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top bracing critique
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10102&t=13401		 Page 1 of 2
Author:  Ken Jones [ Mon Aug 27, 2007 7:13 am ]
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Hello all --


I've been carving away at the bracing in my two OMs, and I thought before I went any further I'd post a couple of images and links to short audio files of the corresponding tap tones. I'd really appreciate some feedback on what you're seeing and hearing, as I still feel that I'm flying blind.



Engelmann



Sitka


Audio files are here: http://www.soundclick.com/bands/pagemusic.cfm?bandID=743213


I did spectrum analyses on them both, sort of before, during, and after carving, and was able to track the descent of the dominant overtones (are these modes?) of each. The sitka top(second grade), currently has an emphasis at 250 hz, or B3. The engelmann is a little lower, at 247 hz, but still at B3.


Am I in the ballpark, or have I taken away too much? The engelmann was definitely floppier than the sitka, so I left a little more brace material.


Anyway, thanks in advance for your insights.


Ken
Author:  JJ Donohue [ Mon Aug 27, 2007 7:23 am ]
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To my ear #1(Sitka) has a more appealing tone than #2 (Engleman). This is purely subjective. It's hard to determine the volume without banging on it myself but neither has the sustain that I would want. IMO, you can loosen it up quite a bit on both.
Author:  KenH [ Mon Aug 27, 2007 7:41 am ]
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I will agree with Hesh on this one. I have opted for a thinner and taller style of bracing. Even when I think I am on the cutting edge of being too little bracing it is turning out that I could have reduced them a little more. If anything, my guitars are getting lighter and with less bracing all of the time.


One suggestion I can offer you is to reduce some of the mass of the transverse brace.

Author:  Michael Dale Payne [ Mon Aug 27, 2007 8:11 am ]
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Pretty much the same here. The upper Transverse brace looks way beefy and almost no need for the upper transverse graft on also the X brace on the sound hole side appears a bit beefy as well.
Author:  Alexandru Marian [ Mon Aug 27, 2007 8:38 am ]
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The best way to find the main mode is to hold the top near a speaker which plays tones. You can use a tone generator, or create a midi with very long note duration and include all halftones from 80 Hz to 350. 

You can either hang it by a hooked index finger through the soundhole, or, this works better for me, use the lower bout wings as support points. What I do is sit down in front of the speaker, place one wing on my toes and support the other wing with a hand. With the other hand i slightly touch the center with my nails. The vibrations will create a noticeable rattle at resonance.

From my limited experience, spectrum analysis of a knock tone gives different results.  However, your main mode is indeed somewhere between 250 and 300 hz judging by how tight they sound.

Keep in mind that when you attach it to a rim, you are adding a lot of stiffness and  that pushes the main mode another 50Hz or so higher. Then with a bridge on, it will go lower. 

I'm afraid I can't help you with specific targets for an OM. I know them for a classical guitar of a certain shape, volume...

Author:  bob_connor [ Mon Aug 27, 2007 8:57 am ]
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What everyone else said.

We seem to be able to squeeze that sustain out of the top that JJ is talking about by feathering the finger and tone bars down at the X-brace. Gotta remember that this is in conjunction with the way we carve the rest of it. Keep flexing the top as you do this to see if you can notice changes in stiffness.

Don't be too apprehensive about getting stuck into the tone and finger braces. They are relatively simple to chisel off and replace if you take too much off and you will probably learn a lot about how far you can take them down by doing this.

You should be able too hear when the top starts to develop a nice resonance and sustain when tapped at various points. If you take too much off that sustain and resonance will diminish.

Interestingly enough Dave and I hold the tops at different nodal points when tapping. It seems our ears are wanting to hear something different from the top.

Cheers

Author:  jhowell [ Mon Aug 27, 2007 9:48 am ]
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Ken--

Thanks for creating the sound files! While I can't offer any meaningful advice, having something to compare my efforts with is a very good thing. For those of us with a total experience of tapping tops in the single digits files like tese are worth their weight in gold.
Author:  Ken Jones [ Mon Aug 27, 2007 12:07 pm ]
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Hey guys --


Many thanks for the feedback. It dawned on me that I neglected to mention my "holding point," which seems to have a huge effect on the energy and resonance of the tops. These were recorded while holding the plate an inch in from the edge of the upper left bout. I think this is partly why they don't sound very resonant. When I suspend the plate with one finger right at the truss rod hole in the shoulder brace, I get a much deeper, more resonant tone. Interestingly, spectrum analysis still shows an emphasis at 250 hz, but an additional bump at around 90 hz.


Where do you guys hold your tops? Also, is there something better, like a soft-faced mallet, that you tap with? I have trouble getting consistent hits with my knuckle. Another question I have is how much does the extra half inch of material around the edge of the plate effect the tone?


Hesh, both tops are in the .110 to .115 range, though since I'm planing/sanding by hand, I did accidentally get the engelmann a little thinner down the middle (.095 to .110), which explains the lower tone.


Terken, my x-braces are 5/16.


I'm going to re-record both tops, holding them as described above. I'll pull some other measurements and post later. Thanks again!


Ken
Author:  James Orr [ Mon Aug 27, 2007 1:55 pm ]
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Hey Ken

I hold between the upper finger brace and the X brace, about halfway
between the edge of the top and edge of the X brace, with the brace side
facing my cheek.

