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From another thread, Rick said  "a heavily scalloped Martin-style dreadnought which has tremendous "whump" in the bass"...

One common theory of bass response (and one that Somogyi teaches) is that it comes primarily from monopole pumping of the top.  Why then would scalloping (breaking up the top into phase/anti-phase) emphasize bass?  I'm not arguing because I've certainly seen a lot of scalloped guitars with great bass.  Just curious about the theory. Are you saying energy that would normally go to the mids is going to the bass?  How, if it's not as capable of monopole?

My experience is that Somogyi's guitars (and mine since taking his class) have tremendous bass but it is different in character from good scalloped braced guitars.  Hard to put into words but maybe more enveloping with less cut.

Rick also said "Guitars without scalloped braces tend to have more midrange punch..."

In my experience, Somogyi influenced guitars tend to have the bottom end without sacrificing mids, something rare indeed in a guitar with scalloped braces.

But again, I'm sure there are some guitars with scalloped braces that have it all going on but I would assume you have to do something to get the mids back.  What is it and why would someone scallop only to have to fix something?  Or is it just that they are building intentionally get the whump with less mids?

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Scalloping loosens up the top which means the main top freq will drop (loose tops lower resonance)

If you carefully read what I wrote, you'll see that what I'm saying is that scalloping the braces causes more mid-range cancellation. Monopole pumping is enhanced by thinning the top around the edges and tapering out that last bit of the bracing. If you do both, you've got yourself a classic Martin.   

I haven't taken Ervin's course, but I've seen a fair number of his guitars come through here for Addam Stark finishing jobs. One thing none of you are talking about is how incredibly thin his backs are.   I've seen guitars on which the backs cannot be more than 2 mm thick...and that's on dreads.   That may have a lot to do with the perceived bass response.   I don't think it's all in the top.

Yep...

Thank you for your response to me on that other thread Rick. It is helpful.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

"scalloping the braces causes more mid-range cancellation"

I would think that anything that "breaks the top up" (areas of drastically differing stiffness) would weaken monopole, even with loose edges.  Are you saying that the scallops only effect mids and don't impede monopole?  Again, not arguing, just trying to understand the theory.  Even though I mostly take an empirical approach to building, I still like to keep theory in mind.

Also, I'm sure Ervin's backs (and everything else) have a big effect on sound but, since taking his class, I have only changed my tops and have gotten a huge increase in bass response.  Having gotten some info about that, I'm now ready to start messing with the backs.  But I'm pretty sure the top is most of it.

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Hi all,

To my ear, my non-scalloped, 70's Martin has more base response than my forward shifted, scalloped J-45 (on which, I happen to really enjoy its midrange).  I appreciate that these two guitars are, perhaps, very poor comparative examples (I'm also bracing myself to hear that neither is a "good" guitar), but at the same time, it's the surest way for me assert that these properties of base/mid/treble in the guitar must at least begin elsewhere in the system.  From where, I surely don't know....yet.

 

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Stuart Brunker

Preface: This is a casual observation.

I know guitars are complex systems and it is dangerous to generalize, but to a large extent aren't choices and adjustments in the brace shapes an attempt to decouple vibration modes (discouraging coupling of resonant frequencies)? That seems to be what Al C. (and others) is getting at when he observes the vibration patterns of a chosen plate/bracing system. Comments?

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

Most of what I've seen Al write about with regard to coupling involves fairly low frequency resonances...top to back main, top to air, etc.   What we're dealing with here is the breakup of the top at mid frequencies into areas that are moving in and out of phase.   A lot of this is in the 200 Hz to 1,000 Hz region...upper bass into real midrange.   

I'm not sure that "coupling" is the right term for Al's work on "closing the ring" with Chladni pattern testing of free plates. I'd like to read what Al might say about this as I hate to stuff words into his mouth.

Don't forget that as we bust up the midrange response, we are still not messing with the air resonance from whence comes much of that bass whump. The bass may appear to increase, but really we're just decreasing the mids. So in this "EQ'ing" of the top, a lot of what we're doing is really filtering...that is taking something away. We're not adding anything unless we increase the efficiency of the guitar top at the same time we're suppressing midrange...resulting in an apparent bass boost.   

I like Sea Scallops with White wine & garlic butter!
Oh !
Sorry wrong forum!

The width & height of the braces along with the thickness of the top(stiffness also)
String guage the guitar is made for all play a role.
Tapering the brace(on the sides) from its glue joint to the top of the brace will take weight from the brace but not the stiffness!Unless you go crazy and have it toothpick width!
There are so many variables that ADD up to the tone the top is able to produce that you have to build each top with the sound your trying to produce-UNLIKE factories that thickness tops all the same and have pre-made braces to glue on.

And of course back & side depth& thickness & stiffness FLAVOR the sound of the guitar!
So much to think about !


www.collinsguitars.com

Whew !
mike

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Mike Collins

Almost all guitar tops I've lookled at, with very few exceptions, show the same four low-order modes when the instrument is assembled. The differences in brace stiffness distribution in this range show themselves as alterations in the relationships between the mode pitches, with only marginal differences in mode shapes. It's only when you get to the higher order modes that there is much evidence of 'break-up' owing to localized stiffness or mass concentrations. These are certainly important to the overall tone of the instrument, but have less influence on the actual power output than you might think.       

