James Orr wrote:
Andy and Dave, would you guys mind mentioning how you go about making flexible backs?

The back of my current guitar is .90 with 1/4" x .650" lutz braces throughout. Does that sound like the right direction?


James,

Arching the backs - I use a 10' radius, and I use a hybrid ladder/X bracing and brace them pretty lightly - about 5-6mm wide by about 12mm high at the centre of the X. Tapered brace shape and I "voice" the braces similar to the top - looking for ringing sustain everywhere I tap rather than any specific notes. The thickness of the back is going to depend on the guitar size and the wood used.
Author:  Mike Collins [ Fri Jul 04, 2008 1:59 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Dave;
holding a "responsive back" against you is NOT the same as a stiff back.
A stiff back will still loose some vibration when held against you but very little compared to the loose one.
Making the top very flexible BUT strong -thick in the middle-stiff in the middle-flexible at the edges allows for fine tuning only one plate
on the guitar.
This is a simplified explanation -BUT I'm simple.
I've made guitars both ways.
I find the stiff back helps with projection & the ability to control unwanted overtones.

I just restored a 1850 Martin-with the real C.F.Martin signature in it!
The back was very responsive-the guitar sounded great as long as you did not hold the back to you.
Which many players do.
By the way i have all the specs. on this guitar if anyone would want to make one.
I did!
The back responded the same way!
Killed the sound when held against the player!
These are just my observations from all the time I've made guitars.
[:Y:]

Mike
Author:  Alan Carruth [ Fri Jul 04, 2008 6:33 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
I'm in both camps.....

Basically, from what I've seen, it looks as though the back can only contribute effectively to the output of the guitar in the 'bass reflex' range; the low range of the guitar. Above about the open G string pitch most of the resonant modes of the back seem to be 'losers'. It's easy to see why. In the low range, if the 'main back' resonant pitch is somewhere near the 'main top' pitch the two will work together to pump air through the soundhole. At higher frequencies, though, the back can't be as effective in producing sound as the top because it's vibrating in a pattern of several smaller areas that are out of phase with each other. It's not going to move much air through the hole, and it's too heavy and stiff to make a lot of sound in the room by itself. Besides, it's facing into your soft tummy!

What I do, then, is to try to get the 'main back' mode working with the 'main top' by paying attention to the relative pitches. Usually you get the strongest useable coupling when the back is about a semitone higher than the top. Above that pitch it's a matter of having enough weight and low enough damping that the back doesn't 'steal' too much energy from the top. The 'dips' in the spectrum caused by the back stealing energy from the top are part of what makes the 'tone color' of the guitar; without them the timbre can be 'flat' and uninteresting. Ovations come to mind....

I've observed that the timbre of a guitar with an active back doesn't seem to change all that much when the back is up against your stomach. There is some loss in the bass, and the volume suffers.
Author:  Parser [ Fri Jul 04, 2008 9:17 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
I do not have the experience that some of the other folks here have, but I do feel that you can built a quality guitar both ways. I think the most important thing is to form your own understanding of how the guitar works, and to optimize the guitar based on your theory.

One potential shortfall that I see from the "back as a reflector" school of thought is that you depend on the top to produce a full range of tones. In my experience, it seems that this means that you are building the top very lightly, primarily to get that extra bit of bass response that some folks get from the back of the guitar.
Author:  LuthierSupplier [ Sat Jul 05, 2008 11:19 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Personally, I'm with Dave W. on this. I like to feel the guitar vibrate while playing. When the back vibrates while holding it, I feel like I'm playing a very responsive guitar. It gives you the feeling that the guitar is alive and does contribute to the overall playing experience. I try to make my guitars light and responsive(even though I don't always succeed).
Author:  Howard Klepper [ Sat Jul 05, 2008 12:58 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Although I wouldn't use your names for the "schools" (it's more about what you want a particular guitar to do than a commitment to some doctrine), I try to have it both ways: stiff in the center and upper bout, compliant around the edges of the lower bout.
Author:  GregG [ Sat Jul 05, 2008 2:15 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Having built several with very active backs and several otherwise, I've opted for a balanced approach and happy with the results. I was trying to figure out how I was going to explain my back construction...then I saw Howard's post...That's it, that is how I build my backs.

Ervin and his crew build with very active backs and certainly it works for a lot of other people too, I've just taken a little different path and like the results....there are so many ways to get it done.....I like to keep an open mind about building because I believe there is always room for improvement no matter where I'm at today.

GG
Author:  Billy T [ Sat Jul 05, 2008 2:55 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
I believe one of the most misleading approaches to testing a guitar by playing, is just that, playing it!

I've started to let other people play the guitar than myself. As a player you're only hearing from a players perspective. It's very easy to tell a guitars distictive sound when someone else does that playing.

Though, generally, there isn't a great difference between the two, I've discovered that in some cases there is. The subtleties between the back in contact as opposed to sitting is an even less pronounced distinction. IMHO!
Author:  David R White [ Sat Jul 05, 2008 5:33 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Quote:
Andy and Dave, would you guys mind mentioning how you go about making flexible backs?


