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25’ radius and mid-range Emphasis
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Author:  bob_connor [ Mon Aug 20, 2007 12:25 am ]
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I was perusing another forum when I came across an interesting response to a question from Bruce Sexauer.

Question was put from a builder that his guitars had a strong mid-range emphasis.

Bruces' reply was "that is the main symptom of building with a 25' dish".

Question was put "can this be softened by further voicing of the top?"

Reply was "I doubt it"

We are just about to change the radius on our tops, not for the above reason, but it has set me thinking about using differing top radii to change the tonal characteristics of the instruments.

Anyone care to comment, discuss, dispel, hypothesise or argue about the merits or not of the above proposition?

Cheers



Author:  Shane Neifer [ Mon Aug 20, 2007 12:39 am ]
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My question would be, "what does Bruce use and why?" Got to start somewhere!

Shane
Author:  bob_connor [ Mon Aug 20, 2007 12:52 am ]
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Don't know why Shane, but I think he uses a 40'

Cheers
Author:  bob_connor [ Mon Aug 20, 2007 1:26 am ]
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Hesh, the only reason at present is to get a better fit (or maybe a quicker and easier fit)

We half radius the transverse brace but still find that we have to do some work on the top between the neck block and the sound hole to get things right. I'm just trying to eliminate this step.

I know you do yours with the flat brace and true up the upper bout to fit but I'd like to keep the full radius on the entire top if I can.

My main interest in all of this is what will the change of radius do to the frequency response of an instrument and how can I leverage this to produce a predictable result.


Cheers



Author:  Dave White [ Mon Aug 20, 2007 1:43 am ]
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Bob,

Take a piece of top wood and tap it. Then bend it into a radius and tap it - the pitch goes up. Then bend it into a tighter radius and tap it - the pitch goes up again.

Same thing if you thin the centre of the top - the pitch goes down.

In a lot of ways it is easier to design/adapt the construction to bring in bass than it is to bring up mids/trebles (just imho by the way) and so I build my tops with the braces radiused to 13' and with the top around 3-3.2mm in the bridge area. There are a whole host of things that I then do in the design to bring the tonal range into the "balance" that I am looking for.

Bruce is a great builder and his building mantra/techniques would appear to go the other way - very flat tops and build in the mid/treble balance from there.

One thing that fascinates me though - if you build a truly flat top, and brace it to make it responsive, string it up and then measure the top a couple of months later (same temperature and relative humidity) is it still a trully a flat top?
Author:  James Ringelspaugh [ Mon Aug 20, 2007 1:52 am ]
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Changing the radius of the top is just one thing in a million things you can do to affect the tone. If someone told me that one particular radius will give one and only one sound and there's nothing you can do about it then I would think at best that's a pretty myopic view of building.

All other things being equal, adding a radius adds stiffness to the top without adding any weight. The smaller the radius the more stiffness. If the plate were over braced and rather massive to start with I wouldn't expect the added stiffness of a radius to make much of a difference in sound, but if the plate were gossamer thin and braced very lightly I could see it making a huge difference in bringing out mid range and high-frequency sound. YMMV.

Author:  bob_connor [ Mon Aug 20, 2007 1:52 am ]
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[QUOTE=Dave White]
One thing that fascinates me though - if you build a truly flat top, and brace it to make it responsive, string it up and then measure the top a couple of months later (same temperature and relative humidity) is it still a trully a flat top?[/QUOTE]

How significant would you reckon this natural change in radius would be contributing in to the guitar "opening up" Dave?

Cheers
Author:  Steve Kinnaird [ Mon Aug 20, 2007 2:57 am ]
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My thinking is somewhat along the lines of Lex's...that the domed top increases the stiffness of the top, and thus the whole thing can be built a wee bit lighter.
Which has always resulted, in my building, in a more profound bass response. It's a function of one's particular bracing scheme to bring out the trebles.
I haven't noticed a rise in mids with a 25' domed top, just a better all-round sound.

Steve
Author:  KenH [ Mon Aug 20, 2007 2:57 am ]
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I build all of my guitars with a 16' back and a 28' top. The only reason I have for this is that I was taught to do it that way by my mentor and the results I get are exactly what I want.  By experimentation I have found that I can tweak the sounds and increase the midrange response by doing several different things as stated above. In all honesty I dont know that any of these tweaks would be at all noticable to the untrained ear, but I can hear the differences. Different bracing is just one of the tweaks. The back bracing also contributes to this as well as the kind of wood the top is made out of.  Some of these tweaks also help to increase the sustain on the guitar. If you ask if I have scientific proof of what I claim, the answer is no. I doubt that few if any builders have gone to that extent. I have had the experience and privelidge of having several of my guitars on hand at once and the ability to play one guitar and compare it to the other. This is what I base my claims on.


