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I’m working on thicknessing the top on my fourth instrument. This one is an L-00 with a lutz top. At 4 guitars I don’t have a lot of deflection data. The top thickness currently is .105 +/- . The deflection ( joined but not cut out, bars on jig @18” and 5lb weight) is .280. I’m not good at describing tone but I’ll call the tap tone lively. Compared to the other tops I’ve worked with (Sitka and Adirondack) it’s weighs a little less. There’s little to no whomp or sheet metal sound when shaking it. I take that sound to be the result of cross grain deflection and the lutz feels quite stiff across the grain. My understanding is cross grain deflection is not a paramount consideration. So thin more for whomp or good to go? My current thought is plane off a few thousandths and see what happens.
Thanks,
David


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I was really quite surprised at my experience in this whole thing. I had been using the sheet metal whomp for years and on many guitars. Then I learned about deflection testing and the first thing I did was thin out to my 'whopm' and then do a deflection test, measure it, and put it in my note book. Several gutiars later I bought the Gore/Gillet books and took a regular piece of spruce and tested it with their method which is only on one half panel. I joined the top, thickness sanded it very accurately to the target result and then did a deflection test and low and behold it had exactly what I would have thicknesses to based on reflection and also it had the same whomp.

I've done this several times now and in fact the results are so much the same I could just do the whomp test. But I still measure tops acoustically and do deflection just because, you know, notes and science and all.

The thing is one man's whomp is another man's whompfph. I just got lucky. But there is a range within the womph that can make a big difference too. I thin out to the point where I just 'barely' start to hear the sheet metal. Soem might go farther and get different results.

Great topic of discussion though and I look forward to hearing other experiences. I have given a great deal of thought to this and still do on every guitar I build... Especially on guitars where the top wood I have is not rectangular and so NONE of these tests work.

A couple of years after I got serious about building in 2004 I was lucky enough to have my stuff in a store where there was the availability of several great professional players with good ears plus some with years of merchandising experience in guitar sales as well.

When I started measuring deflection I had been going partly by specs from the Fox building course and tap tone. When everyone at the store agreed a guitar was really something special I noted the specs and actually wound up making my tops a little thicker with less deflection. It was an ongoing process. I never used the "whoomp" method but I am sure it works.

I recently built my first Mahogany topped OM and used my deflection specs for spruce. It turned out great so that was cool.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Like JF I have found that thinning till the top just has that sheet-metal “whomp” works best for me. I’m keeping them a bit thicker then I was and they seem to sound better. I did deflection testing for a few years but it seemed to be redundant for me. If anybody wants my deflection testing jig stop by and you can have it.


Steve

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Steve Smith
"Music is what feelings sound like"

It seems to me that, to the extent we're all trying to do the same thing, any method that gets us there should converge on similar specs. I'd been measuring stiffness more or less by Gore's method for some time before his book came out, and had worked out a simplified way to calculate the 'right' thickness for a given set of wood that works well for me.

My main method of getting the sound I want is the use of 'free' plate tuning via Chladni patterns: a 'tech' version of 'tap tones'. In my mind this establishes a balance between the stiffness of the top and the braces, so that neither dominates. In a sense, you can think of bracing as a necessary evil: you want just enough to keep the top from folding up too quickly, but not so much as to produce hard spots that keep it from vibrating freely. I've checked this out in a workshop with Dana Bourgeois, and we seem to converge on the same brace profiles.

At one point, several years ago, I sent one of my 'test mule' guitars to Dave Hurd so that he could check it out using his deflection method. He uses a down load on the bridge of the assembled guitar, and checks to see if the contour pattern of top deflection is a smooth 'bullseye': shaving braces to get it right. My top was just right. There's more than one way to skin the cat.

Yep.

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Steve Smith
"Music is what feelings sound like"

Thanks for your comment Alan. This brings up another point. I think you mentioned in the past that you had hearing problems and wore hearing aids. Correct me if I am wrong. At 78 I have been loosing hearing as well and wear hearing aids now. Quite a bit of high frequency loss. I know there is a lot of stuff going on with a guitar that I don't hear any more and if I wear my hearing aids I know that what I hear is probably not what a person with normal hearing appreciates.

How do you deal with that as far as listening to tap tones etc. ? Do you wear your hearing aids when evaluating tonal responses while building?

Thanks

Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Hey Terry, I’ve got hearing aids too. Mine are in all the time unless I'm asleep. For playing music and when I work with guitars I had the audiologist set up a program that disables feedback processing and sets gains so I hear high frequencies at a level similar to an ideal response. Not as good as my younger ears but still works pretty good.

Also should say that the type of hearing aid can make a big difference. My first set were Widex and the music program was just ok. My current set are by Opticon and they do a much better job.