James, I thickness my backs to a standard deflection which is about 80% as flexible as my tops. The last one ended up .085" thick. My back bracing is .25 x .5" - but of course the stiffness is largely determined by how much carving you do. Generally I am building OM or slightly smaller.
Author:  Mike Collins [ Sat Jul 05, 2008 5:37 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
I should not have responded as quickly as I did.
oops_sign
I make Custom guitars!
And some of my clients do want that guitar-body feeling .
I'm not in this to make guitars which only may satisfy me.
It's the players who order them that have to be satisfied.

-BUT if a maker wishes to sell their guitars-I think it's more important to make what the players need!
NOT what I think is right for them!
By that I mean-Make guitars not just to satisfy you -but anyone interested in aquiring one from you.
Do not limit yourself by building only one style!
As Al Carruth said he does both styles of making!
Howard Klepper must know so much about how to match woods and body styles that I WISH he would write a book so we all could !
Mike

[:Y:]
Author:  Alan Carruth [ Sat Jul 05, 2008 5:50 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Mike Collins wrote:
"As Al Carruth said he does both styles of making!"

It's trickier than that: I use both styles in each guitar! I want the back to be 'active' in the 'bass reflex' range, and mostly a 'reflector' above that. Of course, if it's active at all it's not going to be a 'perfect' reflector, but I'm not too happy with the sound of the 'guitars' that that have real reflector backs: you know, those Tupperware things? :) As with everyting in the guitar world, it's a compromise.
Author:  Mike Collins [ Sat Jul 05, 2008 5:55 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Al;
Sorry to simplify your quote!!!!
oops_sign

How do you do it both ways?
Nothing kinky here!!!!!! :lol:

Mike
Author:  Hesh [ Sat Jul 05, 2008 6:32 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Tracy described very well the "feeling" that I like to get from a guitar as well. When I can feel the vibrations of the guitar that I am playing on my body it's a much more immersive experience.

After reading everyone's descriptions I think that I understand what Al is saying and I also think that this is my personal preference too.

What I have not seen here yet is people making the center of the back stiffer and the perimeter more flexible using the bracing alone yet most everyone is indeed doing this. This is what I think...... that I am doing with t*pered/parabolic bracing. The back remains one thickness but it's flexibility is varied by the shape and mass of the braces and their respective locations.

I also like a thinner back to help give me that loving feeling....... How do you like that one Mike........ :D
Author:  James Orr [ Sat Jul 05, 2008 7:14 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
drwhite wrote:
James, I thickness my backs to a standard deflection which is about 80% as flexible as my tops. The last one ended up .085" thick. My back bracing is .25 x .5" - but of course the stiffness is largely determined by how much carving you do. Generally I am building OM or slightly smaller.


That's really helpful, Dave. Thanks! I'm building an OM-ish guitr, too. A tad wider lower bout and narrower waist.

I really need to get going with testing deflection.
Author:  Parser [ Sat Jul 05, 2008 9:10 pm ]
Post subject:  Re: two schools of thought for guitar backs ?
Hesh wrote:
What I have not seen here yet is people making the center of the back stiffer and the perimeter more flexible using the bracing alone yet most everyone is indeed doing this. This is what I think...... that I am doing with t*pered/parabolic bracing. The back remains one thickness but it's flexibility is varied by the shape and mass of the braces and their respective locations.


Hesh, I have similar goals but I look at it from a different angle. Whereas the top is driven from the middle, I believe the back is primarily driven from the sides. I think the air in the box also has an effect on the back, but my opinion is that it is the sides that primarily drive the back. Why do I think this?...take nearly any acoustic guitar and strum a couple strings. While the strings are still going, run a finger down the perimeter of the back of the body. You will always feel the side vibrating primarily at one part of the perimeter of the back (the location changes depending on the strings being strummed). However, you will not always feel the center of the back vibrating. If the air in the box was the primary force driving the back, then I would expect this to be the opposite, i.e., you could expect to find guitars where you can feel vibration in the center of the back, but not along the edge (since the edge would act more as a node).

I make my back braces slightly shorter at the center than they are at the outsides since it is the center of the back that I want to deflect the most. I have built some like this, but not a ton...so far it seems to work!
Author:  Dave White [ Sun Jul 06, 2008 6:31 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Parser wrote:
Whereas the top is driven from the middle, I believe the back is primarily driven from the sides. I think the air in the box also has an effect on the back, but my opinion is that it is the sides that primarily drive the back. Why do I think this?...take nearly any acoustic guitar and strum a couple strings. While the strings are still going, run a finger down the perimeter of the back of the body. You will always feel the side vibrating primarily at one part of the perimeter of the back (the location changes depending on the strings being strummed). However, you will not always feel the center of the back vibrating. If the air in the box was the primary force driving the back, then I would expect this to be the opposite, i.e., you could expect to find guitars where you can feel vibration in the center of the back, but not along the edge (since the edge would act more as a node).