I have been wanting to build at least one guitar with a 16' top AND back just to see what the differences are. I just cant seem to fit that into my schedule right now as commissions are backing up quickly. I am quite interested in other's experiences on this subject. I know that Hesh builds with a more radical radius than I do and I would do just about anything to be able to play one of his guitars just to compare the differences in our guitars.

Author:  Dave White [ Mon Aug 20, 2007 3:41 am ]
Post subject: 
[QUOTE=bob_connor] [QUOTE=Dave White]
One thing that fascinates me though - if you build a truly flat top, and brace it to make it responsive, string it up and then measure the top a couple of months later (same temperature and relative humidity) is it still a trully a flat top?[/QUOTE]

How significant would you reckon this natural change in radius would be contributing in to the guitar "opening up" Dave?

Cheers[/QUOTE]

Bob,

Alan C and Rick are the experts on "opening up", not me. I only make the comment as an observation. All the pictures of the pre-war "cannon" guitars that I have seen show the tops with substantial arching in them in the lower bout area - even though they weren't necessarily built that way originally (although the Larson brothers famously did).

I think of guitars a bit like drums - if a system is under tension and you disturb it, the original tension contributes to the dynamics of the system, and not necessarily in a negative way.

Building with tops with small domes in radius dishes I suspect is something that has evolved to cope with humidity changes and make the building process "easy" (sand out the rimset in the dish to get a fit, no huge problems with neck angle geometry and fingerboard extensions etc)rather than for tonal shaping.
Author:  James Orr [ Mon Aug 20, 2007 3:58 am ]
Post subject: 
Another absolutely important thing to question is what exactly he's
hearing when he thinks of mids. High-mids? Low-mids? What? If the
guitar were a parametric EQ, what frequency range does he hear hyped
too much for his taste?

In Advanced Voicing, John Mayes mentions using a 28' radius over '25 to
smooth a guitar out a hair.
Author:  David Collins [ Mon Aug 20, 2007 5:42 am ]
Post subject: 
When put in to a spruce top, there is no real difference between a 25' and
28' radius. A theoretical 28' radius will become a 25' radius with about a
3%-5% increase in relative humidity, if that. There is a difference if you are
cutting a glass lens, but on a guitar they are the same thing for all practical
purposes.
Author:  Alan Carruth [ Mon Aug 20, 2007 9:36 am ]
Post subject: 
As David Collins says, the difference between a 25' and a 28' radius is negligable for all practical purposes.

Bruce Sexauer is a good builder, but I have to wonder how he can know with such certainty what's going on with your guitar. It's entirely possible that _his_ guitars tend to be mid-rangy (whatever that means) with a 25' radius, but mine aren't (by my definition of 'mid-rangy'), and I've seen plenty of others that aren't.

The sound of any particular guitar is the result of the way all of its parts work together. There are NO independant variables here, everything is effected by everything else. There's sound coming off every part of the guitar at some pitch or other. It doesn't take much of a change in something like brace angle or profile to completely alter the tone of a guitar, and I can't think of any minor change that can't be balanced out by changing some other thing a bit.

The trick is to know what to change. You learn these 'luthier tricks' from experience, mostly, but you can also pick up a lot of good stuff on a list like this. Some of us are working on some scientific approaches to these things, too, and that can be helpful. Once you figure out what you mean when you say 'midrangy' it's often possible to find construction features that correlate with that. So start out by degining in an objective way, if yo ucan, what you mean. Then we can talk about where sound in that pitch range comes from, and that might give you ideas about what you can change.




Author:  John Mayes [ Thu Aug 23, 2007 12:53 pm ]
Post subject: 
actually I like a 30' radius to smooth out the tone a bit over a 25'. Don't
know exactly why it is that way, but in my guitars there is a difference.

On the McPherson guitars we use a 15' radius. Yes you read that right a
15' for the top and a 12' for the back. That is way more domed than a
25' and I don't feel McPhersons are midrangey at all. Quite the opposite
actually.

There is so many variables that is is really hard to pin down one thing that
makes the difference. Best we can do is make a bunch, and try to sort out
the details as we go along trying to improve each and every instrument.

For mid-range honk, yet with good balance I like a light slightly smaller
bodied instrument with a softer back (like Mahogany) and a spruce top.
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