Most of the bending of the top in the monopole mode is taking place in the center of the lower bout. If you reduce the brace height there you drop the stiffness and pitch of the monopole mode, and increase the amplitude. In an article in one of the last numbers of the Catgut 'Journal' Richardson showed that this is much more effective in increasing output of the monopole than thinning the edges.

The monopole, or 'main top' mode, interacts with the 'Helmholtz' or 'main air' mode to form a 'bass reflex couple'. It's like the speaker and ported cabinet familiar to hi-fi in that low to mid range. It's important to keep in mind that when we talk about the 'main air' or 'main top' resonances on a completed guitar, we're talking about two halves of the same resonance couple, and not two seperate things. Many researchers feel that almost all of the power, up to 1000 Hz, is coming from the 'bass reflex couple'. Even though the 'main top' partof the couple is down around 200 Hz or so, it's just so much more effective at pushing air than any of the higher order modes that it predominates even a couple of octaves up.

The scalloped bracing, by moving more air through the top monopole, actually increases the output at the so-called 'main air' pitch an octave or so down from what we're pleased to call the 'main top' mode. At this low frequency the top and the air movement through the hole are certainly out of phase, but the air so predominates that there is more than enough 'residue' to give quite a lot of power. Enhancing the top mode amplitude by scalloping the braces can easily add to that: the gain in air flow is greater than the loss to phase cancellation.

Adding in the back complicates things, but is another way to boost the bass.

Another feature of the usual scalloped brace job is that there are peaks in the braces in the 'wings' outside of, and above and below the line of, the bridge. This extra stiffness will tend to raise the pitch of the two dipole modes. Typically on steel strings these will be at around 275-325 Hz for the 'cross dipole' and 350-375 Hz for the 'long dipole', iirc. Of these two the cross dipole is generally the less effective radiator of sound, owing to the fact that it has two similar size areas out of phase with each other and in close proximity compard to the wavelength it's trying to make in the air. It also, unlike the 'long dipole' doesn't have an inside air mode near in pitch that it can drive, and that can radiate through the soundhole. Energy that goes into the 'cross dipole' is often 'wasted', in the sense that it would have made more sound if it could have gone into a monopole type of motion. Thus the 'cross dipole' is often a 'cuttoff' for the monopole radiation: the spectral peak of the 'main top' mode ends in a 'dip' at the cross dipole frequency. The higher the pitch of the cross dipole the broader the 'main top' peak tends to be, and this often translates into an impression of 'fullness' or 'solidity' in the sound of the guitar, but with perhaps a little less 'clarity'.

The talk on the lists seems to put a wall between 'scalloped' and 'tapered' (or 'parabolic') bracing, and ignores 'straight' bracing almost entirely. In fact, it's much more productive IMO to think in terms of a continuum, from strongly scalloped bracing through straight to parabolic to a sort of triangular or even scooped taper. Looking at the way the different areas of the braces effect the modes on the aseembled instrument, and understanding the way those modes interact to produce the final tone, allows you to work with brace taper (along with patterns and angles, and the properties of the top wood) to fine-tune the sound.       

"If you reduce the brace height there you drop the stiffness and pitch of the monopole mode..."

Alan - this is counterintuitive to me to the extent that reducing mass of the brace should raise the resonant frequency of the associated area of top - I understand the gain in amplitude, the top is less constrained. What am I not getting? Does reducing stiffness of the brace more than offset the loss of mass?

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

Al, thanks for another thorough response.

"If you reduce the brace height there you drop the stiffness and pitch of the monopole mode, and increase the amplitude."

Maybe at issue here is whether one is trying to make the bridge the point of highest excursion (on a 14 fret guitar) or the center of the lower bout.  If the center of the lower bout, then it makes sense that scalloping would increase amplitude.  If the bridge, then maybe not?

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http://www.chassonguitars.com

Some further reading if you were feeling left behind while reading thru Al's comments.

Guitar Acoustics

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Tickle your guitar daily, and it'll tickle you back.

[QUOTE=SteveCourtright] "If you reduce the brace height there you drop the stiffness and pitch of the monopole mode..."

Alan - this is counterintuitive to me to the extent that reducing mass of the brace should raise the resonant frequency of the associated area of top - I understand the gain in amplitude, the top is less constrained. What am I not getting? Does reducing stiffness of the brace more than offset the loss of mass?[/QUOTE]

My understanding is yes, that when the height of the brace is reduced, the effect manifests as a loss in stiffness more than a loss in mass.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

I must say this is a very informative thread for me and much appreciated. I have always intuitively thought that monopole motion is affected more by stiffness at the rim rather than in the top's center. Now via Al Carruth's excellent post I'm seeing that the opposite might be the case. I think perhaps when considering graphs I've paid much more attention to shapes than scale. Oops. If true, it wrecks several ideas I've had...

