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Brace carving effects on monopole/dipole
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=17662		 Page 1 of 1
Author:  Wes McMillian [ Thu Jun 19, 2008 9:43 pm ]
Post subject:  Brace carving effects on monopole/dipole
All the feedback on my OM bracing critique thread got me thinking more. I took to heart the comments about the beefy upper legs of the X (although the main things noted were the height of the inlet ends of the upper bout bracing and discussions on the A-frame bracing, I really can't do anything about those now). What I can still access is the upper legs of the X and shave some more material off. So, again, this got me thinking...

First off, let me say that I know I'm talking over my head here, gathering together bits of information stored in the back of my head from various forums (a little information is a dangerous thing?) so take my comments and questions with that in mind.

If I understand correctly, monopole deals more directly with the up and down piston-like "pumping" action of the soundbox, long dipole deals with the rocking two-pole response longitudinally along the soundbox (with the fulcrum at the saddle or intersection of the X?) and cross-dipole is that some motion as it pertains to the rocking phase motion side to side, along the centerline of the guitar. Anybody feel free to jump in and correct me, here.

So, in my case, where the lower bout is pretty lightly braced and the upper bout pretty heavily braced, is it reasonable to expect a strong long dipole response due to the imbalance on both sides of the fulcrum point, but a diminished monopole response as the heavier upper bout bracing inhibits the overall "pumping" action of the top?

If I'm correct in this assumption, what would the expected outcome of such a build be? Generally speaking, of course. I realize it's more complex than that and that there are no cookie-cutter answers.

If I were to lighten the upper bout bracing, could I expect an increase in monopole action and a decrease in long dipole?

And for the most important (and elusive) aspect of this, if any of these assumptions are correct, what would be the expected change to the player or listener? Yes, I understand the dangers of generalizations here, but a discussion on this could be educational, I think.

So, now that I've opened a whole new can of wiggly fish bait, who wants to bite?

pizza Eat Drink
Author:  Wes McMillian [ Thu Jun 19, 2008 9:48 pm ]
Post subject:  Re: Brace carving effects on monopole/dipole
As a reminder, this is the bracing in question, for a visual reference. Although, this aside, I would really like to get an overall discussion on the theory behind it.




Image


Image
Author:  Brock Poling [ Fri Jun 20, 2008 7:46 am ]
Post subject:  Re: Brace carving effects on monopole/dipole
The dipole that has a fulcrum of the centerline of the guitar is called the "cross-dipole". So in effect there are 3 we are discussing. Monopole, Long Dipole and Cross Dipole. (there are actually more, but my head begins to hurt in considering more than these 3.

Monopole gives you the Amplitude and to some degree your low end oomph.

Long dipole is responsible for your carrying power over distance.

Cross dipole is what gives you the "envelope of sound" that surrounds the player.

Typically a stiffer long dipole will give you more carrying power. But all of this comes at a price. The trick is to get all of this in balance.

Ervin Somogyi has written extensively on this, you might want to find his articles and check them out. I think he has a bibliography on his web site showing where these articles are published.
Author:  Kent Chasson [ Fri Jun 20, 2008 10:36 am ]
Post subject:  Re: Brace carving effects on monopole/dipole
Brock Poling wrote:
Ervin Somogyi has written extensively on this, you might want to find his articles and check them out. I think he has a bibliography on his web site showing where these articles are published.


And a two volume book coming out soon, I hope!

In my opinion, this is one of those areas where it gets really tempting to over-think everything. For instance, if monopole is responsible for bass response then it makes perfect sense that loosening the edges helps the low end but why does scalloping the X also increase bass? In that model, I can't see how it would do anyhting but encourage the bridge to rock thereby increasing long dipole or even tripole. But clearly it works. Maybe I'm missing something.

I think the better way to understand the effect of messing with the braces is by doing just what you said and going in and re-shaping and listening.

If I were to lighten the upper bout bracing, could I expect an increase in monopole action and a decrease in long dipole?