Steve

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Steve Smith
"Music is what feelings sound like"

Here’s to the “whomp”!


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Thanks for the replies. I guess my question comes from the discrepancy from (only 3) previous guitars in terms of long grain deflection vs. thickness at whomp. In the end I left it for now at .100 to .105 thickness, starting to whomp. One face up whomps more then the other. Deflection @.3 (average from either face). It’s also somewhat lighter then previous tops.


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I built 8 L-OOs and half of them were with Lutz tops. My Lutz tops ended up being around .100 and my Adi tops were around .095 and in both cases it was give or take approx. .005". I deflection tested, I used my data but still used my own method of tap tuning to open the tops up as I wanted and I was happy with the results. Not going to detail it here but my tap tuning method begins with unbraced plates during the thicknessing process.... What a concept.

You realize too that this is very subjective in so much as ultimately we all are relying on something we cannot quantify, what we hear as our success meter. There is also an inverse relationship or then can be between plate thickness and all things bracing. Or, in other words the braces and the top are a system that once assembled function collectively making plate thickness only part of the process of a nice, open, resonate top.

It's always going to be possible that the numbers gleaned from one top and testing do not mean that the same results will be had by all tops. The duality of wood.... even from the same tree so to speak.

But your .105 sounds very good to me and it also sounds like you have already benefitted from your deflection testing. Good going David now give as much thought to all things that make a guitar a tool for a musician, set-up, neck angle, action, nuts and saddles, did I say fret work and you will have a killer guitar.

About 5-7 years ago I bought a handful of sitka tops to specifically compare the ideal thickness per Trevor Gore's equation, deflection data along the way (i.e. thickness when it deflected .25, .3, etc), and the warble.

I'm convinced that I don't record the node points for Trevor's approach accurately, because mine are always wildly different than what he consistently records in the book, so I disregard that.

But either way, I consistently got the warble well before getting to .25" deflection or his target. Everyone's mileage varies, but I personally don't consider it in my building anymore and instead look for a deflection between .25-.3" using 18" spacing and 5 lbs.

Yeah I have always had difficulty using the Gore/Gellet method.

As for deflection of course many factors matter. I use a standard red brick (I forgot the weight) and cut my tops to 1 cm outside the shape of the body and place supports 1cm in on each end. I place the brick right on the bridge patch and measure deflection at the x-brace crossing. I do this after installing the rosette but before cutting the sound hole.

The problem comes when you get odd shaped wood that is not square. I have some master grade Red Spruce and Euro that seems to come like that more than Sitka for example. They probably get a better yield when cutting angles into the upper bout else perhaps cutting knots out or something.

That's when you have to draw back on your luthier skills and do old school touch and feel.

I've seen some pics online of deflection rigs where the deflection is measured with the top cut to shape. I guess that would avoid that problem but then I don't see how you can get any real information from that method either. I suppose what it comes down to is pick something and stick with it.

I've only done one flattop; the little Stauffer. The rest are archtops, and violin family instruments, so I do things more like Alan, but without the Chladni patterns. I don't have a speaker!

I did check the belly on the Stauffer for defection. It moved what everyone said. It never whomped. I braced it lighter than the plan; both front and back. Sounds great. I have some more flattops to do, I'll see what I come up with.

I tap all the time. It's cool to hear the wood talk. Violin makers have tones and weights they use as measurements of stiffness, and lightness; trying to get light and stiff. It might work for bellies to get to a starting point; maybe the whomph is a starting point; because you have to brace to make it strong enough. If you use different back woods, or slab, and quartered; the tones can be all over the place. Flamed birch came out very low. Other woods are higher.

I get arch tops to where they feel right, and have nice resonance; just like violins, and cellos. Then you can tune them on the ribs, or on a form so the sides are not free. If you do it on the form, you can dial in bracing before the top is glued on. You can still do minor things on the outside if you want to later.

I got this from Area tuning by Keith Hill, a harpsichord, and violin maker. I used to have links for it on the computer. Checking his site I don't see it. Ah. He has a book. His area tuning hints, is on the GAL, but I don't have a membership. His idea seems to work. I don't go for specific tones as he does; just an evenness. The arching adds the tones in. He seems to think like I do on most things. Some is very heady, but not technical at all.

Something different.

My 15 inch arctop belly is mostly done on the inside, and getting there outside. The edges are still very thick. Tapping it for a low mode; balancing on the middle of the plate, and tapping the edge I found something interesting. Tapping at the lower edge I got d# (first fret 4th string, and tapping on the lower bout edge gives G# on the low E. A fifth apart. Two low modes.

Tapping is just fun.

Is whompping fun?

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