[QUOTE=Alan Carruth]Energy that goes into the 'cross dipole' is often 'wasted', in the sense that it would have made more sound if it could have gone into a monopole type of motion. Thus the 'cross dipole' is often a 'cuttoff' for the monopole radiation: the spectral peak of the 'main top' mode ends in a 'dip' at the cross dipole frequency. The higher the pitch of the cross dipole the broader the 'main top' peak tends to be, and this often translates into an impression of 'fullness' or 'solidity' in the sound of the guitar, but with perhaps a little less 'clarity'.[/QUOTE]

My recent thinking has been that the increased stiffness at the peaks of scalloped braces would not only inhibit cross-dipole motion, but also enhance overtones which is at least one reason why clarity would be sacrificed and a more 'complex' and louder sound achieved ('louder' to our ears since we tend to register higher frequencies more so than low). I wonder if the extra energy in the system due to reduced dipole motion goes more towards clarity, volume, or attack and delay?

Kent:
I don't know if you've seen the thread on the 'cube rule', but that basically explains it. Stiffness goes as the cube of the height, so if you make a brace twice as tall it's eight times as stiff for the same width, but only twice as heavy. The stiffness/mass ratio rises.

lex_luthier asked:
" I wonder if the extra energy in the system due to reduced dipole motion goes more towards clarity, volume, or attack and delay?"

You're treading perrilously close to metaphysics, there. Things like 'clarity',and even 'volume' are really subjective. Not only are there no clear metrics, but different people will hear the same guitar differently in these respects. One of the big tricks in making custom instruments is learning to translate what folks say they want into terms you can work with. From time to time you can really get messed up.

Alan, I think my question must not have been clear.

The goal of scalloping is to make the area behind the bridge less stiff.  As you say, that is clearly one way to increase monoplole amplitude.  But it seems the goal with that method is to encourage an area behind the bridge to be the point of most excursion in amplitude.

On a 14 fret guitar, that area would be about at the wide point of the lower bout.  Clearly that makes sense if you only look at the cross axis, perpendicular to the strings.

But if you look at the long axis, parallel to the strings, wouldn't the point of most excursion naturally want to be at the bridge, not behind it?  One reason is that the bridge is closer to the center of that axis than the widest point of the lower bout is.  Another reason is the mass of the bridge.  Attach a weight to a bungee cord and pluck it anywhere.  The weighted area becomes the point of most excursion no matter where it is on the length of the cord.

And that's on a 14 fret.  On a 12 fret, with the bridge centered on the wide part of the lower bout, it makes even more sense to me to enhance the natural tendency or the bridge to be the point of most excursion.

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Why do you want the bridge to be the point of maximum excursion? In terms of getting the strings to work correctly, you want the bridge to be absolutely stationary; any motion of the string ends contributes to intonation problems, amoung other things. Of course, if the bridge didn't move at all there would be no sound produced, so we end up compromising.

And so it is with the top bracing. Certainly the 'main top' mode is the primary producer of power, and you want it to move as much air as possible. Absolutely the bridge is a huge mass concentration, and will tend to move more than any other part of the top. But that's not all the top is doing, and it's not the only thing that contributes to the tone. Small differences in the brace profile will probably not make a lot of difference in the way the top vibrates in its fundamental mode, but could make a world of difference at some other frequency.

Science and analysis can only take you so far. At some point you have to get into the art of figuring out what compromises you will make. Those compromises will define 'your' sound, and there's no 'right' way to do it, although there seem to be plenty of 'wrong' ways.       

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http://www.chassonguitars.com

Well, yeah, the bridge sure _wants_ to move on guitars, and maybe that's a good reason to _make_ it, but there are also good reasons to make it _not_ move.

Acoustic guitar bridges are generally stiff enough and heavy enough to offer a pretty good imedance mismatch with the string, so that the string will 'know' how long it is. It's not like the 'cello, say, where at some pitch the top reacts back on the bridge so strongly that it 'forgets' how to vibrate in the fundamental mode, and jumps up an octave. Once it stops making the fundamental the top vibration dies out, the string suddenly 'remembers', and builds the top vibration to the point of feedback again: the 'wolf' howls.

Sometimes, though...... One of my customers came back after a year or two with a funny buzz at D on the B string. The buzz tracked the pitch if you retuned, so it was a resonance issue. However, it was only on that string. We spent about four hours checking all the 'usual suspects' with no luck. Finally I recorded the pluck on my computer and ran it through an FFT. Every third peak was 'split': instead of coming in at a single picth the 3d, 6th, and 9th partials were at two pitches close together, and the buzz was the difference frequency. It turned out there was a strong resonance in the top at around 880 Hz with a moving area at the point where the B string crossed the bridge. A few shavings off the upper end of the upper tone bar moved the node line closer to the saddle and changesd the pitch just a little, and the problem went away with little change in the timbre of the guitar. I saw him the other night, and he still uses that little cedar/mahogany OM: currently it's in a 'Nashville' setup, but I've heard it in 'drop' tunings down to C. Whatever you do to it, it just says:"OK boss!" and gets on with the program.