Another question: What is the goal for this guitar? Strong attack? Lots of
overtones? Strumming? Fingerstyle? Flatpicking?
Author:  Ken Jones [ Mon Aug 27, 2007 3:33 pm ]
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O.k., the new audio files are up, this time supporting each top at the truss rod hole. They're located here: http://www.soundclick.com/bands/pageartist.cfm?bandID=743213  the sitka comes complete with guest appearance


Do they still sound too tight?


I'm using the Antes plans, but thank goodness I checked the archive first -- thanks again for all that good info. I reduced the x-braces to 8 x 15 mm, as well as the upper tone bar. Lower tone bar was cut down to 8 x 12 mm, fingers 8 x 7 mm, and shoulder brace is 14 x 18 mm with a rounded section profile.


Hesh -- I checked again just to make sure, and the plans do show a .125" gap between the upper tone bar and the bridge plate. Maybe its been revised? Also, the gap looks greater in the photo than it really is.


James -- Great question. The sitka is for an old buddy just learning to play, so definitely mostly strumming on that one, and the other will be mine until I sell it, so a fairly even mix of strumming, flat and fingerpicking.


I appreciate very much everyone taking the time to help me out here, I'd be lost without your guidance. If you're ever in Asheville, I'll buy you a tall cold beverage of choice.


Ken
Author:  James Orr [ Mon Aug 27, 2007 4:12 pm ]
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I love the guest appearance! Hahaha!

I think the Sitka is fine where it is for a strumming guitar. It's a little
tight, but I wouldn't know when to tell you where to stop. You should
check out John Mayes' Advanced Voicing DVD. It has some great
examples for you to hear about where to go for different goals.
Author:  bob J [ Mon Aug 27, 2007 9:01 pm ]
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THANK ALL OF YOU. This is one of the best discussions of tuning the top I have read and seen
Author:  CraigSz [ Mon Aug 27, 2007 11:50 pm ]
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Great discussion everyone. This really helps me to get a better handle on bracing and associated tap tones. Thanks


 


Craig.
Author:  JJ Donohue [ Mon Aug 27, 2007 11:59 pm ]
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Ken...they both still seem tight to me. There are 2 issues that make it difficult for me to comment with certainty:

1) I'm still not sure that you're holding the plate in an optimized nodal spot. I always hold mine on the treble side, upper bout, between the UT brace and the upper leg of the X brace...near the edge. Perhaps the TR spot is the same but I've never compared the two.

2) There is an inconsistency of tapping force between the 2 tops to my ear. I am one who uses a piano hammer and feel that I can get a consistent force when tapping.

In general...I feel that both tops are still very tight and can stand more brace shaving...but I'm not certain because of my concerns.

BTW...what do you think? How do they feel as far as flexibility now?

Keep trying...this is a good exercise for all of us.
Author:  WaddyThomson [ Tue Aug 28, 2007 1:39 am ]
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Am I missing something?  When I went to hear the new tap tones, there were no audio files.  The page was there, but no links.  I wantta hear too!   
Author:  Ken Jones [ Tue Aug 28, 2007 2:26 am ]
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Waddy --


I accidentally put the intro page link instead of the page with the files. Clicking on "music" would take you there, or just click here: http://www.soundclick.com/bands/pagemusic.cfm?bandID=743213


Thanks JJ, terken, and James for the input. I'm gonna have to get me a piano hammer. I'm thinking that I might shave down the UT into a parabolic shape similar to Hesh's tutorial pics. Hopefully that will not only remove some unneeded mass, but also loosen up the tops that extra little bit. Does this seem logical?


K
Author:  James Orr [ Tue Aug 28, 2007 7:21 am ]
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Keep in mind that the more your top resonates when tapped, the more
overtones you'll have and less fundamental. Guys, correct me if I'm
wrong. That might not be what you want because it'll get pretty washy
when you strum.
Author:  bob_connor [ Tue Aug 28, 2007 9:01 am ]
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I hold the top on the bass side of the lower bout about an inch and a half in from the edge and around the same distance from the upper tone bar.(between the upper tone bar and the X brace.

The exact position will vary from top to top.

I tap all over the top with the pad of my middle finger in the initial stages of carving listening for areas that lack sustain or sound "thuddy".

When I'm reasonably happy with the consistency of sustain over the top I will hone in on tapping the bridge plate.

When you tap it here you should hear two tones that are reasonably distinct.

There is a higher pitched sound that is the initial attack of your finger on the soundboard and a lower rumble.

It is the higher pitched tone that I am listening to and that is the one that I want the most resonance and sustain from.

My reasoning is that the strings are attached to the bridge/bridge plate and when the strings are strummed we want to hear resonance and sustain.

Tapping over the bridge plate should set the entire top into motion in a similar fashion to strumming the strings.

This is my way of doing it and it seems to work for me.

Mind you, after I have voiced a top I leave it for Dave to have a listen to and vice versa.

He holds it in the upper bout and taps. I'ved tried it like this and don't like the sound I'm trying to listen to. It is much lower in pitch. If he feels it is not quite right yet his usual comment is "it doesn't ring enough".

He'll have a bit of a carve and when he is happy with what he is hearing, I generally find that the tone that I've been listening to has also improved.

Cheers




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