I don't know but I bet you would get more bass :)
Author:  SniderMike [ Fri Jun 20, 2008 12:08 pm ]
Post subject:  Re: Brace carving effects on monopole/dipole
My thought would be that you're going to get plenty of monopole pumpage, since it's not restricted until where your upper x arms meet the linings, which is right where there's usually a big UTB anyway. It seems to me like the long dipole is what is going to be restricted by the upper half of the x being stiffer. At least that's how I'm seeing it in my head.
Author:  Alan Carruth [ Fri Jun 20, 2008 2:28 pm ]
Post subject:  Re: Brace carving effects on monopole/dipole
Yup; easy to over-think this stuff alright....

According to Richardson, loosening up the bracing in the center under the bridge does more to increase air pumping by the monopole than loosening up the edges. Think about the 'scalloped' bracing in all of those thumpy Bluegrass Dreads, as opposed to the 'tapered' bracing the fingerstyle guys tend to like, that gives less bass.

Many in the guitar research community think that the monopole, and the Helmholtz air mode, working together to make up the 'bass reflex couple', account for most of the power output of the guitar all the way up to 1000 Hz or so. Maximizing the monopole can actually help the _high end_ response, as Wright showed in his thesis. Think about the sound of a reso, which is all 'monopole' (and ofetn a Helmholtz mode that only bats can hear).

Scalloped bracing, with the main X and finger braces peaked out from the center, would seem to me to be likely to boost the pitch of the 'cross dipole' mode, by stiffening the 'wings' of the top outboard of the bridge. It's a little hard to talk about the exact acoustic effect of this; my take is that it tends to make a more 'fundamental' and 'solid' sound that is not as 'forward'. Of course, everybody has their own way of describing tone.

The 'long dipole' mode can couple constructively with the 'A-1' air resonance. I talked a bit about that in the article on the 'corker' guitar that just came out in American Lutherie. Making the upper arms of the X stiff, and losening up the lower arms, will most likely alter the location of the node line for the long dipole. This tends to be near the bridge location, but can be above or below it. This, in turn, can change the effective mass of the mode, and the phase relationship between the driving force of the strings and the top motion. Does your head hurt yet? Mine does. See Wright's thesis for more on this.

Just to add something else to think about: sometimes on asymmetrically braced tops I've seen diagonal dipoles on the assembled guitar, rather than 'cross' and 'long' dipoles. In fact, I've even seen the two diagonals, PLUS a long dipole, owing to the coupling with the A-1 modes. Just another fly in the ointment.....
Author:  Brock Poling [ Fri Jun 20, 2008 7:20 pm ]
Post subject:  Re: Brace carving effects on monopole/dipole
Alan Carruth wrote:
The 'long dipole' mode can couple constructively with the 'A-1' air resonance. I talked a bit about that in the article on the 'corker' guitar that just came out in American Lutherie.


BTW, I really enjoyed that article. :)
Author:  Wes McMillian [ Sat Jun 21, 2008 9:40 pm ]
Post subject:  Re: Brace carving effects on monopole/dipole
Thanks, everybody. I always read these types of threads over and over until things start slowly coming together in my head. The light bulb's lit, but it's real dim right now. I do intend to play with this one after it plays in a little by shaving the upper legs. It will be interesting to see what "real world" response, if any, is seen.

Al, if you read this, maybe you can pop in this other thread which wound up in a discussion of A-frame bracing. The following comment was made there:

"I wasn't aware that Martin and Al Carruth tunnelled the UTB. I would like to hear his rationale...hopefully he will chime in. "

viewtopic.php?f=10101&t=17650

Thanks again!
Author:  Steve Saville [ Sat Jun 21, 2008 10:39 pm ]
Post subject:  Re: Brace carving effects on monopole/dipole
Alan Carruth wrote:
Yup; easy to over-think this stuff alright..........


Maybe for you! :lol:

I can't over think it because I get confused real fast. [uncle]
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