I make my back braces slightly shorter at the center than they are at the outsides since it is the center of the back that I want to deflect the most. I have built some like this, but not a ton...so far it seems to work!


Parser,

I just tried this plucking each open string in turn (tuning was DADGAD). I'm not sure what you are defining as the "centre", but I had my hand on the back in the centre of the lower bout roughly below the bridge and I'm feeling vibration there with each plucked open string - stronger movement as the strings go lower.
Author:  Larry Drover [ Sun Jul 06, 2008 8:21 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Sound is the result of something making a noise and the loudness of it depends on the hardness of the material it bounces off....In a Canyon the sound of your voice will re-vibrate on and on if there are lots of solid walls to bounce of....I can compare it to a rubber ball thrown at a brick wall, it comes back at you almost as fast as it was thrown...Throw the ball at a curtain and it drop straight down.....Is there anything to be learned from this in guitar making?....I think so....My thought ,for what its worth, heavier backs are better, thicker sides are harder to bend ,but the downside, they make the guitars heavier and uncomfortable to play.....Larry
Author:  Alan Carruth [ Sun Jul 06, 2008 10:15 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Mike asked"
"How do you do it both ways?"

It's just the way the back works. Unless you want to make a really rigid bowl, like a thick lute back or an Ovation, the back is going to vibrate at some frequencies within the tuning range of the guitar. Only the lowest mode, or maybe the lowest two, where there is one large area that is doing most of the moving, will be able to pump air through the soundhole. The higher modes have alternate areas that mostly cancel each other out. Besides, as I say, the back is usually much stiffer and heavier than the top, so for a given amount of energy input it's not going to move as much or put out as much sound. And, of coutrse, it's facing the wrong way.

If you make a normal back really stiff and heavy the 'air pumping' function will be at a much higher frequency than many of the top resonant modes, and the back won't be very effective even at it's best. If you can get the lowest back resonant mode down near the range of the 'main top' resonance, the top will be able to drive the back effectively, and the back will help the top in moving air in the 'bass reflex' range. This gives you a stronger low end, and also tends to 'spread out' the spectral peak around the pitch of the 'main top' resonant mode, which tends to give a 'fuller' and 'more solid' sound, IMO.

Above that 'main back' resonant pitch you tend to want the back to be pretty inactive. Making the back heavy helps. Also, if you can use a material that has low inherent damping the resonant peaks will be 'tighter'; not spread over such wide frequency bands. This helps limit the amount of energy that the back wastes. Thus the common preference for dense, low damping woods like rosewoods. Light backs, like mahogany, may work a bit differently, but it's hard to say. Probably they move more air in the bass reflex range, and may have a more positive effect on output in thwe low range as a whole, although I think they'd tend to be bigger losers as you go furhter up in pitch. Just an opinion.

Parser wrote:
"Whereas the top is driven from the middle, I believe the back is primarily driven from the sides. I think the air in the box also has an effect on the back, but my opinion is that it is the sides that primarily drive the back. "

The measurements I've made suggest strongly to me that most of the energy that drives the back comes fron air pressure changes inside the box, with little or none coming from the sides. For example, when you open a 'port' and vent one of the internal air resonant modes, the energy that shows up as sound in the room comes from the internal pressure of the mode. The sound outside the guitar at that pitch is louder, but inside it's not as loud. This results in less motion of the back at the frequency of that mode. OTOH, adding weight to the sides does not seem to effect the way the back moves very much.
Author:  Parser [ Sun Jul 06, 2008 11:56 am ]
Post subject:  Re: two schools of thought for guitar backs ?
Alan Carruth wrote:
Mike asked"
Parser wrote:
"Whereas the top is driven from the middle, I believe the back is primarily driven from the sides. I think the air in the box also has an effect on the back, but my opinion is that it is the sides that primarily drive the back. "

The measurements I've made suggest strongly to me that most of the energy that drives the back comes fron air pressure changes inside the box, with little or none coming from the sides. For example, when you open a 'port' and vent one of the internal air resonant modes, the energy that shows up as sound in the room comes from the internal pressure of the mode. The sound outside the guitar at that pitch is louder, but inside it's not as loud. This results in less motion of the back at the frequency of that mode. OTOH, adding weight to the sides does not seem to effect the way the back moves very much.


Thanks for posting Alan, I was hoping you'd peek back in!

I'm not sure I understand your reasoning with regard to function of the ports and how that relates to how much energy is transferred to the back via the air in the box. Also, when you open a port, it would seem that you are making a large change to the fundamental air resonant modes (more or bigger holes would seem to equate to a different Helmholtz frequency).

If the sides are not driving the back, then how do you explain the buzz that you can feel along the perimeter of the back? Assuming a standard back bracing pattern, how would the air alone drive the back at the edge? I can form a good picture in my mind of how the air could add to "buzz" at the center of the back, but not at the edge. Thus, I have to assume the sides are adding something.

As for the weight of the sides, I would bet if you drastically increased it you would get an audible difference...but only on a guitar with an active back.

Great discussion, thanks again for your thoughts.
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